Re: Replacement for a DEC 7474 Chip
On Tue, May 17, 2022 at 12:35 AM ben via cctalk wrote: > Did DEC not use a few Non TTL chips to reduce I/O loading on the bufferd > lines? > DEC used non-TTL buffer chips for bus interface (Omnibus, Unibus, Qbus, and external buses like Massbus). Most of the other SSI/MSI logic chips are TTL or TTL-compatible. TTL buffers were usually used where higher fanout was needed on a module, or on non-bus backplane connections.
Re: Replacement for a DEC 7474 Chip
On 2022-05-16 1:50 p.m., Eric Smith via cctalk wrote: On 2022-May-15, at 3:53 PM, Eric Smith wrote: I specifically said 74x74. Early TTL flipflops were very crude by comparison. On Mon, May 16, 2022 at 11:28 AM Brent Hilpert via cctalk < cctalk@classiccmp.org> wrote: pre-TTL != early TTL No, but 7470, 7472, 7473, and 74948 were _very_ early and were also very crude, as were their later L and H variants. 7474 was slightly later, and less crude. It should also be noted that the 7400 series was NOT the first commerical TTL integrated circuits. The earlier TTL flip-flops were even more crude, but I imagine the engineers that used them were nevertheless delighted at the advance over RTL and DTL. Did DEC not use a few Non TTL chips to reduce I/O loading on the bufferd lines? Ben.
RE: Replacement for a DEC 7474 Chip
You are right, I thought I remembered someone else saying on this thread that he couldn't find a datasheet for the 7474 so I didn't look, but actually I found one quite easily! From: Paul Koning Sent: 16 May 2022 22:08 To: r...@jarratt.me.uk; Robert Jarratt ; cctalk@classiccmp.org Cc: Rick Murphy Subject: Re: Replacement for a DEC 7474 Chip On May 16, 2022, at 4:59 PM, Rob Jarratt via cctalk mailto:cctalk@classiccmp.org> > wrote: I have several 7474s, including one marked DEC 7474. Sadly, I fear that shipping from the US is likely to be prohibitive. I have ordered some 7474s. However I am getting a bit lost in the discussion now. From a simple fan out point of view, would an S be sufficient to replace it? I can think of two ways to answer that: (1) ask and see if you get an answer, hopefullly a correct one, (2) find the data sheet for the 74x74 you're looking at, and the devices it is driving, and do the calculation. Sum up the max input currents of the driven devices, compare with the min output current of the 74x74. If the output current is >= the load current, you have the needed fanout. paul
Re: Replacement for a DEC 7474 Chip
> On May 16, 2022, at 4:59 PM, Rob Jarratt via cctalk > wrote: > >> I have several 7474s, including one marked DEC 7474. Sadly, I fear that >> shipping from the US is likely to be prohibitive. > > I have ordered some 7474s. However I am getting a bit lost in the discussion > now. From a simple fan out point of view, would an S be sufficient to replace > it? I can think of two ways to answer that: (1) ask and see if you get an answer, hopefullly a correct one, (2) find the data sheet for the 74x74 you're looking at, and the devices it is driving, and do the calculation. Sum up the max input currents of the driven devices, compare with the min output current of the 74x74. If the output current is >= the load current, you have the needed fanout. paul
RE: Replacement for a DEC 7474 Chip
> -Original Message- > From: cctalk On Behalf Of Rick Murphy via > cctalk > Sent: 16 May 2022 17:28 > To: cctalk@classiccmp.org > Subject: Re: Replacement for a DEC 7474 Chip > > On 5/15/2022 4:16 AM, Eric Smith via cctalk wrote: > > On Sat, May 14, 2022, 16:09 ben via cctalk wrote: > > > >> On 2022-05-14 11:50 a.m., Nigel Johnson Ham via cctalk wrote: > >>> AFAIR LS can only drive one unit TTL load. > >>>> paul > >> LS is 4 TTL, 4 ma low. > >> Was there a trick of forcing the output of D flip flip to clear it? I > >> was wondering if this is what kills all the 7474's? > >> > > I don't think that worked on any TTL (or CMOS) 74x74 flip flops, > > except maybe by accident if you shorted the output enough to draw Vcc > > down (or ground up) enough to disrupt the FF, and then you have other > problems. > > > > Despite the logic diagram showing feedback from the outputs, all 74x74 > > have buffered outputs. The recent TI data sheets show an equivalent > > schematic only for the 74LS74. I can't at the moment find one for the 7474. > > > > It seems likely to me that early pre-TTL logic families like RTL might > > have had FFs with unbuffered outputs, but I haven't checked. > > I have several 7474s, including one marked DEC 7474. Sadly, I fear that > shipping from the US is likely to be prohibitive. I have ordered some 7474s. However I am getting a bit lost in the discussion now. From a simple fan out point of view, would an S be sufficient to replace it? > > -Rick
Re: Replacement for a DEC 7474 Chip
On 2022-May-15, at 3:53 PM, Eric Smith wrote: > I specifically said 74x74. Early TTL flipflops were very crude by comparison. On Mon, May 16, 2022 at 11:28 AM Brent Hilpert via cctalk < cctalk@classiccmp.org> wrote: > pre-TTL != early TTL > No, but 7470, 7472, 7473, and 74948 were _very_ early and were also very crude, as were their later L and H variants. 7474 was slightly later, and less crude. It should also be noted that the 7400 series was NOT the first commerical TTL integrated circuits. The earlier TTL flip-flops were even more crude, but I imagine the engineers that used them were nevertheless delighted at the advance over RTL and DTL.
Re: Replacement for a DEC 7474 Chip
On 2022-May-15, at 3:53 PM, Eric Smith wrote:
> On Sun, May 15, 2022, 13:03 Brent Hilpert via cctalk
> wrote:
> On 2022-May-15, at 1:16 AM, Eric Smith via cctalk wrote:
> > On Sat, May 14, 2022, 16:09 ben via cctalk wrote:
> >> On 2022-05-14 11:50 a.m., Nigel Johnson Ham via cctalk wrote:
> >>> AFAIR LS can only drive one unit TTL load.
> paul
> >> LS is 4 TTL, 4 ma low.
> >> Was there a trick of forcing the output of D flip flip
> >> to clear it? I was wondering if this is what kills all
> >> the 7474's?
> >
> > I don't think that worked on any TTL (or CMOS) 74x74 flip flops, except
> > maybe by accident if you shorted the output enough to draw Vcc down (or
> > ground up) enough to disrupt the FF, and then you have other problems.
> >
> > Despite the logic diagram showing feedback from the outputs, all 74x74 have
> > buffered outputs.
>
> Per TI schematics from 1969: 74 standard, H and L series flip-flops are
> unbuffered. Or at least many of them are/were, in their then-original form.
> Including 7475, 7490, etc. The output transistors connect both to the pins
> and wrap back to form the FF or other purposes.
>
> Collector-triggering was discussed a some years ago on the list in regards to
> a pdp8 front panel where DEC used collector-triggering on 74175's (IMO, bad
> design practice). From (my) empirical tests at the time, it turned out some
> 74S (Schottky) parts could be collector-triggered. However, between standard,
> LS, and S types, behaviour could vary with manufacturer and production date.
>
>
> > The recent TI data sheets show an equivalent schematic
> > only for the 74LS74. I can't at the moment find one for the 7474.
>
> > It seems likely to me that early pre-TTL logic families like RTL might have
> > had FFs with unbuffered outputs, but I haven't checked.
> I specifically said 74x74. Early TTL flipflops were very crude by comparison.
S = { "", "L", "H", "S", "LS", "F", "AS", "ALS", "AC", "ABT", etc. }
x ∈ S
"", "L", "H", "S" ∈ S
"L", "H", "S" != ""
7474: x == ""
pre-TTL != early TTL
Re: Replacement for a DEC 7474 Chip
On 5/15/2022 4:16 AM, Eric Smith via cctalk wrote: On Sat, May 14, 2022, 16:09 ben via cctalk wrote: On 2022-05-14 11:50 a.m., Nigel Johnson Ham via cctalk wrote: AFAIR LS can only drive one unit TTL load. paul LS is 4 TTL, 4 ma low. Was there a trick of forcing the output of D flip flip to clear it? I was wondering if this is what kills all the 7474's? I don't think that worked on any TTL (or CMOS) 74x74 flip flops, except maybe by accident if you shorted the output enough to draw Vcc down (or ground up) enough to disrupt the FF, and then you have other problems. Despite the logic diagram showing feedback from the outputs, all 74x74 have buffered outputs. The recent TI data sheets show an equivalent schematic only for the 74LS74. I can't at the moment find one for the 7474. It seems likely to me that early pre-TTL logic families like RTL might have had FFs with unbuffered outputs, but I haven't checked. I have several 7474s, including one marked DEC 7474. Sadly, I fear that shipping from the US is likely to be prohibitive. -Rick
Re: Replacement for a DEC 7474 Chip
I specifically said 74x74. Early TTL flipflops were very crude by comparison. On Sun, May 15, 2022, 13:03 Brent Hilpert via cctalk wrote: > On 2022-May-15, at 1:16 AM, Eric Smith via cctalk wrote: > > On Sat, May 14, 2022, 16:09 ben via cctalk > wrote: > >> On 2022-05-14 11:50 a.m., Nigel Johnson Ham via cctalk wrote: > >>> AFAIR LS can only drive one unit TTL load. > paul > >> LS is 4 TTL, 4 ma low. > >> Was there a trick of forcing the output of D flip flip > >> to clear it? I was wondering if this is what kills all > >> the 7474's? > > > > I don't think that worked on any TTL (or CMOS) 74x74 flip flops, except > > maybe by accident if you shorted the output enough to draw Vcc down (or > > ground up) enough to disrupt the FF, and then you have other problems. > > > > Despite the logic diagram showing feedback from the outputs, all 74x74 > have > > buffered outputs. > > Per TI schematics from 1969: 74 standard, H and L series flip-flops are > unbuffered. Or at least many of them are/were, in their then-original form. > Including 7475, 7490, etc. The output transistors connect both to the pins > and wrap back to form the FF or other purposes. > > Collector-triggering was discussed a some years ago on the list in regards > to a pdp8 front panel where DEC used collector-triggering on 74175's (IMO, > bad design practice). From (my) empirical tests at the time, it turned out > some 74S (Schottky) parts could be collector-triggered. However, between > standard, LS, and S types, behaviour could vary with manufacturer and > production date. > > > > The recent TI data sheets show an equivalent schematic > > only for the 74LS74. I can't at the moment find one for the 7474. > > > It seems likely to me that early pre-TTL logic families like RTL might > have > > had FFs with unbuffered outputs, but I haven't checked. > >
Re: Replacement for a DEC 7474 Chip
> On May 15, 2022, at 2:51 PM, Chris Quayle via cctech > wrote: > > ... > Original 74 series TTL can still be found, Ebay often, new old stock > and other surplus vendors. > > Would need to take care replacing with later ttl series, as the gate > delay, switching times and fanout probably won't match. Always use the > original series if you can... That's sensible advice. When you can't, faster devices with adequate fanout should work. The exception would be in circuits with hairy timing, where the design relies on the actual stage delays of the components. Most people don't build that way, for good reason. Seymour Cray is the best known exception; for example, the CDC 6600 design is very thoroughly dependent on the circuit and wire delays, and either slower OR faster will break things. And actually, it isn't clear to me why the thing ever works; the design files clearly demonstrate clashing signals that somehow apparently don't show up in reality. paul
Re: Replacement for a DEC 7474 Chip
On 2022-May-15, at 1:16 AM, Eric Smith via cctalk wrote: > On Sat, May 14, 2022, 16:09 ben via cctalk wrote: >> On 2022-05-14 11:50 a.m., Nigel Johnson Ham via cctalk wrote: >>> AFAIR LS can only drive one unit TTL load. paul >> LS is 4 TTL, 4 ma low. >> Was there a trick of forcing the output of D flip flip >> to clear it? I was wondering if this is what kills all >> the 7474's? > > I don't think that worked on any TTL (or CMOS) 74x74 flip flops, except > maybe by accident if you shorted the output enough to draw Vcc down (or > ground up) enough to disrupt the FF, and then you have other problems. > > Despite the logic diagram showing feedback from the outputs, all 74x74 have > buffered outputs. Per TI schematics from 1969: 74 standard, H and L series flip-flops are unbuffered. Or at least many of them are/were, in their then-original form. Including 7475, 7490, etc. The output transistors connect both to the pins and wrap back to form the FF or other purposes. Collector-triggering was discussed a some years ago on the list in regards to a pdp8 front panel where DEC used collector-triggering on 74175's (IMO, bad design practice). From (my) empirical tests at the time, it turned out some 74S (Schottky) parts could be collector-triggered. However, between standard, LS, and S types, behaviour could vary with manufacturer and production date. > The recent TI data sheets show an equivalent schematic > only for the 74LS74. I can't at the moment find one for the 7474. > It seems likely to me that early pre-TTL logic families like RTL might have > had FFs with unbuffered outputs, but I haven't checked.
Re: Replacement for a DEC 7474 Chip
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Today's Topics:
1. Replacement for a DEC 7474 Chip (Rob Jarratt)
2. Re: Replacement for a DEC 7474 Chip (John Robertson)
3. Re: Replacement for a DEC 7474 Chip (Paul Koning)
4. Re: Replacement for a DEC 7474 Chip (Nigel Johnson Ham)
5. Re: Replacement for a DEC 7474 Chip (ben)
6. Re: Replacement for a DEC 7474 Chip (Chuck Guzis)
7. Re: Replacement for a DEC 7474 Chip (ben)
8. Re: Replacement for a DEC 7474 Chip (dwight)
9. Re: Replacement for a DEC 7474 Chip (Chris Zach)
10. RE: Replacement for a DEC 7474 Chip (Rob Jarratt)
11. Re: Replacement for a DEC 7474 Chip (Eric Smith)
12. Re: Replacement for a DEC 7474 Chip (Eric Smith)
13. Re: Replacement for a DEC 7474 Chip (Chuck Guzis)
--
Message: 1
Date: Sat, 14 May 2022 18:11:32 +0100
From: "Rob Jarratt"
To: "General Discussion: On-Topic and Off-Topic Posts"
Subject: Replacement for a DEC 7474 Chip
Message-ID:<007b01d867b5$a8c90f00$fa5b2d00$@ntlworld.com>
Content-Type: text/plain; charset="us-ascii"
Hello,
I have found a bad DEC 7474 chip on my M7133 board. Clearly it is a
7474 D flip flop. The problem is I don't know which modern series
would be the best one to replace it with. I am sure I have seen a list
somewhere of modern equivalents for some DEC chip numbers, but I can't
remember where.
If it helps at all, on the PDP 11/24 printset it is E78 on page K6 of the
schematic (p157 of the PDF).
Picture of the failed chip here:
https://rjarratt.files.wordpress.com/2022/05/damaged-dec-7474-4_li.jpg
Can anyone tell me what the best modern equivalent is likely to be?
Thanks
Rob
--
Message: 2
Date: Sat, 14 May 2022 10:41:34 -0700
From: John Robertson
To: Rob Jarratt via cctalk
Subject: Re: Replacement for a DEC 7474 Chip
Message-ID:
Content-Type: text/plain; charset=UTF-8; format=flowed
On 2022/05/14 10:11 a.m., Rob Jarratt via cctalk wrote:
Hello,
I have found a bad DEC 7474 chip on my M7133 board. Clearly it is a
7474 D flip flop. The problem is I don't know which modern series
would be the best one to replace it with. I am sure I have seen a list
somewhere of modern equivalents for some DEC chip numbers, but I can't
remember where.
If it helps at all, on the PDP 11/24 printset it is E78 on page K6 of the
schematic (p157 of the PDF).
Picture of the failed chip here:
https://rjarratt.files.wordpress.com/2022/05/damaged-dec-7474-4_li.jpg
Can anyone tell me what the best modern equivalent is likely to be?
Thanks
Rob
You are stuck with using an original 7474 family assuming this is
driving other early TTL. 74LS74, and others simply don't have the drive
capability to work. You can use 74S74 or 74F74 as they have the same
output current, the "S" is Schottky, and the other is "F"aster.
I find that https:///unicornelectronics.com is a reliable source of TTL.
John :-#)#
Original 74 series TTL can still be found, Ebay often, new old stock
and other surplus vendors.
Would need to take care replacing with later ttl series, as the gate
delay, switching times and fanout probably won't match. Always use the
original series if you can...
Chris
RE: Replacement for a DEC 7474 Chip
What ever you do, don't use a Fairchild part. When I worked for Intel in the 80's, we finally band using Fairchild for any latching device. They failed on pullup current, even when the parts were sent back and they claimed they were good. We just gave up on them, we couldn't hold production while they figured it out. We had a similar problem with PowerOne, a manufacture of power supplies. I've heard the same thing about Intel, Shugart, Verbatim, Wabash, Tandy, Apple, Commodore, VW, Ford, Chevy, . . . Just about EVERY company has released a bad product, and had their reputation damaged. A better warning is: "Between xxx and yyy, we had a lot of bad parts from zzz." SOME have cleaned up their act, and have made good products subsequently.
RE: Replacement for a DEC 7474 Chip
Folks, I probably have some, and now I have found my ICL House Codes sheet I may even be able to identify them. However I won't be in the same country as my stock for a couple of weeks. ... and they are in the UK so getting them stateside may take some time. Dave G4UGM > -Original Message- > From: cctalk On Behalf Of Chuck Guzis via > cctalk > Sent: 15 May 2022 16:12 > To: cctalk@classiccmp.org > Subject: Re: Replacement for a DEC 7474 Chip > > On 5/15/22 01:16, Eric Smith via cctalk wrote: > > > I don't think that worked on any TTL (or CMOS) 74x74 flip flops, > > except maybe by accident if you shorted the output enough to draw Vcc > > down (or ground up) enough to disrupt the FF, and then you have other > problems. > > Checking my old Moto databook, the old FFs appear to be capable of > having the outputs jammed. At first glance, both the 7472 and 7473 FFs > had unbuffered outputs. Most early latches, likewise. > > Of course, the very early TTL was density-constrained, so FFs like the > MC512/MC563 are of course without buffered outputs. > > --Chuck
Re: Replacement for a DEC 7474 Chip
On 5/15/22 01:16, Eric Smith via cctalk wrote: > I don't think that worked on any TTL (or CMOS) 74x74 flip flops, except > maybe by accident if you shorted the output enough to draw Vcc down (or > ground up) enough to disrupt the FF, and then you have other problems. Checking my old Moto databook, the old FFs appear to be capable of having the outputs jammed. At first glance, both the 7472 and 7473 FFs had unbuffered outputs. Most early latches, likewise. Of course, the very early TTL was density-constrained, so FFs like the MC512/MC563 are of course without buffered outputs. --Chuck
Re: Replacement for a DEC 7474 Chip
On 5/14/22 23:04, ben via cctalk wrote: > Faster is not better, bigger is bett... ops wrong topic. > > 74H while slower, and 74S tend to have reliable delay times looking at > gates with min to max delays < 3 ns. This helped devices to track > the same speed across a system. > A 74ABT00 is 1 to 4 ns depending on the temp,phase of the moon,and what > ever. How much margin do I need to add? Ok I guess for connecting a 386? Sure, but if the concern was output sink current, the S or AS family can certainly provide it. It all depends on the position of the IC in the circuitry. Depending on application, you might conceivably get away with a 74HC74; that is some parameters may not matter. I might still have a couple of DM7474's kicking around, but they're so old, I wouldn't depend on them. I even have a few Fairchild 74F74s... --Chuck
Re: Replacement for a DEC 7474 Chip
On Sat, May 14, 2022, 16:09 ben via cctalk wrote: > On 2022-05-14 11:50 a.m., Nigel Johnson Ham via cctalk wrote: > > AFAIR LS can only drive one unit TTL load. > >> paul > LS is 4 TTL, 4 ma low. > Was there a trick of forcing the output of D flip flip > to clear it? I was wondering if this is what kills all > the 7474's? > I don't think that worked on any TTL (or CMOS) 74x74 flip flops, except maybe by accident if you shorted the output enough to draw Vcc down (or ground up) enough to disrupt the FF, and then you have other problems. Despite the logic diagram showing feedback from the outputs, all 74x74 have buffered outputs. The recent TI data sheets show an equivalent schematic only for the 74LS74. I can't at the moment find one for the 7474. It seems likely to me that early pre-TTL logic families like RTL might have had FFs with unbuffered outputs, but I haven't checked.
Re: Replacement for a DEC 7474 Chip
Those should be fine. Only the more complex parts had issues, not the simple gates. On Sun, May 15, 2022, 02:04 Rob Jarratt via cctalk wrote: > Oh dear, while I was ordering an original 7474 I ordered some other parts > that were connected to the same bad chip in case other chips are damaged, > and I ordered a Fairchild 74LS08! I will ask them to change it for a > Motorola part they also have. > > > -Original Message- > > From: cctalk On Behalf Of dwight via > cctalk > > Sent: 14 May 2022 23:36 > > To: Paul Koning via cctalk > > Subject: Re: Replacement for a DEC 7474 Chip > > > > What ever you do, don't use a Fairchild part. When I worked for Intel in > the > > 80's, we finally band using Fairchild for any latching device. They > failed > on > > pullup current, even when the parts were sent back and they claimed they > > were good. We just gave up on them, we couldn't hold production while > > they figured it out. > > We had a similar problem with PowerOne, a manufacture of power supplies. > > Since it was a custom supply, we had to send someone to their plant to > fix > > their final test. > > Dwight > > > > > > > > From: cctalk on behalf of Nigel Johnson > > Ham via cctalk > > Sent: Saturday, May 14, 2022 10:50 AM > > To: Paul Koning via cctalk > > Subject: Re: Replacement for a DEC 7474 Chip > > > > AFAIR LS can only drive one unit TTL load. > > > > I may have some 7474, even of that vintage, if you cannot find any > anywhere > > else. > > > > cheers, > > > > Nigel > > > > > > Nigel Johnson, MSc., MIEEE, MCSE VE3ID/G4AJQ/VA3MCU Amateur Radio, > > the origin of the open-source concept! > > Skype: TILBURY2591 > > > > > > On 2022-05-14 13:48, Paul Koning via cctalk wrote: > > > > > >> On May 14, 2022, at 1:41 PM, John Robertson via > > cctalk wrote: > > >> > > >> On 2022/05/14 10:11 a.m., Rob Jarratt via cctalk wrote: > > >>> Hello, > > >>> > > >>> I have found a bad DEC 7474 chip on my M7133 board. Clearly it is a > > >>> 7474 D flip flop. The problem is I don't know which > > >>> modern series would be the best one to replace it with. I am sure I > > >>> have seen a list somewhere of modern equivalents for some DEC chip > > >>> numbers, but I can't remember where. > > >>> > > >>> If it helps at all, on the PDP 11/24 printset it is E78 on page K6 > > >>> of the schematic (p157 of the PDF). > > >>> > > >>> Picture of the failed chip here: > > >>> https://rjarratt.files.wordpress.com/2022/05/damaged-dec-7474-4_li.j > > >>> pg > > >>> > > >>> Can anyone tell me what the best modern equivalent is likely to be? > > >>> > > >>> Thanks > > >>> > > >>> Rob > > >>> > > >> You are stuck with using an original 7474 family assuming this is > driving > > other early TTL. 74LS74, and others simply don't have the drive > capability > to > > work. > > > I know LS has less fanout, but is it not able to drive plain 74xx at > all? That > > doesn't sound right. If the circuit in question runs near the fanout > spec > of > > plain 74 the yes, 74LS won't work. > > > > > > Spec sheets and the actual schematic will give a definitive answer. > > > > > >paul > > > > > > > >
RE: Replacement for a DEC 7474 Chip
Oh dear, while I was ordering an original 7474 I ordered some other parts that were connected to the same bad chip in case other chips are damaged, and I ordered a Fairchild 74LS08! I will ask them to change it for a Motorola part they also have. > -Original Message- > From: cctalk On Behalf Of dwight via cctalk > Sent: 14 May 2022 23:36 > To: Paul Koning via cctalk > Subject: Re: Replacement for a DEC 7474 Chip > > What ever you do, don't use a Fairchild part. When I worked for Intel in the > 80's, we finally band using Fairchild for any latching device. They failed on > pullup current, even when the parts were sent back and they claimed they > were good. We just gave up on them, we couldn't hold production while > they figured it out. > We had a similar problem with PowerOne, a manufacture of power supplies. > Since it was a custom supply, we had to send someone to their plant to fix > their final test. > Dwight > > > > From: cctalk on behalf of Nigel Johnson > Ham via cctalk > Sent: Saturday, May 14, 2022 10:50 AM > To: Paul Koning via cctalk > Subject: Re: Replacement for a DEC 7474 Chip > > AFAIR LS can only drive one unit TTL load. > > I may have some 7474, even of that vintage, if you cannot find any anywhere > else. > > cheers, > > Nigel > > > Nigel Johnson, MSc., MIEEE, MCSE VE3ID/G4AJQ/VA3MCU Amateur Radio, > the origin of the open-source concept! > Skype: TILBURY2591 > > > On 2022-05-14 13:48, Paul Koning via cctalk wrote: > > > >> On May 14, 2022, at 1:41 PM, John Robertson via > cctalk wrote: > >> > >> On 2022/05/14 10:11 a.m., Rob Jarratt via cctalk wrote: > >>> Hello, > >>> > >>> I have found a bad DEC 7474 chip on my M7133 board. Clearly it is a > >>> 7474 D flip flop. The problem is I don't know which > >>> modern series would be the best one to replace it with. I am sure I > >>> have seen a list somewhere of modern equivalents for some DEC chip > >>> numbers, but I can't remember where. > >>> > >>> If it helps at all, on the PDP 11/24 printset it is E78 on page K6 > >>> of the schematic (p157 of the PDF). > >>> > >>> Picture of the failed chip here: > >>> https://rjarratt.files.wordpress.com/2022/05/damaged-dec-7474-4_li.j > >>> pg > >>> > >>> Can anyone tell me what the best modern equivalent is likely to be? > >>> > >>> Thanks > >>> > >>> Rob > >>> > >> You are stuck with using an original 7474 family assuming this is driving > other early TTL. 74LS74, and others simply don't have the drive capability to > work. > > I know LS has less fanout, but is it not able to drive plain 74xx at all? That > doesn't sound right. If the circuit in question runs near the fanout spec of > plain 74 the yes, 74LS won't work. > > > > Spec sheets and the actual schematic will give a definitive answer. > > > >paul > > > >
Re: Replacement for a DEC 7474 Chip
I'll have to check when I get back from Norway but I believe I replaced thirty or so flip flops with 74ls74 chips with no problems. This was on my pdp8l and granted Dec used a lot of 4 and 8 input nand gates to drive loads On May 15, 2022 12:08:55 AM GMT+02:00, ben via cctalk wrote: >On 2022-05-14 11:50 a.m., Nigel Johnson Ham via cctalk wrote: >> AFAIR LS can only drive one unit TTL load. >>> paul >LS is 4 TTL, 4 ma low. >Was there a trick of forcing the output of D flip flip >to clear it? I was wondering if this is what kills all >the 7474's? >Ben. > > > -- Sent from my Android device with K-9 Mail. Please excuse my brevity.
Re: Replacement for a DEC 7474 Chip
On 2022-05-14 10:53 p.m., Chuck Guzis via cctalk wrote: On 5/14/22 15:35, dwight via cctalk wrote: You are stuck with using an original 7474 family assuming this is driving other early TTL. 74LS74, and others simply don't have the drive capability to work. I know LS has less fanout, but is it not able to drive plain 74xx at all? That doesn't sound right. If the circuit in question runs near the fanout spec of plain 74 the yes, 74LS won't work. Spec sheets and the actual schematic will give a definitive answer. I've got a whole parts drawer full of 74S74s, which I believe exceed the output current capabilities of the original 7474. There's also the 74ABT74, if you want a beefer BiMOS version. Heck, I can probably come up with a couple of crufty old 74H74s... --Chuck Faster is not better, bigger is bett... ops wrong topic. 74H while slower, and 74S tend to have reliable delay times looking at gates with min to max delays < 3 ns. This helped devices to track the same speed across a system. A 74ABT00 is 1 to 4 ns depending on the temp,phase of the moon,and what ever. How much margin do I need to add? Ok I guess for connecting a 386? Ben.
Re: Replacement for a DEC 7474 Chip
On 5/14/22 15:35, dwight via cctalk wrote: >>> You are stuck with using an original 7474 family assuming this is driving >>> other early TTL. 74LS74, and others simply don't have the drive capability >>> to work. >> I know LS has less fanout, but is it not able to drive plain 74xx at all? >> That doesn't sound right. If the circuit in question runs near the fanout >> spec of plain 74 the yes, 74LS won't work. >> >> Spec sheets and the actual schematic will give a definitive answer. >> I've got a whole parts drawer full of 74S74s, which I believe exceed the output current capabilities of the original 7474. There's also the 74ABT74, if you want a beefer BiMOS version. Heck, I can probably come up with a couple of crufty old 74H74s... --Chuck
Re: Replacement for a DEC 7474 Chip
What ever you do, don't use a Fairchild part. When I worked for Intel in the 80's, we finally band using Fairchild for any latching device. They failed on pullup current, even when the parts were sent back and they claimed they were good. We just gave up on them, we couldn't hold production while they figured it out. We had a similar problem with PowerOne, a manufacture of power supplies. Since it was a custom supply, we had to send someone to their plant to fix their final test. Dwight From: cctalk on behalf of Nigel Johnson Ham via cctalk Sent: Saturday, May 14, 2022 10:50 AM To: Paul Koning via cctalk Subject: Re: Replacement for a DEC 7474 Chip AFAIR LS can only drive one unit TTL load. I may have some 7474, even of that vintage, if you cannot find any anywhere else. cheers, Nigel Nigel Johnson, MSc., MIEEE, MCSE VE3ID/G4AJQ/VA3MCU Amateur Radio, the origin of the open-source concept! Skype: TILBURY2591 On 2022-05-14 13:48, Paul Koning via cctalk wrote: > >> On May 14, 2022, at 1:41 PM, John Robertson via >> cctalk wrote: >> >> On 2022/05/14 10:11 a.m., Rob Jarratt via cctalk wrote: >>> Hello, >>> >>> I have found a bad DEC 7474 chip on my M7133 board. Clearly it is a >>> 7474 D flip flop. The problem is I don't know which modern series >>> would be the best one to replace it with. I am sure I have seen a list >>> somewhere of modern equivalents for some DEC chip numbers, but I can't >>> remember where. >>> >>> If it helps at all, on the PDP 11/24 printset it is E78 on page K6 of the >>> schematic (p157 of the PDF). >>> >>> Picture of the failed chip here: >>> https://rjarratt.files.wordpress.com/2022/05/damaged-dec-7474-4_li.jpg >>> >>> Can anyone tell me what the best modern equivalent is likely to be? >>> >>> Thanks >>> >>> Rob >>> >> You are stuck with using an original 7474 family assuming this is driving >> other early TTL. 74LS74, and others simply don't have the drive capability >> to work. > I know LS has less fanout, but is it not able to drive plain 74xx at all? > That doesn't sound right. If the circuit in question runs near the fanout > spec of plain 74 the yes, 74LS won't work. > > Spec sheets and the actual schematic will give a definitive answer. > >paul > >
Re: Replacement for a DEC 7474 Chip
On 2022-05-14 11:50 a.m., Nigel Johnson Ham via cctalk wrote: AFAIR LS can only drive one unit TTL load. paul LS is 4 TTL, 4 ma low. Was there a trick of forcing the output of D flip flip to clear it? I was wondering if this is what kills all the 7474's? Ben.
Re: Replacement for a DEC 7474 Chip
AFAIR LS can only drive one unit TTL load. I may have some 7474, even of that vintage, if you cannot find any anywhere else. cheers, Nigel Nigel Johnson, MSc., MIEEE, MCSE VE3ID/G4AJQ/VA3MCU Amateur Radio, the origin of the open-source concept! Skype: TILBURY2591 On 2022-05-14 13:48, Paul Koning via cctalk wrote: On May 14, 2022, at 1:41 PM, John Robertson via cctalk wrote: On 2022/05/14 10:11 a.m., Rob Jarratt via cctalk wrote: Hello, I have found a bad DEC 7474 chip on my M7133 board. Clearly it is a 7474 D flip flop. The problem is I don't know which modern series would be the best one to replace it with. I am sure I have seen a list somewhere of modern equivalents for some DEC chip numbers, but I can't remember where. If it helps at all, on the PDP 11/24 printset it is E78 on page K6 of the schematic (p157 of the PDF). Picture of the failed chip here: https://rjarratt.files.wordpress.com/2022/05/damaged-dec-7474-4_li.jpg Can anyone tell me what the best modern equivalent is likely to be? Thanks Rob You are stuck with using an original 7474 family assuming this is driving other early TTL. 74LS74, and others simply don't have the drive capability to work. I know LS has less fanout, but is it not able to drive plain 74xx at all? That doesn't sound right. If the circuit in question runs near the fanout spec of plain 74 the yes, 74LS won't work. Spec sheets and the actual schematic will give a definitive answer. paul
Re: Replacement for a DEC 7474 Chip
> On May 14, 2022, at 1:41 PM, John Robertson via cctalk > wrote: > > On 2022/05/14 10:11 a.m., Rob Jarratt via cctalk wrote: >> Hello, >> >> I have found a bad DEC 7474 chip on my M7133 board. Clearly it is a >> 7474 D flip flop. The problem is I don't know which modern series >> would be the best one to replace it with. I am sure I have seen a list >> somewhere of modern equivalents for some DEC chip numbers, but I can't >> remember where. >> >> If it helps at all, on the PDP 11/24 printset it is E78 on page K6 of the >> schematic (p157 of the PDF). >> >> Picture of the failed chip here: >> https://rjarratt.files.wordpress.com/2022/05/damaged-dec-7474-4_li.jpg >> >> Can anyone tell me what the best modern equivalent is likely to be? >> >> Thanks >> >> Rob >> > You are stuck with using an original 7474 family assuming this is driving > other early TTL. 74LS74, and others simply don't have the drive capability to > work. I know LS has less fanout, but is it not able to drive plain 74xx at all? That doesn't sound right. If the circuit in question runs near the fanout spec of plain 74 the yes, 74LS won't work. Spec sheets and the actual schematic will give a definitive answer. paul
Re: Replacement for a DEC 7474 Chip
On 2022/05/14 10:11 a.m., Rob Jarratt via cctalk wrote: Hello, I have found a bad DEC 7474 chip on my M7133 board. Clearly it is a 7474 D flip flop. The problem is I don't know which modern series would be the best one to replace it with. I am sure I have seen a list somewhere of modern equivalents for some DEC chip numbers, but I can't remember where. If it helps at all, on the PDP 11/24 printset it is E78 on page K6 of the schematic (p157 of the PDF). Picture of the failed chip here: https://rjarratt.files.wordpress.com/2022/05/damaged-dec-7474-4_li.jpg Can anyone tell me what the best modern equivalent is likely to be? Thanks Rob You are stuck with using an original 7474 family assuming this is driving other early TTL. 74LS74, and others simply don't have the drive capability to work. You can use 74S74 or 74F74 as they have the same output current, the "S" is Schottky, and the other is "F"aster. I find that https:///unicornelectronics.com is a reliable source of TTL. John :-#)# -- John's Jukes Ltd. 7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3 Call (604)872-5757 (Pinballs, Jukes, Video Games) flippers.com "Old pinballers never die, they just flip out"
