Re: Transporting an LGP-30

[Brent Hilpert]

On 2016-Dec-23, at 4:14 PM, Cory Heisterkamp wrote:
>> On Fri, Dec 23, 2016 at 11:45 AM, Chuck Guzis  wrote:
>> 
>>> On 12/23/2016 08:06 AM, Cory Heisterkamp wrote:
 Hi Folks,
 
 I recently became the owner of an LGP-30, supposedly in 'working'
 condition. However, the machine is roughly 2000 miles from me and
 will need to be transported by freight. Before it's palletized, are
 there any special precautions I should take to ensure its safe
 travel. I'm especially worried about the drum (drum lock?), but
 haven't been able to find a maintenance or setup doc.
 
 Anyone out there with experience or can offer a few pointers?
 
>>> 
>>> Is this the LGP-30 that came up for auction in Canada?
>>> 
>>> Personally, I'd buy a plane ticket and supervise the packing.  There
>>> are,  IIRC, 110 or so tubes, a drum and other goodies that should
>>> probably be removed and shipped separately.   Nothing like getting a box
>>> o' broken glass to make life interesting.
>>> 
> 
> Well, I think you guys have convinced me that a trip is in order. Better safe 
> than sorry with a piece of equipment like this.
> 
> Yep Chuck, this is the CA machine. I was surprised it never reared its head 
> on classiccmp the past few days. -C

Was this ebayed?

Just found this mention on VCFed from a week ago:

http://www.vcfed.org/forum/showthread.php?55364-1956-Binary-Desk-Computer-LIBRASCOPE-LGP-30

I see Vancouver is mentioned:
"I've been told that this was the oldest computer in Vancouver"

As a lifelong area resident, I'd be interested in hearing anything known of the 
lineage, where it came from and when it left Vancouver.
Would love to have worked on it if it was once nearby.
Congrats to Cory for the acquisition, should be an exciting project.

Re: PDP-6s at MIT

[jim stephens]

On 12/24/2016 12:00 PM, Noel Chiappa wrote:

 > their KA10 had a paging box, made by System[s] Concepts

Speaking of which, here's a photo of the display panel from it:

   http://ana-3.lcs.mit.edu/~jnc/tech/jpg/SysConKAPanel.jpg

The meter didn't show up well in that, and it's too cool to miss, so here:

   http://ana-3.lcs.mit.edu/~jnc/tech/jpg/SysConMeter.jpg

is a shot of it.

Noel


I was looking at some photos on FB posted by a friend and ran across 
Richard Greenblatt's oral history.  It has some mentions of what the 
PDP6's and PDP 10's were used for.


http://archive.computerhistory.org/projects/chess/related_materials/oral-history/greenblatt.oral_history.2005.102634500/102657935-05-01-acc.pdf

I've not had time to read other than to think these are perhaps 
interesting if you guys didn't run across them.  I think they may have 
been used by the Project Mac folks.  Perhaps posting this over on the 
Multics thread might stir up some information if you want more.  A lot 
of the original guys are there.  I'd be glad to do it if no-one else on 
the thread here can do so.


Thanks for the interesting thread.

thanks
Jim

And Merry Christmas from LA

OT: Please help - Neighbors home burned this Christmas eve

[drlegendre .]

Please forgive this, list-members, I know it's way OT:

https://www.gofundme.com/jeff-kris-kidder-xmas-eve-fire

My super great neighbors across the street lost their home to fire this
evening. =(

These folks are the best of the best.. as the saying goes "Couldn't have
happened a nicer guy!". Please spread it around as you feel is appropriate.
Like most of us, we're still kind of in shock over it.

I can only imagine how Jeff & Kris are.. thanks for your forbearance.

Re: Altair 8800 name Was: Re: Altair 680 Expansion Boards?

[Eric Smith]

On Sat, Dec 24, 2016 at 8:10 AM, Peter Corlett  wrote:

> On Fri, Dec 23, 2016 at 04:14:28AM -0700, Eric Smith wrote:
> > The same trick works perfectly well with a 6502, and in fact was
> invented by
> > Don Lancaster using a 6502 years before the ZX80 was designed. That
> doesn't
> > really explain the choice of the Z80.
>
> Interesting. I was of the understanding that the Z80 video hack didn't
> work on
> the 6502 due to the latter being slightly pipelined and so the instruction
> fetch cycle couldn't be abused in the same way. Perhaps Sinclair couldn't
> get
> it to work on some dodgy 6502s that fell off the back of a lorry.
>

The details aren't identical, but on the 6502 it's abusing the fetch of
both an opcode and an immediate operand.

If there was nothing in it, then it's a mystery why the Z80 was selected.
> However, Sinclair wasn't exactly one for making rational design decisions
> based
> on technical merit or industry best practice.


As much as I like the 6502, the Z80 is better in some ways, particularly if
you want to do a lot of 16-bit add and subtract.

Re: Other SMS300/8X300/8X305 uses?

[Eric Smith]

On Sat, Dec 24, 2016 at 8:43 AM, Al Kossow  wrote:

> see if he'll dump the firmware. it's identical to the DCA IRMAprint
>

OK, I've asked. Will let you know.

Re: ICL7611 op-amp (was: Motorola MC14081B)

[Adrian Graham]

On 24/12/2016 19:04, "Brent Hilpert"  wrote:

> On 2016-Dec-24, at 4:07 AM, Adrian Graham wrote:
>> On 24/12/2016 03:48, "Brent Hilpert"  wrote:
>> 
>>> The ICL7611 is, I expect, a very-low-power (Intersil's niche) op amp.
>>> Together
>>> with the CMOS 4081 the circuit appears tailored for low-power operation.
>>> Is it supplied by the battery?
>>> It may require the battery presence for stable supply at time of power-up to
>>> get reliable reset operation from this power-on-reset circuit.
>> 
>> That's a very good point so now I'm going to have to scrabble around to find
>> the battery to find out what voltage it was since I removed it ages ago and
>> may have recycled it. I don't recall any markings on it though, it looked
>> like a 'normal' 3 terminal NiCAD wrapped in blue plastic. I did take hi-res
>> pictures of it however.
>> 
>> The 5V input at R395 does head off towards the battery location before it
>> hits the resistor so I'll trace that out too.
> 
> 
> The circuit makes sense if the battery is connected to the Vcc line of the
> ICL7611, 4081, etc.,
> D1 would be backflow prevention into the +12V line .
> R416 would be current limiting providing ~ 8mA trickle charge of the NiCds.

Looking at the back of the board the battery WAS originally connected to the
7611 and 4081 at manufacturing but then that connection was physically cut
out and a jumper wire added to the line that supplies the MM58174AN RTC
that's next to them, which is derived from the 12V DRAM supply.

Pity, I was kind of hoping that if I added the/a battery back in then the
7611 would be happy at power up but it looks like it's not involved at all.
Could I add a bit of adjustability by replacing R397 with a 100K pot?

*embarrassed face* The resistor formerly known as R416 is actually R424,
seen here top right in a hybrid front/back mix of the board around the
battery which includes the CMOS chips:

http://www.binarydinosaurs.co.uk/STCExecutelBatterySectionHybrid.jpg

You can see the cutout under the upper left hand battery terminal and the
blue jumper wire running to the 4081. The two green resistors upper left in
the cluster of 5 are R398 and R406. All the red caps are decouplers.

> Speaking of such equipment, our museum has one of these buried somewhere, a
> Nortel Displayphone:
> http://classiccmp.org/dunfield/disphone/index.htm
> I don't think it has all the apps of the Executel,
> it's phone + on-screen phone directory & phone apps + RS232 terminal with
> 300bps modem.
> I hope to work on it and document it sometime.

Nice!

Hope you have a great xmas :D

-- 
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?

Re: PDP-6s at MIT

[Noel Chiappa]

> their KA10 had a paging box, made by System[s] Concepts

Speaking of which, here's a photo of the display panel from it:

  http://ana-3.lcs.mit.edu/~jnc/tech/jpg/SysConKAPanel.jpg

The meter didn't show up well in that, and it's too cool to miss, so here:

  http://ana-3.lcs.mit.edu/~jnc/tech/jpg/SysConMeter.jpg

is a shot of it.

Noel

Re: ICL7611 op-amp (was: Motorola MC14081B)

[Brent Hilpert]

On 2016-Dec-24, at 4:07 AM, Adrian Graham wrote:
> On 24/12/2016 03:48, "Brent Hilpert"  wrote:
> 
>> The ICL7611 is, I expect, a very-low-power (Intersil's niche) op amp. 
>> Together
>> with the CMOS 4081 the circuit appears tailored for low-power operation.
>> Is it supplied by the battery?
>> It may require the battery presence for stable supply at time of power-up to
>> get reliable reset operation from this power-on-reset circuit.
> 
> That's a very good point so now I'm going to have to scrabble around to find
> the battery to find out what voltage it was since I removed it ages ago and
> may have recycled it. I don't recall any markings on it though, it looked
> like a 'normal' 3 terminal NiCAD wrapped in blue plastic. I did take hi-res
> pictures of it however.
> 
> The 5V input at R395 does head off towards the battery location before it
> hits the resistor so I'll trace that out too.


The circuit makes sense if the battery is connected to the Vcc line of the 
ICL7611, 4081, etc.,
D1 would be backflow prevention into the +12V line .
R416 would be current limiting providing ~ 8mA trickle charge of the NiCds.

Speaking of such equipment, our museum has one of these buried somewhere, a 
Nortel Displayphone:
http://classiccmp.org/dunfield/disphone/index.htm
I don't think it has all the apps of the Executel,
it's phone + on-screen phone directory & phone apps + RS232 terminal with 
300bps modem.
I hope to work on it and document it sometime.

Re: Other SMS300/8X300/8X305 uses?

[Al Kossow]

On 12/24/16 7:43 AM, Al Kossow wrote:
> see if he'll dump the firmware. it's identical to the DCA IRMAprint
> 

the pcb is identical

Re: PDP-6s at MIT

[Noel Chiappa]

> From: Lars Brinkhoff

> I have this on AI:

>> the semi-original AI (that is, the KA-10 rather than the PDP-6) was
>> .. was given to a bunch of hackers from Concourse

Oh, right, now that you mention it, I very vaguely recall this.

I'm not sure why I thought they had taken a KA too - I think I may have been
confused by this email:

  Date: Fri, 16 Sep 88 00:16:04 EDT
  From: Peter Lothberg 
  Subject: The "crack team", is dissasembling MX, for it's trip to Sweden
  To: info-...@ai.ai.mit.edu

  The crack team has begun to work;
  ...
  (As the system will not fill the container more than 40% or so, we vold like
  donations of other stuff, like Lisp-machines, AAA terminals, a IMP,
  Conection machines, retired 2060's etc, (I'm not joking...))

which does talk about taking other stuff. But I have this persistent memory
that they took a KA - maybe I should try and get ahold of Peter and see? There
is a later email:

  Date: 11-Nov-88  0:39:51 +0100
  From: Peter_Lothberg 
  To: bug-...@ai.ai.mit.edu
  Subject: The container and MX

  Arrived to Stockholm and we unpacked the container on wendsday.

  The container has, sure shaked, the cardboard paper that we put between
  the cabinets, has bloue spots.

  But, everything was in the same position that we left it, so, hopfully it is
  not hurt by the transport, or the cold here.

  We have put the machine on several places, while we are waiting for our new
  machine room to be completed.

which makes it sound like they only got the KL? It's also possibly I am mixing
two memories, and remembering this:

  Date: Sat, 3 Oct 87 23:02:14 EDT
  From: Alan Bawden 
  Subject: The operating system that wouldn't die! EE!
  To: info-...@ai.ai.mit.edu

  I thought I would take this opportunity to spread the word about something
  that I don't think has been very widely publicized. Some of you may recall
  that a while ago some fellows in Sweden contacted us about running ITS on
  various PDP-10's that they owned? Well, we mailed them a set of tapes for
  bringing up ITS on their 2020, which they were able to do without too much
  trouble .. That all happened over a year ago. Recently we learned that these
  guys have successfully -built- ITS paging hardware for their KA-10, and have
  ITS up and running there as well! Totally Amazing.

Pretty astonishing accomplishment, that.

   Noel

Re: General public machines (Was: Altair 8800 name Was: Re: Altair 680 Expansion Boards?

[geneb]

On Fri, 23 Dec 2016, Cameron Kaiser wrote:


Commodore's Z80 in the 128 was due to unnecessary fear that they might lose
market share to CP/M, when IBM should have been their big worry.
I don't know all of the details of the ST/Amiga technology swap, but BOTH
were too late, if the primary goal was competing with IBM.


That might be Commodore marketing - Bil Herd said that he threw the Z-80
into the design essentially because he could. :)  He's done a few talks on
how the C-128 came about.  It's pretty interesting.


It also saved the 100% compatibility problem (in this case, with Commodore's
CP/M cartridge by designing it onto the board, and with Commodore Magic Voice,
which fouled banking by altering the memory configuration lines in realtime:
the 8502 would crash, but the Z80's activity would not be detected and the
C= key could be checked to force a C64 memory map).


Yep.  The group actively kept one step ahead of management to keep them 
from screwing with the project too badly. :)


g.

--
Proud owner of F-15C 80-0007
http://www.f15sim.com - The only one of its kind.
http://www.diy-cockpits.org/coll - Go Collimated or Go Home.
Some people collect things for a hobby.  Geeks collect hobbies.

ScarletDME - The red hot Data Management Environment
A Multi-Value database for the masses, not the classes.
http://scarlet.deltasoft.com - Get it _today_!

Re: Transporting an LGP-30

[Brad H]

 Original message 
From: william degnan  
Date: 2016-12-24  3:35 AM  (GMT-08:00) 
To: cctech  
Subject: Re: Transporting an LGP-30 

> Well, I think you guys have convinced me that a trip is in order. Better
safe than sorry with a piece of equipment like this.
>
> Yep Chuck, this is the CA machine. I was surprised it never reared its
head on classiccmp the past few days. -C

Believe me, I for one was interested, and I was in contact with the
seller.   The expense and effort, as much as >I'd love to work on restoring
>this, was too great to pull the trigger.

I thought about it also.  I would have liked to have brought it back to BC and 
kept it here in Canada.  But I began to wonder how it had ended up with these 
guys and talking to them I had my doubts about the amount of care taken in 
moving it around.  And I assumed freight would be insane anyway.  

Re: Other SMS300/8X300/8X305 uses?

[Al Kossow]

see if he'll dump the firmware. it's identical to the DCA IRMAprint

On 12/23/16 8:22 PM, Eric Smith wrote:
> https://twitter.com/yesterbits/status/812415257616457728
> 

Re: Altair 8800 name Was: Re: Altair 680 Expansion Boards?

[Peter Corlett]

On Fri, Dec 23, 2016 at 04:14:28AM -0700, Eric Smith wrote:
> On Fri, Dec 23, 2016 at 3:59 AM, Peter Corlett  wrote:
>> People who know Uncle Clive's unwillingness to spend a penny more than he
>> has to on bulding computers may wonder why they selected the relatively
>> expensive Z80 over the 6502, but it was because they managed to trick the
>> Z80's address-fetcting and instruction decoding cycle into generating video
>> on the ZX80 and ZX81, and thus saved more money elsewhere.
> The same trick works perfectly well with a 6502, and in fact was invented by
> Don Lancaster using a 6502 years before the ZX80 was designed. That doesn't
> really explain the choice of the Z80.

Interesting. I was of the understanding that the Z80 video hack didn't work on
the 6502 due to the latter being slightly pipelined and so the instruction
fetch cycle couldn't be abused in the same way. Perhaps Sinclair couldn't get
it to work on some dodgy 6502s that fell off the back of a lorry.

> Also, by the time the ZX80 was introduced, both the Z80 and 6502 were
> basically dirt cheap. Any premium price the Z80 had once commanded had long
> since evaporated.

If there was nothing in it, then it's a mystery why the Z80 was selected.
However, Sinclair wasn't exactly one for making rational design decisions based
on technical merit or industry best practice...

Clone of 98228A ROM for 9825T information posted.

[Paul Berger]

I have posted a package of information and ROM images from my project to 
clone the 98228A ROM on the VintHPcom group at groups.io.  Progress on 
laying out a PCB is going slowly as I have little experience and the 
learning curve for using Kicad to lay out a board seems pretty steep.



Paul.

Hello,

This email message is a notification to let you know that a file has 
been uploaded to the Files area of the vinthp...@groups.io group.


*File:* 98228A.zip

*Uploaded By:* Paul Berger

*Description:*
Package with information and ROM images from my project to clone the 
rare 98228A ROM that supports both 9885 and 9895 8" diskette drives.


You can access this file at the URL:
https://groups.io/g/VintHPcom/files/HP9825/98228A.zip

Cheers,
The Groups.io Team

_._,_._,_

Re: Transporting an LGP-30

[william degnan]

> Well, I think you guys have convinced me that a trip is in order. Better
safe than sorry with a piece of equipment like this.
>
> Yep Chuck, this is the CA machine. I was surprised it never reared its
head on classiccmp the past few days. -C

Believe me, I for one was interested, and I was in contact with the
seller.   The expense and effort, as much as I'd love to work on restoring
this, was too great to pull the trigger.

What are you going to do with this?  I did not notice if a Flexowriter was
included.

Here are my notes on the LGP 30
http://www.vintagecomputer.net/browse_thread.cfm?id=596

Re: General public machines (Was: Altair 8800 name Was: Re: Altair 680 Expansion Boards?

[Liam Proven]

On 24 December 2016 at 05:02, geneb  wrote:
>> Commodore's Z80 in the 128 was due to unnecessary fear that they might
>> lose market share to CP/M, when IBM should have been their big worry.
>> I don't know all of the details of the ST/Amiga technology swap, but BOTH
>> were too late, if the primary goal was competing with IBM.
>>
> That might be Commodore marketing - Bil Herd said that he threw the Z-80
> into the design essentially because he could. :)  He's done a few talks on
> how the C-128 came about.  It's pretty interesting.

Seconded. I don't think CBM was scared of CP/M at all. I think it
maybe thought it was a handy extra.

Some of the story is here:

https://hackaday.com/2013/12/09/guest-post-the-real-story-of-hacking-together-the-commodore-c128/

I am somewhat irritated by Bil Herd's claim that it was the last 8-bit computer.

The MSX TurboR was arguably the greatest Z80 home computer:
https://en.wikipedia.org/wiki/MSX#MSX_turboR

It came out in 1990:
https://www.msx.org/wiki/MSX_Turbo_R

But its R800 CPU is arguably 16-bit although MSX-DOS doesn't use that.
https://en.wikipedia.org/wiki/R800_(CPU)

It is in part based on the Z800:
https://en.wikipedia.org/wiki/Zilog_Z800

The SAM Coupé was pure Z80 machine, a lovely British design, a
much-enhanced ZX Spectrum 48, and it came out in 1989 and went on sale
in 1990:
https://en.wikipedia.org/wiki/SAM_Coup%C3%A9

The Acorn BBC Master wasn't all-new but neither was the C128. The
Master was an elegant upgrade to the BBC Micro, and was released in
1986:
https://en.wikipedia.org/wiki/BBC_Master

I'm sure there are many more.

So no, Herd is definitely wrong. The C128 was _not_ the last new 8-bit
computer. It wasn't even the last new Commodore 8-bit computer -- the
C65 was arguably that (and a more logical successor to the C64, IMHO).


-- 
Liam Proven • Profile: https://about.me/liamproven
Email: lpro...@cix.co.uk • Google Mail/Talk/Plus: lpro...@gmail.com
Twitter/Facebook/Flickr: lproven • Skype/LinkedIn/AIM/Yahoo: liamproven
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RE: ICL7611 op-amp (was: Motorola MC14081B)

[Dave Wade]

> -Original Message-
> From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Adrian
> Graham
> Sent: 24 December 2016 12:09
> To: General Discussion: On-Topic and Off-Topic Posts
> 
> Subject: Re: ICL7611 op-amp (was: Motorola MC14081B)
> 
> On 24/12/2016 04:34, "Glen Slick"  wrote:
> 
> > On Fri, Dec 23, 2016 at 7:48 PM, Brent Hilpert 
wrote:
> >>
> >> The op amp is configured as a schmitt trigger or comparator with
> hysteresis:
> >> There is no negative feedback so it is operating at full gain
> >> and functions like a comparator.
> >> However there is positive feedback via R412 (*1), this adds
> >> hysteresis to the trip point(s).
> >> (Brief hackneyed, not rigorous, theory of op: As the input
> >> differential varies past the trip point, the output pulls the + input
> >> further above or below the point at which it just tripped, so
> >> the inputs now have to 'overcome' a greater differential
> >> to trip as the input differential varies in the opposite
> >> direction.)
> >>
> >
> > A recent EEVblog video has some whiteboard explanation of using
> > positive feedback for hysteresis if anyone is curious...
> >
> > EEVblog #941 - Schmitt Trigger Tutorial
> > https://youtu.be/Ht48vv0rQYk?t=13m43s
> 
> Ah cool, I was watching some of Dave's tutorials yesterday but in one of
> them he lost me completely towards the end, talking about Virtual Ground
> on op-amps.

That's how op-amps are typically used in linear mode. If you 

1. Have a split rail PSU
2. Ground the +ve input
3. Connect the output back to the inverting input,

You get a unity gain circuit.  Add resistors and you can adjust the gain as
required. 

Dave


> 
> Cheers,
> 
> --
> Adrian/Witchy
> Binary Dinosaurs creator/curator
> Www.binarydinosaurs.co.uk - the UK's biggest private home computer
> collection?
> 

Re: ICL7611 op-amp (was: Motorola MC14081B)

[Adrian Graham]

On 24/12/2016 04:34, "Glen Slick"  wrote:

> On Fri, Dec 23, 2016 at 7:48 PM, Brent Hilpert  wrote:
>> 
>> The op amp is configured as a schmitt trigger or comparator with hysteresis:
>> There is no negative feedback so it is operating at full gain and
>> functions like a comparator.
>> However there is positive feedback via R412 (*1), this adds
>> hysteresis to the trip point(s).
>> (Brief hackneyed, not rigorous, theory of op: As the input
>> differential varies past the trip point, the output pulls the + input
>> further above or below the point at which it just tripped, so the
>> inputs now have to 'overcome' a greater differential
>> to trip as the input differential varies in the opposite direction.)
>> 
> 
> A recent EEVblog video has some whiteboard explanation of using
> positive feedback for hysteresis if anyone is curious...
> 
> EEVblog #941 - Schmitt Trigger Tutorial
> https://youtu.be/Ht48vv0rQYk?t=13m43s

Ah cool, I was watching some of Dave's tutorials yesterday but in one of
them he lost me completely towards the end, talking about Virtual Ground on
op-amps.

Cheers,

-- 
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?

Re: ICL7611 op-amp (was: Motorola MC14081B)

[Adrian Graham]

On 24/12/2016 03:48, "Brent Hilpert"  wrote:

>> On a whim I managed to solder new legs onto the old LM385Z and it works,
>> giving 1.2V at IN-, but the output is still only 0.2V.
>> 
>> I don't mind admitting I'm stumped :)
> 
> 
> The op amp is configured as a schmitt trigger or comparator with hysteresis:
> There is no negative feedback so it is operating at full gain and functions
> like a comparator.
> However there is positive feedback via R412 (*1), this adds hysteresis to the
> trip point(s).
> (Brief hackneyed, not rigorous, theory of op: As the input differential varies
> past the trip point, the output pulls the + input
> further above or below the point at which it just tripped, so the inputs now
> have to 'overcome' a greater differential
> to trip as the input differential varies in the opposite direction.)

OK, that explanation helps a bit. My brain still has trouble 'slowing down'
how these components work, like when I first dropped from 3GL programming to
assembly. Fortunately there are a lot of tutorials out there.

> One input to the (now) comparator is the 1.2V from V407 regulated down from
> some power rail.
> The other input is the ~ 1/4 voltage divider down from +5V formed by R398 &
> R406 (netting 1.34V @ 5V).
> 
> It appears the idea is that as the +5 supply ramps up at power-on the
> comparator trip (and hence release of reset) is delayed till the +5 reaches
> something around +4.5V.

That's what I figured yesterday which is why I was surprised that
reinstating the LM385Z didn't make it all spring into life.
 
> The ICL7611 is, I expect, a very-low-power (Intersil's niche) op amp. Together
> with the CMOS 4081 the circuit appears tailored for low-power operation.
> Is it supplied by the battery?
> It may require the battery presence for stable supply at time of power-up to
> get reliable reset operation from this power-on-reset circuit.

That's a very good point so now I'm going to have to scrabble around to find
the battery to find out what voltage it was since I removed it ages ago and
may have recycled it. I don't recall any markings on it though, it looked
like a 'normal' 3 terminal NiCAD wrapped in blue plastic. I did take hi-res
pictures of it however.

The 5V input at R395 does head off towards the battery location before it
hits the resistor so I'll trace that out too.
 
> The diode-resistor pairs at the 4081 AND gate inputs turn the AND gates into
> 'asymmetric edge delay gates': the resistor together with some capacitance
> delays the switching of the gate for an edge of one direction, while the diode
> shorts the delay for an edge of the other direction.

Gotcha, I wondered why the diode was there.

> If there is no cap at the input, they must be relying on the gate capacitance
> of the CMOS inputs, making for a pretty short delay.
> 
> *1: Are you sure that's a 220‡ resistor? It's awfully small compared to the
> impedance of the voltage divider feeding it.

My bad, it's a 1.4Mohm and tests ok.

Thanks!

-- 
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?