OK. I think I know where some of this confusion I'm having is coming from.. All these descriptions are misleading in
that they (Wiki) call pin 2 TX. On wiki's page, both DCE and DTE are TX=2. The table below shows the same except that
TX is defined from the DTE's point of view. Thus DTE to DTE needs 2&3 crossed, and DTE and DCE needs 2-2 & 3-3. If your
device is a DCE, then pin 2 is really a RX input, not a TX output.
DB-25 DB-9 Common EIA
Pin # Pin # Name Name CCITT DTE-DCE Formal Name
----- ----- ----- ----- ----- ------- -------------------
1 FG AA 101 - Frame Ground
2 3 TD BA 103 ----> Transmitted Data, TxD
3 2 RD BB 104 <---- Received Data, RxD
4 7 RTS CA 105 ----> Request To Send
5 8 CTS CB 106 <---- Clear To Send
6 6 DSR CC 107 <---- Data Set Ready
7 5 SG AB 102 ---- Signal Ground, GND
8 1 DCD CF 109 <---- Data Carrier Detect
Stanley, Did I get it right this time?Sorry to all for my previous ramblings.Bob----- Original Message ----- From:
"jimlux" <jim...@earthlink.net>
To: "Discussion of precise time and frequency measurement" <time-nuts@febo.com>
Sent: Sunday, May 23, 2010 8:58 PM
Subject: Re: [time-nuts] Z3805 utility, Was: AW: (no subject)
Stanley Reynolds wrote:
<snip>
Dec computers / terminal servers were as I described, but many brands
were different. Still have a BOB aka break out box with LEDs to
indicate levels, matching transmit and receive is easy, getting the
hardware flow control / signaling right was a little more difficult.
straight cable = pin to pin
crossed cable = null modem = swapped pins
The phrase "null modem" comes from no modems or the configuration
that allows two singular ports to be connected, this cable would
cross the receive and transmit pins, and some would call it a cross
over cable. A null modem cable would be used to connect two computers
together and a program like kermit used to transfer files.
Yep.. DTE cable to DCE communications medium(phoneline) DCE to DTE
DCE == Modem (e.g. a Bell 202 or 212, for instance)
There were the flow control (RTS/CTS) used to turn around a half duplex link. And, there are also the secondary
transmit and receive (for a low rate reverse channel). If you were receiving data from the link (DCE), you'd assert
RTS, and when the modem had switched, it would tell you CTS, and off you'd go. (fancy modems used the reverse channel
to send the request to the far end, which would acknowledge... others just use a fixed time delay) There are also
pins for the clock (since some of these modems were used on synchronous data links).
the "crossover" occured in the DCE to DCE link (that is, you'd transmit from
one DCE to the other DCE's receiver)...
the nominal cable between DTE and DCE was straight through. With no real convention on male/female.. most devices had
female sockets, and the cables usually were male male plugs. IBM PCs had male on the chassis for DTE, as did some
PDT-110 (VT-100/LSI-11 smart terminals), but most other terminals (the LSI ADM-x, Hazeltines, etc.) all seemed to have
female, as did the TI 800 series printer/terminals.
So, a "null modem" was a cable that emulated the DCE to DCE connection..
there are/were various strategies on how sophisticated the reverse is.. do you also send the secondary channel? What
about clocks? Most folks ignored all that and used RTS/CTS
Or you strap RTS to CTS on your side, the other side does the same.
I think the phrase "standard cable" which could be null or straight
depending on the use is the confusing part.
Phone cables RJ11 and RJ45 swap the wires which is standard. Network
cables match the wires with the same color always on the right which
is standard. But even when a phone cable is standard it is not
interchangeable with a standard network cable. Again we have a need
for cross as well as straight network cables.
And, to make things worse, there are different "pair" arrangements.
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