----- Original Message ----
From: Robert Benward <[email protected]>
To: Discussion of precise time and frequency measurement <[email protected]>
Sent: Sun, May 23, 2010 4:38:43 PM
Subject: Re: [time-nuts] Z3805 utility, Was: AW: (no subject)
Jeff,
Generally for a given DTE, there is a swap of 2&3 when going from DB9 to DB25
and vice-versa. For a normal serial cable, DTE to DCE, pins 2&3 are swapped.
RS-232 states that pin 2 is the TX data, DCE or DTE, on a DB25. It's TX on pin
3 for a DB9. Therefore to hook a DCE to a DTE, you must swap pins 2&3. The
fact that you are going from 2-2 and 3-3 probably means you've inserted a null
modem in between, and you are using a standard serial cable.
In the old days... it was a Data Terminal Equipment (DTE) that communicated
with Data Communications Equipment (DCE) to the mainframe. Stanley's
assessment is correct, although I'm not sure if the mainframes of yesteryear
were DTEs.
<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.
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.
So a standard cable for connecting a terminal to a Dec computer would be a null
cable. A standard cable for connecting a terminal to a IBM mainframe could be
straight. But I disagree that a standard cable can always be straight or null.
Stanley
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