[time-nuts] Z3805 utility, Was: AW: (no subject)

Stanley Reynolds stanley_reynolds at yahoo.com
Mon May 24 00:41:35 UTC 2010

----- Original Message ----
From: Robert Benward <rbenward at verizon.net>
To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
Sent: Sun, May 23, 2010 4:38:43 PM
Subject: Re: [time-nuts] Z3805 utility, Was: AW: (no subject)

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.


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.


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