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

Robert Benward rbenward at verizon.net
Mon May 24 03:22:54 UTC 2010

My experience with the term "straight through" is that I've seen RS-232 cable that have the ground pin connected to the 
shell.  In a "straight through" the pins are one to one and the only thing connected to the shell would be the shield if 
one is available.


----- Original Message ----- 
From: "Bob Camp" <lists at rtty.us>
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Sunday, May 23, 2010 9:21 PM
Subject: Re: [time-nuts] Z3805 utility, Was: AW: (no subject)

> Hi
> Long ago I decided to go with the terms "straight" and "null modem" for the cables I use. NM and ST are easy to mark 
> and hard to confuse.
> Bob
> On May 23, 2010, at 8:58 PM, jimlux wrote:
>> 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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