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

jimlux jimlux at earthlink.net
Mon May 24 00:58:45 UTC 2010

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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