On Feb 6, 4:08, r. 'bear' stricklin wrote:
> I just wired a handful of these adapters for my own asynch comms server.
> Crossing TX and RX, as well as RTS and CTS, was a "no-duh". But what the
> heck do I do with DSR, DCD, and DTR? Part of my problem is I don't
> understand exactly what those signals are used for and how they relate to
> each other.
>
> I looked at some PC "null modem" cables and they seem to tie DSR and DCD
> at one end, to DTR at the other. I'd do this exact thing for my setup but
> the comm server only offers DCD and DTR signals.
>
> How do I handle DSR, DCD, and DTR in this situation? I tried leaving them
> unwired but things seemed unhappy.
The way I usually do it is to cross wire like this:
DTR -------------------- DSR
\
DSR -- ------ DCD
\
DCD -------------------- DTR
Another way is to simply link all three together on each connector (with no
DTR/DSR/DCD connections between the connectors).
I usually also cross over RTS and CTS, or tie them together at each end.
The logic is roughly like this: terminal connected to modem; terminal
raises DTR to indicate that it's active (Data Terminal Ready) and it wants
some attention. If modem is powered up, it replies by raising DSR (Data
Set Ready) to indicate some kind of life. If it grabs a line, makes a
connection, and gets a carrier tone in response, it raises DCD (Data
Carrier Detect) to say so.
I say "roughly" because it's actually different for full-duplex and
half-duplex, and neither is quite what I've described above. However, that
conveys the general sense. So in a situation where you have two terminals
(or any other equivalent device, like a PC, printer, in fact any DTE or
Data Terminal Equipment rather than DCE -- Data Communications Equipment)
you create a "null modem" in between. When one side raises DTR, the other
sees DSR, and vice-versa. When ever one sees DSR, it knows there's
something there and it also sees DCD and thinks it's OK to transmit and
receive (well, strictly speaking, only to receive).
--
Pete Peter Turnbull
Network Manager
University of York
Received on Wed Feb 06 2002 - 13:36:20 GMT