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A.J. · Guest

Hi all,

I'm attempting to interface serial devices using standard Category 5
cables, since they are cheap, widely available, and can be manufactured
in long lengths. To achieve this, I have obtained several adapter kits
that convert D-Sub 9 and D-Sub 25 connectors to RJ45 connectors.

I am using the EIA-561 standard for wiring serial devices together,
with a small modification. I am constructing the RJ45 adapters using
the following pinout:

RJ45 pin............Signal

1 (blue)............DSR (Data Set Ready)
2 (orange)..........DCD (Carrier Detect)
3 (black)...........DTR (Data Terminal Ready)
4 (red).............GND (Signal Ground)
5 (green)...........RxD (Receive)
6 (yellow)..........TxD (Transmit)
7 (brown)...........CTS (Clear to Send)
8 (white)...........RTS (Request to Send)

The difference between my pinout and EIA-561 is that I use Pin 1 for
DSR (Data Set Ready) instead of RI (Ring Indicator).

Now, I wish to connect two PCs together using a null modem serial
configuration (wherever an Ethernet port is not available). The nice
thing about using EIA-561 is that I can make the null modem cable as
long as I like by finding a longer Category 5 cable. I plan to use a
straight-through EIA-561 adapter on one end, and a null-modem adapter
on the other end, wired according to the following pinout:

RJ45 pin............Signal

1 (blue)............not connected
2 (orange)..........DTR (Data Terminal Ready)
3 (black)...........DCD (Carrier Detect)
4 (red).............GND (Signal Ground)
5 (green)...........TxD (Transmit)
6 (yellow)..........RxD (Receive)
7 (brown)...........RTS (Request to Send)
8 (white)...........CTS (Clear to Send)

This will establish the following connections between both ends:

DSR -- not connected
DCD -- DTR
DTR -- DCD
GND -- GND
RxD -- TxD
TxD -- RxD
CTS -- RTS
RTS -- CTS

I have actually built both of these adapters and they seem to be
working, at least between a PC and an Apple IIGS.

However, I have a few questions about this configuration:

1. Is there a consequence to omitting DSR in the null modem adapter?
Will it reduce compatibility between particular devices?

2. How important is the DSR signal for serial communications,
especially in a null-modem configuration?

3. Can I use two of my straight-through adapters in order to connect a
modem to a PC? Will omitting RI cause any problems with modern modems?

4. Should I have just wired Pin 1 in both adapters to be RI instead of
DSR?

Thanks,
--A.J.

Scott Packard · Guest

You just want to get data from one comm port to the other, right?
There are *many* things on the 'net for this so you should really
just search. Even blackbox.com has info, I'm sure, on making
this type of cable.
Basically, DSR loops locally back to DTR, RTS loops locally to CTS,
your cable carries RX and TX, and carries GND.
CD may or may not be used (depending on your comms program).
If you need it, just tie it in with the local DSR-DTR.
The length limitation is what you'll have do deal with, since you're
using 12V unbalanced (for RS-232, I don't know what voltage EIA-561 is) to
do signalling, the cable capacitance eventually won't allow you to go
beyond a certain length.
It really sounds like you're going for long length comms (like over 25')
and it's much less of a headache to just Ethernet the boxes.

Regards, Scott

On Sun, 24 Apr 2005 14:31:10 -0700, A.J. wrote:

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