RJ45 to serial DE-9

From: Andreas Freiherr <Andreas.Freiherr_at_Vishay.com>
Date: Wed Feb 6 07:28:58 2002

> Serial ports on RJ45 are not standardized. Your best bet is to buy an
> adapter you can wire yourself.

Right. And even for the DB-9 there are at least two pin assignments. The
more common is that of PeeCees, another was used on some MicroVAX
console interfaces by DEC.

The simplest serial interface uses only RX, TX and ground. For this
setup, you must use some kind of software handshake to allow the
receiving side to stop the sender before an input buffer overflows.
Using this three-wire setup, you can go for ETX/ACK (think I have seen
this with some HP machine) or for XON/XOFF (common with DEC and Unix,
probably others). The difference is not only with the ASCII characters
used, but also with the sequence of events. Details beyond scope here
(?).

RTS (request to send) and CTS (clear to send) are modem control lines
that are often used for handshaking. RTS is the DTE (data terminal
equipment) signalling to the modem (DCE, data communications equipment)
that it wants to send something, CTS is the modem telling the terminal
to do so.

DSR (data set ready) and DTR (data terminal ready) are modem and
terminal telling each other "I'm here for your service". These are
another possible choice for hardware handshake.

DCD (data carrier detect) is usually a modem telling the terminal "we're
online". With some ports / some setups, this signal must be active
before anything can be sent out or received.

So, if your connection will work does not only depend on the wires but
also on the settings at each end of them. If you can select XON/XOFF
handshake, you may be able to go with just RX, TX, and ground, but if
you need hardware handshake (many PeeCees), you need to connect the
particular signals used with your particular device.

Tying DCD to DTR is a loopback to make the terminal "see" a carrier all
the time.

If the classic "DTR feeds DSR and DCD" setup does not work for you, then
maybe DTR is not asserted? - You can check this with a simple voltmeter.

By watching the TX line, you should be able to tell if this particular
side is talking to the other side at all. Otherwise, you cannot tell if
your keyboard input is sent, but not echoed, or if just the echo is not
sent.

I do know people whose first estimate on the time required to get a
serial interface running is always "2 workdays". When answering so, they
usually hear a "huh? two whole days???", but they often need that time -
and it's not because they don't know their job!

Things that come to my mind: the "Commodore" is a PC? - Then it will
probably prefer hardware handshake, while the Lantronix may want to use
XON/XOFF, so the Lantronix may not assert CTS and/or DSR. Consequently,
the PC may not be able to send any data. Can the Lantronix be talked
into using RTS/CTS or DTR/DSR handshake (maybe it calls it "modem
control")?

--
Andreas Freiherr
Vishay Semiconductor GmbH, Heilbronn, Germany
http://www.vishay.com
Received on Wed Feb 06 2002 - 07:28:58 GMT

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