Link Communications RLC-4 Manual Download Page 23

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1-5

Serial Port Interfacing

The RLC-4 has a full duplex serial port for interfacing to a computer or serial terminal; it is the
DB-9 connector closest to the DB-25 I/O connector and power plug. This connection can be made
with just a cable, a telephone modem, a packet TNC, or any other method that will send the serial
characters back and forth (some have even used Telnet over the Internet). You can use this serial
interface to monitor and program the controller. See Appendix F for more information about
controlling from the serial port.

Using a Telephone Modem

There are two ways to control the RLC-4 over the phone line. If the RLC-4 has an autopatch, you
can control it using the reverse autopatch and entering commands with the telephone's DTMF pad.
This works well for small changes, but if you want to do a lot of programming remotely, it is nice
to do it with a computer. You can do this by putting an external modem on the controller and
calling it from a modem attached to your computer (either internal or external). The modem should
be connected to the controller's serial port - it has nothing to do with the autopatch. You can either
run separate phone lines to the autopatch and the modem, or you can split the phone line and use it
for both (one at a time, of course).

You can either have your computer call the modem and have it answer or you can have the
controller call your computer. If you want to call the controller and have the modem answer and
you plan to use the reverse autopatch, you will have to have a way of determining which one
answers the phone. The most common way is to set the modem to answer on three rings, then to
change the reverse patch setup to make it answer on either 1 or 5 rings (you can do that with a
couple of macros). Whichever one answers first will make the connection. It is also possible to use
a call routing interface (similar to a fax switch) to detect whether you are calling with a modem or
not or to make you press DTMF digits to get to the modem.

The other option is to make the controller call your computer. This is usually done by sending a
DTMF command to the controller to tell it to call you, which means you can't connect with the
modem if there is a problem with your receiver. The DTMF command usually calls a macro that
sends "ATDT <phone number>" to the modem. The advantage of having the controller call you is
that it doesn't conflict with the reverse patch and since the phone number is hard coded in the
macro, you can be sure that no one else will call the controller with their modem and get control
(since the modem can be set not to answer). If you plan to use this method, you may have to turn
off line feed sending with command 060, since most modems will abort the dialing process if they
receive any more serial characters after "ATDT<phone number><carriage return character>" and
the controller usually sends both a carriage return and line feed at the end of each line.

Before connecting the external modem to the controller, you should connect it to your computer or
serial terminal and enter some setup commands. Even if you already have a modem in your
computer, you need to hook up the modem that will go on the controller to set it up. You can use
a standard modem cable for this, or build one according to the chart below. Note that this cable is
different than the one that you will use between the controller and the modem, even if the
connectors are the same (pins 2 and 3 are swapped).