to this standard. The only feature lost is the 20mA interface, which is
obsolete and unlikely to ever be needed. Cut the tracks leading to pins 1 (was
drive out, now CD in), 9 (was 20 mA in, now DSR in), and 12 (was 20mA out, now
DTR out). Then connect pin 2 to pin 12, giving 5V on the DTR out signal. Check
your connections carefully as you will not wish to blow up your Nascom. Some
cables connect 5, 6, 8 and 20 at the terminal end. If the terminal puts 12V on
its DTR then this could be fed back to the N2 5V supply with disastrous
results, so be careful!
Special Purpose Leads
It is possible to build special leads to connect a host to a host or a
terminal to a terminal. In each case every device believes the other to be what
In both cases pin 7 (ground) is connected to pin 7 at the other end. Pin 2
at one end is connected to pin 3 at the other, and vice versa, since the inputs
and outputs must be reversed. Do not connect any other wires between the
The host to host lead could be used to connect two computers together,
using their printer sockets – if they are wired up correctly! At each end,
connect pin 6 (DSR out) to pins 4 and 20 (RTS in and DTR in). This fools each
host into believing that it is connected to a terminal.
The terminal to terminal lead has pin 20 (DTR out) connected to pins 5, 6
and 8 (CTS in, DSR in, CD in) at each end. This fools each terminal into
believing it is connected to a host.
There are many articles explaining various aspects of the RS232 interface.
There is a useful section in
D. R. Hunt’s article
in the 80-BUS news Vol 1
Issue 3, pages 32 to 36.
When the computer is acting as a host, the normal operating system
software can use the serial interface to drive a printer. But it is not so
obvious what to do then the computer is to act as a terminal device. In this
case a program is run which makes the computer act as a terminal. A
comprehensive terminal emulation program is given in full below. Another
program which is very useful is MODEM7, which is available from the CP/M Users
Group, and is easily made to operate on a Nascom or a Gemini. This allows
files to be transferred down phone lines (using modems or acoustic couplers),
and it automatically recovers from line errors by patiently retransmitting the
data when needed.
Terminal Emulation Software
It can be a problem to find software which allows a computer to act as a
simple terminal. This is not as simple as you might at first think, and
therefore an example for the Gemini
is printed below.
Now for the program which turns a Gemini into a dumb terminal. This is not
as simple as it seems for several reasons. Firstly as soon as the speed gets
high enough, the display cannot keep up with the incoming data, and characters
start to be lost. Also this program allows characters to be echoed to a serial
printer which may be very slow, for example 10 cps. Another complication is
that the terminal may operate in half duplex or full duplex. This program
allows for all these options, as you can see from the comments at the start of
it. It can be assembled using M80.