80-Bus News

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 The only feature lost is the 20mA interface, which is obsolete and unlikely to ever be needed.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 it expects.

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 devices.

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.

Other Articles

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.

Software

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 IVC 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.