System 80 Networking Modification

Some System 80 users pushed the machine to its limits in new and innovative ways. One example was this home-grown networking option in use at Waiopehu College, Levin, New Zealand (and others) in the early 1980s. This modification was designed and implented by Murray Balchin (see the link to my email interview with Murray below). Murray, who sold and repaired computers as a business, lived in the Manawatu settlement of Rongotea. The software for the Networking ROM was written by a student at the local University (Massey University).

Physical description

The modification allowed a number of 32k internally expanded System 80s (clients) to read and write files to disk drive(s) fitted to a single master machine (the host). Hardware in the client machines took the form of a daughter board mounted on the RF output box which interfaced with the external cassette port, replacing some of its componentry (figure 1). The board was powered directly from the power lines on the CPU board. The external cassette port was used to connect all machines by a single shared cable. Included in the modification was a small audio socket, which exited the machine from the bottom of the case, underneath the external cassette port. These plugged into the Dick Smith monitors, which had audio input.

Figure 1. System 80 (client) boards showing network modification

Client machine software was provided from an EPROM, occupying memory locations 3000H-35FFH and fitted to the expansion ROM socket on the CPU board. This is the same socket that held the EPROM providing the extra ROM functions of the Blue Label and Mark II models. A student using the system would boot up to the following menu which said:

NETWORK : By Small Business Systems Ltd.

<C> Clear the Program
<T> Transmit a Program
<R> Run a Program
<B> Return to BASIC

Command?

Close up pictures of the modification are contained in this zip file, which can be downloaded and viewed by those wishing to study it further. A code dump of the Networking EPROM is available here. The zip file also includes a quick disassembly listing courtesy of Leslie Ayling.

History of the modification and theory of operation

This is best told by Murray Balchin himself, in the reply to questions asked in an email interview on 18th November, 2010. Click here to read the transcript.

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