System 80 / FreHD Adaptor

The Dick Smith System 80 was a TRS-80 Model 1 clone that was popular in New Zealand and Australia in the early 1980's.

While the machine was largely software compatible, it is a different hardware design.  This is particularly true for the expansion bus on the back of the machine and the design of the expansion interface.

This difference in expansion design means that the FreHD cannot connect directly to the keyboard or the expansion interface of the System 80.

The TRS-80 Model 1 and the companion 26-1132 Model 1 Hard Disk Adaptor made FreHD connection easy.  The 26-1132 connects to the expansion connector on the left side of the expansion interface.  This expansion connector is a duplicate of the connector on the back of the keyboard with the exception of the dynamic memory refresh signals.

The System 80 expansion interface does not duplicate the keyboard expansion connector for further expansion.  In fact it doesn't provide any externally accessible expansion ports that could be used for a FreHD connection.

What the System 80 Expansion Interface does provide is an internal 50 pin connector that is intended for an optional S-100 bus adaptor.

In the two expansion interfaces I have seen this 50 pin connector is an edge connector socket mounted on the PCB.  The connector pinout is documented in the Video Genie EG-3014 technical manual available from http://www.classic-computers.org.nz/system-80/manuals_expander_EG3014_technical_manual_1981.pdf

The first challenge for a prototype was to connect using this 50 pin connector.  I didn't want to make a board without knowing the final design so created an adaptor for the edge connector socket that provided a pin header connector to the test board.  An old ISA bus parallel port board was sacrificed to provide the edge connector.

On top of the edge connector was soldered a 50 pin header and spacers to ensure the edge connector would insert into the socket and make contact.

The initial attempt at an adaptor board was to build a wiring adaptor. Unlike the Model 1 which combined the Z80 /WR, /RD and /IORQ signals to generate /IN and /OUT (and needed to be seperated back to the standard signals by the 26-1132), the System 80 provides the standard signals so all that was required (or so it seemed) was to match these to a Model III/4 type expansion pinout.

Problem was it didn't work reliably.

After a lot of testing by Ray Whitehurst (a System/80 enthusiest from Australia), head scratching and review of the Expansion Interface schematics we were able to get a reliable adaptor.

In addition to the pin mapping it also required:

  • Termination resistors on the data, address, /IORQ, /RD and /WR lines immediately after the S-100 connector.
  • Following the termination resistors added a 74LS245 buffer on the data lines.

Ray"s First Working Board

Final Prototype with SIP Terminating Resistors

System 80 Expansion Interfaces are very difficult to find in New Zealand.   I am very grateful to Mark Barlow of Techvana for lending me his Expansion Interface and Ray Whitehurst for all the prototype boards he assembled for the FreHD validation.

 

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Contact Andrew Quinn

jaquinn@ihug.co.nz http://twitter.com/jaquinn