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

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.


TRS80 Model 1 Fixup - Part 3

So with a power supply, working power switch, restored Expansion Interface and a suitable case, things were looking good for a quick test of the Model 1 / FreHD adaptor and move on to the next project.

As they say on the billboard advertisements for Tui beer in New Zealand.....

I do however now have a lot better understanding of the WD1771 floppy disk controller and how it is integrated with the Model 1 EI.

For those that like to jump to the end of the book.... the problem was C62 (a 33uf electrolytic capacitor) in the "later revision" EI (not sure how EI's are actually named and probably a different component reference in the early version). Replacing this cured all the boot problems.

The path to discovering this is the interesting bit.

The behavior of the machine was that it would boot "sometimes".... perhaps 10% of the time. It didn't make any difference if it was a boot from power on or pressing reset.  If the boot failed it would just hang with a blank screen.  If it did boot then LDOS would run perfectly but a reset or power recycle and it was back to the "will it boot" problem.

Went through all the basic problem solving steps by changing the following:
1. Floppy drive (both 3.5" and 5.25")
2. Drive cable
3. Keyboard to EI cable
4. WD1771 controller
5. Added a Percom Data Seperator

None of these improved the situation to any extent.

One interesting difference that pushed me down an incorrect path was that sometimes when the boot failed the disk heads would step inwards to the center of the disk and stop when the drive hit the stops.  This made me suspect the 7416 driver that controls the direction and step lines.  I replaced this with a 7406 but it didn't fix the boot problem.  I discovered afterwards that this behavior would only occur with certain WD1771 chips. I have two different types... an original RS part that I got from Ian and some later "1982" NOS WD1771's that I got on eBay.  Only the "1982" versions exhibited the stepping behavior.

Spent some time writing some simple basic programs to test the FDC.  It was with these that I discovered that I could cause a similar problem.  Some requests to the FDC to move to different tracks would not complete and the WD1771 status register would report "busy" until it eventually timed out and the busy flag was cleared.  Further digging with the logic analyzer showed that the WD1771 "Ready" line was being deasserted before the command completed.

In the WD1771 datasheet it says that "Ready" is how the drive tells the controller that it is ready for the next command (such as stepping commands when seeking a track).

In the Model 1 (and possibly the Model 3/4) "Ready" doesn't come from the drive (because mini-floppy drives don't seem to have a concept of "ready") and is generated in the EI.  When a drive is selected by writing to 0x37E0 drive select latch this is loaded into Z47 (a 74LS175 flip flop).  The DS outputs are OR'ed to generate the "Ready" so this will high if any drive is selected.

Because "mini floppy" drives are not intended to run continually Z33 is used to clear Z47 and deselect all drives approximately 2-3 seconds after the drive is selected. Z33 is a 74LS123 and uses R25 and C62 to generate a "delay" during which the drive select is active.  If the drive is not re-selected this will timeout and "Ready" will clear.

As C62 had aged it's value has changed (reduced) so Z33 was timing out early and as far as the WD1771 was concerned the drive had gone off line.

This caused problems at boot because the bootstrap code is pretty simple and didn't expect that it needed to reselect the drives during the process because the C62 would keep the drive selected.  The reason it worked sometimes was just luck and where the heads happened to be from the last failed attempt.

Replacing C62 with a new 33uf Tantalum capacitor ($2.50 at Jaycar in New Zealand) fixed the problem and all disks I have now boot.


The offending C62....

... and it's replacement.


TRS80 Model 1 Fixup - Part 2

Included in Ian's box of "things from the junk box" for FreHD testing was a Model 1 Expansion Interface PCB.

Ian tells me that a common problem is people shipping Expansion Interfaces with the power transformer still inside.  Add an enthusiastic courier driver and you are almost guaranteed to have a broken case. This Expansion Interface had such treatment so had ended up a junk box parts donor.

Apart from the missing case, WD-1771 floppy disk controller and connection cable to the keyboard, the Expansion Interface board looked complete and in good condition.

For anyone who finds a parts donor Expansion Interface board and wants to get it going, as of 2013 the WD-1771 floppy controllers are still available on eBay as are the 4116 dynamic RAM chips.  Best to search around because the prices vary a lot.  Unless you are hung up on period authenticity then the vintage style packaging with the really high prices are not required!

With the chips replaced and a cable made up the initial indications where that the Expansion Interface worked fine.

At this point I realized why they put these in cases... where to put the monitor when you just have a PCB? Things get very spread out.

Taking inspiration from the current trend in acrylic project cases for things like the Raspberry Pi I went looking for a source of acrylic sheet and discovered CTS Plastics in Tauranga.

Here is my solution to the no-case problem.

    2 sheets of 6mm grey perspex plastic

    5 x 50mm bolts, assorted nuts and washers.

Total cost about NZ$25.


Much safer than having the EI board floating around on the table, looks smart and with 6mm perspex it is very strong so the monitor can sit on top.


The dimensions and drilling information is in the attached pdf for anyone wanting to build their own.

The drill details are:
    4 holes (1 at each corner) 20mm from the edges.
    2 holes 143mm from the left edge.  One is 82mm from the bottom and the other 202mm.
    1 hole 38mm from the right edge.  This is 142mm from the bottom.
The two pieces will be mounted together with spacers so the holes need to be accurately drilled on both sheets.







TRS80 Model 1 Fixup - Part 1

With the success of the FreHD on the TRS-80 Model III and 4 it was time to start testing on the Model 1.

The first step was finding a Model 1 and Expansion Interface.  While they may be plentiful in the US they are a fairly uncommon machine in New Zealand.

Luckily just across the Tasman Sea is Ian Mavric's TRS-80 OldTimer Center.  Ian is a big supporter of the TRS-80 community and the FreHD project in particular, offering both partial kits and completed boards for the FreHD so when we started talking about Model 1 testing he went hunting through his well stocked junkbox to help out.


"I have found you some bits" said the email.... "An early Model 1 keyboard and an EI board. No power supplies so you will have to sort that out. I think they work but you know something about electronics so I am sure you can sort them out"

"How hard can that be?"


While waiting for the parts to arrive I started sorting out the power supply.  Most of my microcomputers use 5VDC and 12VDC so I wasn't expecting much difficulty there.  Then I discovered that while the Model 1 may use 5VDC internally (and -5VDC for the 4116 dynamic RAM), the power supply bricks need to provide 20VDC and 16VAC. No opportunity to recycle an old PC supply here and keep the Model 1 fairly close to a stock configuration.

The solution was an excellent power supply design by Dean Bear that is documented on  There is a good writeup on this design in the September 2013 issue of TRS8BIT available at


For my test setup I built the power supply on prototype board and mounted it in a Jaycar case. As the TRS8BIT article warns, there are high voltages in this project and if you are not comfortable work with this I suggest checking with Ian Mavric to see if a kit or built up unit is available.




With a suitable power supply now ready and a box of parts having arrived it was time to see what Ian had sent over.  It had been 35 years since I had used a Model 1 and my initial reaction was that it was a lot thinker than I remembered.


The machine powered up with the usual Mem Size? prompt but the power switch was difficult to latch on.

Holding it in was fine for the initial test but made using the machine difficult so this was the first thing to address.

With the latching pushbutton not being easily available here I used a miniature three pole toggle switch from Jaycar instead. This is easy to wire into the PCB holes previously used by the pushbutton and mounts nicely in the existing case hole. One day when I find the real pushbutton I can return to a more stock configuration.





This closeup thanks to Dean Bear shows the PCB holes clearly.



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