Wednesday, 23 April 2014

Percom Doubler II

My first TRS-80 floppy disk drive was an Aerocomp branded 40 track, single sided drive. The available storage was minimal so I ended up buying two additional 80 track drives. The Percom Doubler was available around that time but I could not afford it. Instead, I bought a Percom Separator. The Raspberry Pi interface can read double density and I hope soon to be able to write double density, but without double density on the TRS-80 there was no way to be sure that double density support is really working correctly.

Fortunately I found schematics for a Percom Doubler II here and decided to build one. The provided layout is best suited for a professionally built board with plated through holes. Since I would be etching and drilling it myself I re-imported the schematic into KiCad and rearranged the layout to allow more room between ICs and fewer vias.

Some of the components are difficult to find. I was fortunate to already have several MB8876A ICs which are identical to the FD1791 using in the original doubler. A company called Mikes Arcade was able to supply the required PROM and program it as well.

Board after exposure and developing.
Ferric Chloride warming up in a water bath.

Replaced socket with pin connector.
Board in position.
After considerable testing, resoldering, checking of components and replacing older capacitors on the expansion interface the Percom Doubler II clone is working. I created an 80 track single density LDOS system disk using the Raspberry Pi, booted from it and then created an 80 track double density system disk on the TRS-80. Note that this disk actually contains a single density track zero as the Model I ROM will only boot from single density. The boot sector and system configuration files then enable double density support. The final test was being able to successfully boot from the new double density boot disk.