In drafting stage

PAL Coder - v.1 (1 PROM + passive elements)

Another PAL coder that can be used with CoBra, this time a very ingenious circuit. I used it successfully with the green CoBra mainboard. Since I did not get to build it myself today, with recently made components, I declare it for now "In drafting stage". I bought it already assembled sometime in the '90s when I was living in the campus of the Electronics Faculty. I'm not really sure anymore where I got it, but I tend to believe it's from a booth on the street which was right across the building I was living in and which was known as "Star 5". The circuit is ingenious in that it uses only one active element, an integrated circuit which is a PROM. I don't know if there is any other PAL coder schematic for a "home computer" more simple than this. Seems to be the perfect incarnation of the expression "component saving". The coder even came with instructions on how to connect it to a home computer, printed on a tiny piece of paper. You can see below a photo of the circuit the way I had installed it back then on the green mainboard as well as the scanned image of the paper with instructions which I successfully managed to keep to this day.

At that time (after I bought it and before installing it on the green mainboard) I examined its circuit board and extracted the schematic, which I also show below as a scanned image of the piece of paper I drew it on in pencil. As far as I remember two of the colors were mistakenly switched in the printed instructions, which I noticed when I connected it to CoBra and I saw the colors were wrong.

The schematic extracted by me back then is drawn in pencil, In blue I recently added in the scanned image (image editing) the conclusion I reached today regarding the PROM type, after I successfully managed to read it using my memory programmer, to which I gave the command to read a 74S287 PROM. I also corrected the value of a resistor (180 Ohm) which I had extracted wrong back then (220 Ohm).

Recently (April 20, 2013) I decided to try somehow and determine what exactly could be the integrated circuit in this coder. The printed instructions said it would be a PROM, so at least the category was known. What was left to determine was capacity and organization (how many bits for output). At this point I had a discussion with YO3GHM and he came up with the suggestion that it could be the same type as the one used in the PROM PAL coders he had seen being used in HCs, that is a 4 bit PROM, the kind of 74S287. 74S287 has Tri-State outputs and 74S387 has Open-Collector outputs. I decided to try somehow to read the PROM with my memory programmer. In order to do this, in the beginning I was planning to unsolder the IC off the circuit board. But then the thought crossed my mind that instead of basically destroying the circuit by doing so, there would be a better solution, so that after I attempt reading it I could still bring the circuit easily back to its working state. So I started working and I cut all circuit tracks going to the pins of the IC. Then I soldered some resistor and diode terminal leftovers, perpendicularly to the back of the circuit board directly on the IC pins, as seen below.

"Processed" this way, I inserted the circuit (with some difficulty) in the memory programmer socket and I gave it the command to read a 74S287 PROM. Below you can read the relevant part of the log:

L0097: Selected device: National Semicond. DM74S287.
L0098: Buffer checksum in range of [0h..FFh]: 00000F00  - Byte sum (x8)
L0100: >> 20.04.2013, 21:20:51
L0101: Reading device: National Semicond. DM74S287.
L0102: Device insertion test ...
L0103: Reading device ...
L0104: Verifying device with buffer ...
L0105: Reading device - done.
L0106: Elapsed time: 0:00:02.1
L0107: Buffer checksum in range of [0h..FFh]: 00000900  - Byte sum (x8)
L0108: Statistics info: Success:1  Failure:0  Other failure:0  Total:1

So the reading ended successfully, confirming that it's a 4 bit PROM with 256 locations. The binary image read by the programmer can be downloaded as a ZIP archive. Now, if we examine the schematic extracted, I believe it would rather be an open-collector PROM (like 74S387), since all outputs are connected to Vcc through low value resistors.