As my 502 board came with no Basic ROMs, I decided to figure out a simple to adapt my board to run 2716 EPROMs instead. The 502 board comes standard with 2316 ROMs. 2316 ROMs come in a variety of different types. The Apple II, Commodore 2001, and old vIdeo games, to name a few, all use 2316's. However, I believe that companies could order 2316's from the manufacturer in different configurations, with the differences being wether the chip select lines were active high or low (I could be a little confused about the exact details). Unfortunately the 2716 EPROMs chip selects are opposite to the 2316's used in the OSI, otherwise the pinouts are basically compatible.

There are two Pins (18 and 20) on each EPROM that will need to be dealt with, and how you deal with them differs on wether you are installing the Monitor ROM or the four BASIC ROMs.

I will discuss each one separately...

THE MONITOR ROM

Fortunately replacing the Monitor ROM with a 2716 EPROM is easy, and OSI has provided simple onboard jumpers to do this. Once these two jumpers are changed you will be able to simply drop in a 2716 EPROM. Easy!

Pin 20:

The next part is a little more tricky. Pins 20 from the ROMs pass through a 7404 inverter at U17 which lies between the ROMs and a 74139 at U16. As I previously mentioned the 2716 does not need the CS signal inverted, therefore the 7404 is not needed. I decided the simplest and cleanest way was to remove the 7404 and put a socket in its place. Then I used small wires to jumper the socket. This way if I ever need to put the 7404 back in, there is a socket ready and waiting for it. The socket is jumpered in the following way:

3 - 4        5 - 6        8 - 9        10 - 11

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Under U8 (BASIC ROM 1). Cut the line that connects pins 18 and 21. It's a little tricky to get to as it’s under the socket.

Solder side. Run a jumper between U8 pin 18, and U9 pin 18 (bottom arrow). Also jumper between the area where you previously cut the trace and the GND line (top arrow).

CUt the trace on this jumper area.

That's all. The board should now be ready for 2716 EPROMs. I used the SYN600 Monitor ROM and BASIC ROM images found here (http://osiweb.org/software.html) to program my EPROMs. I also used the ‘fixed BASIC 3 ROM’ image, as it resolves some garbage collector issues that the original ‘BASIC 3 ROM’ had.

I have been possibly a little verbose in my instructions, but the process is actually very simple.

Remove U17. Install a socket, and use wires to jumper the connections.

Reference image showing the jumper locations to change for a 2716 Monitor ROM.

Reference image showing the jumper locations to change for a 2716 Monitor ROM.

Schematic showing the 4 Basic ROMs.

502 board now running Basic.

THE 4 BASIC ROMs

This is where things get a little more difficult….but not too much.

OSI has not made it easy to replace the BASIC ROMs with EPROMs, with no easy jumpers like those for the Monitor ROM. Again, pins 18 and 21 will need to be dealt with. I will explain each pin individually.

Pin 18:

OSI seems to have dealt with pins 18 and 20 differently between BASIC ROM 4 and BASIC ROMs 1,2, and 3. Looking at the schematic it seems that BASIC 4 was able to be replaced with a 2664. Not sure what a 2664 is, but this means that first we have to find a way to tie pins 18 on all the BASIC ROMs 1, 2, 3, and 4 together, and then connect this network all to GND. Here's how:

On the parts side there is a single trace under U8 (BASIC ROM 1) which connects pins 18 and 21, you will need to cut this, but be aware that it's a little hard to get too as it is under the right side of the socket. The schematic actually shows these pins to be connected on each ROM but this is not the case, and only this one line needs to be cut.

There is a jumper area above and to the left of U9 (BASIC ROM 4). Cut this.

On the solder side you will need to run a small jumper wire between U8 pin 18, and U9 pin 18. This ties pin 18 of all 4 BASIC ROMs together. You will also see the solder side of the jumper area above U9, where you previously cut the trace. Use another short jumper wire to jumper between here and the GND line.

This takes care of pins 18 for all the 4 ROMs. They will now all be connected nicely to GND.