Digital Interface Systems 16k RAM expansion

[azog]

I recently acquired a "Digital Interface Systems" 16k RAM expansion, in the infamous "unknown condition". It has eight 6116 RAM chips (2k by 8 bit), a TTL decoder (74xx139) and two banks of DIP switches (one is 8 switches, the other is 2 switches).

I took the dare and plugged it into my VIC-20. At first, I thought the DIP switches were just switching each individual RAM chip, but either there is bad RAM (quite likely), or the decoding isn't that simple (equally likely)

I tried random DIP switch positions, but most of them return the standard 3.5k RAM, and only one starts the VIC-20 with just over 11k RAM.

Does anyone know of a manual for this? I could probably trace it out and figure out the decoding, but I figured I'd ask here first.

For the curious, an image is here:

image.jpg (171.57 KiB) Viewed 989 times

[buzbard]

I haven't seen that particular cartridge before but it looks to be setup similar to a Commodore 16k RAM cart. Given that, I'm assuming that the bank of 8 switches is used to select the address of each of the 8k blocks.

Switches 1-4 "should" select the address of the first 8k block and 5-8 should select the address for the second 8k block. Never select 2 switches in the same block or same address. (for example, if you select switch 1 don't use 2-5) hope that's not too confusing.

So,
switches 1 and 5 should select BLK1 ($2000)
switches 2 and 6 should select BLK2 ($4000)
switches 3 and 7 should select BLK3 ($6000)
switches 4 and 8 should select BLK5 ($A000)

And the bank of 2 switches are probably write/enable for each of the 8k banks.

If you set switch 1 and 6 to the on position you should get 8k at address $2000 and 8k at $4000 = (16k + the internal 3.5k) or 19967 bytes.

[ral-clan]

Sorry to resurrect this old thread, but I've had the exact same cartridge for several years, and never have gotten it to do anything either.
I should add that the printing on the circuit board in the upper right actually identifies it as a RAM/ROM cartridge.

I get similar results in my random dip-switch settings as the original poster does. So it's less likely that we both have bad units.

[buzbard]

Here's small program that might help. ramcart.prg
Load it with:

LOAD"RAMCART.PRG",8,1
NEW

Then flip the switches one at a time and type:

SYS828

It will give a list of Blocks that contain RAM. Take note of what each switch reports.

You shouldn't have to reset each time you flip a switch.

The program just pokes a value of 255 into addresses $2000, $4000, $6000 and $a000 then waits a couple seconds then reads the values to see if they've changed.

[ral-clan]

Thanks! I'll give this a try sometime this week and report back.