Denial

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:

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.

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.

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.

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