256KB Memory Card IBM

ibm-cga-card-fixedI have an IBM 5150 which I restored. It had a bad CGA card and thanks to Ebay I could fix it 🙂

Nice isn’t it?





Now, in that machine there’s a card: 256KB Memory Card and when you turn on the IBM it counts to 256KB. That’s the onboard memory. I was wondering why doesnt’t it count to 256, since 256 + 256 = 512KB.

There are 8 dip switches on that card. On this site you will find this information:

“Switches 1 to 4 control the starting address of the RAM.

Starting address
64 KB
1=on, 2=on, 3=on, 4=off
128 KB
1=on, 2=on, 3=off, 4=on
192 KB
1=on, 2=on, 3=off, 4=off
256 KB
1=on, 2=off, 3=on, 4=on
320 KB
1=on, 2=off, 3=on, 4=off
384 KB
1=on, 2=off, 3=off, 4=on
448 KB
1=on, 2=off, 3=off, 4=off
 Not applicable if more than 192 KB is enabled on the card  (because conventional RAM past 640K would be created).
512 KB
1=off, 2=on, 3=on, 4=on
 Not applicable if more than 128 KB is enabled on the card  (because conventional RAM past 640K would be created).
576 KB
1=off, 2=on, 3=on, 4=off
 Not applicable if more than 64 KB is enabled on the card  (because conventional RAM past 640K would be created).


And it also says: It is your choice as to whether you fit 1, 2, 3 or 4 modules, however you need to adjust switches 5/6/7/8 on the card per:

Module 0 only : 5=on, 6=off, 7=off, 8=off
Modules 0/1 : 5=off, 6=on, 7=off, 8=off
Modules 0/1/2 : 5=off, 6=off, 7=on, 8=off
Modules 0/1/2/3 : 5=off, 6=off, 7=off, 8=on

Now, I tested a lot of these settings. The standard setting that it had was:
1011 0001 So this means: starting address 256KB and use all the four banks, makes sense but no effect whatsoever on the machine.

I kept that last 4 switches to 0001 (use al banks) and tried all kinds of settings on the first 4 dip switches:

0000 no image on the screen!
0001 no image on the screen!
0010 no image on the screen!
0011 no image on the screen!
0100 ERROE: “parity check 2 ?????”
0101 ERROE: “parity check 2 ?????”
0110 ERROE: “parity check 2 ?????”
0111 visual, no effect
1000 visual, no effect
1001 visual, no effect
1010 visual, no effect
1011 original setting, no effect
1100 visual, no effect
1101 visual, no effect
1110 visual, no effect
1111 visual, no effect

So, I’m no step further. If you have any ideas, please contact me 🙂


Commodore 128 repair resources

Commodore 128 with 1571 floppy drive

Power Supply Unit (PSU):

Looking at Commodore 128 case:

GROUND ——-4         3—– 9VAC
5———- 9VAC
+5VDC —–1       2—– NO CONNECTION

Or in mirror when looking at the plug of the PSU:

9VAC ——-3         4—– GROUND
5———- 9VAC
NO CONNECTION   —–2       1—– +5VDC

Checking if your PSU is working:
– measure on pin 1 and 4: DC (in dutch: gelijkstroom) 5V
– measure on pin 3 and 5: AC (in dutch: wisselspanning) 9V

Welkom op mijn blog / site!

Hoi, mijn naam is Angelo,

Ik ben een retro computer verzamelaar (van met name Commodore computers), ben eigenaar van TASK4 Studios en ben ook toetsenist in diverse bands.

Op deze pagina’s vind je vooral informatie over retro computers en gadgets. Voor muziek kun je meer vinden op Soundcloud en Forza Coverband. En de rest vind je op Facebook.