IBM 51xx Memory Expansion Card / IBM / 256KB

Introduction

This 256KB memory expansion card is from IBM and is named "256KB MEMORY CARD". This card supports a maximum of 256 KByte conventional memory and consists of a maximum of four "square" modules of 64 KByte each. It also appears on the Internet under the name: Short memory expansion card 64-256 KByte or something like that. eBay is offering an item for about $400 right now (Jan 2018). Is it really that rare or is the price exaggerated a little bit?

The IBM 256KB Memory Expansion Option is available for the IBM Personal Computer family of products. The system board must be populated to the maximum 256KB of RAM before the IBM 256KB Memory Expansion Option can be installed.
IBM Personal Computer: IBM 256KB Memory Expansion Option (6138355)

DIP switch settings

I use the expansion card with the following RAM configuration:

• IBM XT 5150 ......: 4 x 64 = 256 KB (starting address)
• 256KB MEMORY CARD : 4 x 64 = 256 KB
• -- DIP switch -------
• -- 1 2 3 4 5 6 7 8 --
• -- 1 0 1 1 0 0 0 1 -- // 0 = OFF, 1 = ON

Switches 1 to 4 control the starting address and switches 5 to 8 the four memory modules of the expansion card.

Switch 1 to 4

Perform the following simple calculation:

• Divide starting adress by 64
• Convert to binary
• Fill up with zeroes to 4 digits
• Invert it

• Example 256 KB : 64 >> 4 >> 100 >> 0100 >> 1011 >> ON OFF ON ON
1. ON
2. OFF
3. ON
4. ON

Switch 5 to 8

5. ON: Select 64 KB
6. ON: Select 128 KB
7. ON: Select 192 KB
8. ON: Select 256 KB

• Example 256 KB:
5. OFF
6. OFF
7. OFF
8. ON

Links

• Minus Zero Degrees: IBM 256KB Memory Expansion Option

Manuals

• IBM 256KB Memory Expansion Option (en)