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Working Around 5161 Expansion Unit Wait State - CONFIRMED


Experienced Member
Jul 25, 2009
Palm Beach, Florida
The expansion unit is such a cool device, but it comes at a price - an extra wait state...

I understand that decoupling the bus and re-creating it on an external device would have a penalty, but why should the on-board (i.e. system unit in IBM speak) have to suffer as well? I would love to put my "fast" (video, SCSI, etc) devices in the system unit, and my "slow" (floppy, serial, parallel, etc) devices in the expansion unit. The clue was in the extender card's DIP switches. They are to set the amount of memory in the system unit (so that it doesn't have to incur the wait state penalty). The documentation shows up to 640k, but in researching the memory map (http://www-01.ibm.com/support/docview.wss?uid=pos1R1000255&aid=1) I had an idea! What if I followed the same Boolean logic and set the memory range to include the video/VGA range (792k)? So I set it to 832k (the next highest 64k block), and voila! my system board VGA runs at full speed!

Below is a cheat sheet for these extended switch settings:

704k off, on, off, off
768k off, off, on, on (Full video range, but not video bios)
832k off, off, on, off (Full video range, vga bios, and some adapter bios range)
896k off, off, off, on

Just be careful not to turn off the wait state for any device that actually is in the expansion unit - no idea what would happen???