Replacing CPU board

Besides the small differences between a 16500A and 16500B model, the 16500A has two 720k floppy drives and no slot for an IDE drive or a LAN card, the CPU slot is compatible with a 16500B CPU board, which is a good starting point converting an old A-model to a little faster B-model, including an onboard CompactFlash card on a small CF-IDE adapter directly plugged into the IDE connector.

What isn’t directly compatible after the CPU board upgrade are the floppy drives (16500B only supports one 1.44M drive) and the monitor connector. It might look complicated when looking at the 40 pin monitor connector on the A-model CPU board. But believe me, only 5 signals are used:

GND -  2    1 - GND
GND -  4    3 - V Sync
GND -  6    5 - GND
GND -  8    7 - H Sync
GND - 10    9 - GND
GND - 12   11 - GND
GND - 14   13 - GND
GND - 16   15 - GND
GND - 18   17 - GND
GND - 20   19 - GND
GND - 22   21 - Red
GND - 24   23 - GND
GND - 26   25 - GND
GND - 28   27 - GND
GND - 30   29 - Green
GND - 32   31 - GND
GND - 34   33 - GND
GND - 36   35 - GND
GND - 38   37 - Blue
GND - 40   39 - GND

This is the pinout of the B-model monitor connector:

GND - 1    2 - H Sync
GND - 3    4 - V Sync
GND - 5    6 - Blue
GND - 7    8 - Green
GND - 9   10 - Red

Below a picture of the experimental hookup between B-model CPU board and monitor connector:

As the 16500A frame has no space for a harddrive, well, there is if you remove the back floppy drive, a more convenient way is to use a direct pluggable CF-IDE adapter, not only that the system would boot up faster, but also since compatible 3.5″ harddrives are hard to get these days.

This CF-IDE adapters are available from pcengines.ch and fit directly onto the CPU board:

As the IDE port has no +5V power line, we need to add two wires, one on the bottom of the CPU board, and one on the CF adapter. As pin number 20 is the key pin, or even sometimes used for supplying +5V, this is the way to go:

And here finally the CF adapter directly plugged onto the B-model CPU board: