(while the following discusses the IBM 5110 A1 board, much of the same applies also to the IBM 5100 A1 board)
What is the A1 Board
The “A1 Board” is the term used in the IBM reference manuals for the IBM 5100/5110 to refer to a backplane that interconnects all the processing boards.
The board consists of both wire traces and yellow patch wiring. The wire traces are “common” connections that apply to all configurations of the system. The yellow patch wirings are either post-board-production updates or set as-needed connections for the specific configuration of the system.
The “A1 board” is somewhat like a very early motherboard. It is an array of connected pins that allows the system cards to exchange signals. There are board traces, but also IBM used yellow patch cables between many of the pins. These are not expansion cards like in the modern sense, rather each card has a specific purpose and must be located in its assigned slot.
NOTE: The Controller/Processor is compatible between these two models, but the remaining cards likely are not.
Initially it looks like a mess. But there is an organization to the arrangement of the board. The pins of each card, along with the pins of each perimeter connector (Y1 – Y4 and Z1 – Z4) simply “pass-thru” from one side of the board to the other. This way, any pin of the system can be accessed while the A1 board is “closed” (folded in). Looking closely at the “pin side” of the A1 board, it is arranged as a grid as follows:
Each horizontal (Y and Z) and vertical (A-N) “group” consists of 24 pins (2 rows of 12). Some parts of the manual will refer to “slot A2” or “slot F4” which generally implies the one whole half of the card. Within each vertical group (A-N), the two rows of pins in that group are referenced by “DB” “JG” “PM” “US” (a sticker is placed on the power supply as a reminder to this arrangement). “DJPU” being the left side and “BGMS” being the right side.
Some of the hardware is compatible between the IBM 5100 and IBM 5110.
Notes near the front of the IBM 5110 MAP manual include the following information:
The Auxiliary Tape Unit Adapter Card is a card found inside the external tape unit, called the “5106.” Early editions of 5106 do not have these jumper settings, which means the 5106 tape unit is not always compatible with the 5110.
The Processor was called the “Controller” in the IBM 5100 manual. The notes below indicate that this processor could be made compatible with either system by the following jumpers:
Below is an example of two IBM 5110 configured Processor cards. The jumpers described above are on the right-side edge of the card.
Is the RWS (RAM) compatible between the 5100 and 5110? No.
The 5100 uses 8KB cards per “half-slot” and has four slots allocated for RWS. 8x8KB = 64KB.
The 5110 uses 16KB cards per “half-slot” and has two slots allocated for RWS. 4x16KB = 64KB.
Right away, we see the Storage Address, Card Select, and control lines are all different pins.