The P188 was a board to put an 8088 processor on a standard S100 bus. The
board ran in 3 modes, allowing maximum flexibility in S100 systems of
different configurations. The board emulated all necessary S100 bus signals.
Numerous jumpers allow the configuration of the card to run different
operating modes, as well as with static and dynamic memory.
The 8088 processor was from four to six times more powerful than an 8080A.
Providing 5 MHz processing speed, with an external bus of 8 bits, it had a
16-bit internal architecture. Also, 8-bit and 16-bit signed and unsigned
arithmetic in binary or decimal, including multiply and divide. The 8088
addressed 1 megabyte of memory. The 8088 was an 8086 with an 8-bit external
data bus instead of a 16-bit bus. In applications that manipulate 8-bit
quantities extensively, or that are execution-bound, the 8088 can approach
to within 10% of the 8086's processing throughput. With pipelined
architecture, allowing instructions to be perfected during spare bus cycles,
and internal 16-bit data path, and a compact instruction format, the 8088
realizes its high performance.
The ACOM P188 board operated in 3 modes:
1. As a stand-alone processor on the S100 bus
2. As a slave processor to a Z80, 8080 or other S100 processors
3. In a multi-processor mode with one or more additional processors on the
As a slave processor to a second processor card on the bus, control can be
transferred via software commands. When the "transfer command line" on the
board is activated, the P188 will issue a HOLD command. Upon receipt of a
HOLD ACK from the master processor, the P188 will take over the bus and
start executing instructions at memory location FFFF0H. The P188 can
transfer control back to the master processor by releasing the transfer
control line. It is necessary for the transfer control line to be driven by
an I/O port external to the P188. It could also be controlled from an
external switch as well as an I/O port.
In systems where it was necessary to have 2 or more processors that can
become masters, the P188 could be jumped to allow all interface lines to be
removed from the bus. It was necessary for the other processors in the
system to also be able to remove all lines from the bus. This can be
accomplished on most existing processor boards at the time with a slight
modification. Thus, implementing a true multi-processor environment.
The bus interface of the P188 allowed for 48 mille-amp output current sink.
They performed well in large systems with or without bus terminators. Also,
all control inputs had an RC filter and Schmitt-trigger to improve noise
An LED indicator on the P188 card (top right) was on when it was in control
of the bus. Two prototype IC locations were provided on the card to help
implement special requirements. A 26'pin connector was provided on top of
the card to interface auxiliary control functions, such as the transfer
The manual for this board was very helpful in putting together a 8/16 bit
S-100 system and provided simple code to switch CPU's. It also describes how
to modify the board for dynamic RAM systems.
The manual and schematic for this board can be obtained