The SCP 200B as it was called, had a clean design that was actually taped by a
The board consisted of an Intel 8086 CPU chip with the electrical and
mechanical hardware to interface it to the S-100 bus. It could
be used to upgrade the performance of current S-100 systems by swapping CPU
cards or it could form the foundation of a new 16-bit computer.
How the board handled some of the S-100 signals was as follows:
S-100 Pin 24, The Master Clock. All timing signals were derived
from an on-board 24 MHz oscillator. The clock rate of the 8086 chip itself
was either 4 or 8 MHz. A lower-priced version of the board was capable of 4
MHz operation only, while the higher-priced version has switch selection of
either a 4 or 8 MHz clock. All 4 MHz-only CPU cards were factory upgradable
to 4/8 MHz. Interestingly, when the board first came out, the fastest
generally available version of the 8086 Intel offered was 6MHz.
The S-100 bus Pin 49, is always 2 MHz. This was generated on the board.
While pSTVAL* ( pin 25) was equal to the clock frequency of the CPU (4 or 8
MHz). pSTVAL* and the master clock were equal in phase, but
pSTVAL* may be configured to be disabled (tristate) by CDSB* (pin 19) while
of course the master clock was never disabled.
The 8086 allowed 3 clock cycles for memory access (minus delays and setup
time). At 8 MHz, 250 ns memory is required while at 4 MHz, over 500 ns
access time is allowed. If more time is required, the card could be
switched to insert a wait state in every bus cycle, SCB used the IEEE-696
pin 98 (ERROR) to indicate the clock speed t -- high for 8 MHz, low for 4
MHz. This may be used to control wait state generators of peripherals.
Clearly not not standard!
The 8086 CPU chip is a 16-bit processor which was interfaced to the S-100 bus
by the support logic in either of two modes: 8-bit mode or 16-bit mode. The
mode was selected dynamically at the beginning of each bus cycle, depending
on CPU status and bus response, as follows:-
If the processor wished to read or write 16 bits, sXTRQ* (pin 58) on the bus
was driven low. The addressed device could then respond by pulling low
SIXTN* (pin 60) if it could perform a 16-bit parallel transfer. Timing for
SIXTN* was the same as the READY lines (RDY/XKDY) -- it had to be valid 50 ns
before the rising edge of the master clock that ends pSYNC. An exception to
this was if a wait state was requested (externally or on-board), SIXIN*
could be delayed until near the end of the last wait state. SIXTN* was
latched on the rising edge of the master clock, either near the end of pSYNC
or near the end of the last wait state, any changes after this time were
If, at the time SIXTN* was latched, it was high (inactive), then sXTRQ* was
removed and a double 8-bit mode transfer was selected. Otherwise, sXTRQ* was
kept low and 16-bit mode was selected.
Should the 8086 CPU card be used in a system which previously defined a
conflicting use for bus pins 58 or 60, either or both of these lines could
be disconnected from CPU logic. In this case, 16-bit mode was not possible
but the board could at least in theory work in 8 bit mode.
All 24 IEEE-696 S-100 address lines were driven by the CPU card; however
since the 8086 chip itself generates only 20 bits of address, A20-A23 were
always driven low.
Traditionally, S-100 memory cards have decoded only the lowest 16 address
lines, limiting their address space to 64K bytes. Ordinarily, use of such
cards in a system with more than 64K was not possible because these cards
will appear in the same relative position in each 64K block. However,
special provision was made on the 8086 CPU card, selected by a jumper:
Whenever a memory location ABOVE the lowest 64K is addressed, PHANTOM* (pin
67) was driven low, which may be used to disable memories with a 16-bit only
address. Thus these memories appear only in the lowest 64K, when PHANTOM* is
not being driven. This Phantom driving circuitry on the CPU card
continued to function during DMA operations. Thus DMA controllers that use a
24-bit address did not have to be concerned with the presence of memories
using only a 16-bit address. This was a neat trick and very useful to
people that already have an investment in S-100 memory boards.
The proposed IEEE-696 standard allowed and the 8086 CPU chip supported a
16-bit I/O address. However, traditionally the lower 8 bits and upper 8 bits
of the address have been the same, and unfortunately many devices have been
designed which decode their I/O address from the upper 8 lines. To maintain
compatibility with such devices the support logic on the board could be
jumpered to throw away the high 8 bits of the 8086's I/O address and
substitute the low 8 bits to allow these old boards to work. Clearly
not a lasting situation as IO space on the S-100 bus was becoming crowed.
Soon after Intel introduced the 8086 they introduced "co-processors" to run
with the chip. There were two, an 8087 math coprocessor and a less common
8089 I/O co-processor. For scientific and statistical type
applications the 8087 was quite useful in speeding up program run time.
Later SCP and many other manufactures had a spare 40 pin socket beside the
8086 for co-processors on their CPU boards.
SCP in fact had a small daughter board you could add to the above 200B board
to accommodate a co-processor. Such a setup is shown below.
The very detailed and thorough manual for the SCP 200B board can be obtained
SCP also supplied a "Zapple" like ROM monitor program to used with the
The manual for this monitor can be obtained
Bill Machrone wrote up a brief review of the board in 1981 for Microsystems
(Vol 2, #4, p22) it can be seen