System Monitor Option: This is the base of the PLUS features. No
other PLUS options are possible without this. When fabricated, the
MEM8PLUS board is actually two boards that are initially attached to
each other. First is the 10” by 6” S-100 Main Board. (Yes, 6 x 10 is
one inch higher than the standard, but it is still within the
IEEE-696 specification, and the extra inch provides space for some
front panel components that need to be accessible when the board is
plugged in.) This contains all the components to operate the MEM8
part of the design, plus many of the parts of the PLUS section.
Then, on top of the Main board is a 10” by 4” board called the
Display Board. The two may be used as-is, or, for certain physical
configurations, like bringing the displays on the display board to a
front panel, they may be snapped apart and then connected together
with two inexpensive 40-pin ribbon cables. The display board, in its
minimum configuration as a systems monitor board, has 16 LEDs or
four hexadecimal displays to show the contents of the Address bus, 8
LEDs or two hexadecimal displays to show the contents of the Data
Input bus, 8 LEDs or two hexadecimal displays to show the contents
of the Data Output bus, and 15 LEDs to show the important status and
control signals on the bus. A switch on the board turns the displays
ON or OFF.
Extended Address System Monitor Option: This adds 8 LEDs or two
hexadecimal displays to show the contents of the extended address
bus, A16 to A23. This option would be useful when the 24-bit memory
option (described above) is selected, or the Memory Management
option is installed.
Logic Probe Option: A simple TTL logic probe is provided. It reads
only TTL levels and displays on a single LED.
Front-panel Operations Option: This option extends the System
Monitor Option to a fully-functional front panel. All these features
are enabled by this option:
Hardware CPU Stop and Start. A switch, when set to STOP
will stop the running CPU on the next machine (M) cycle.
When set to RUN, the CPU will start running again at
A momentary pushbutton switch will step the CPU one M
cycle when the CPU is stopped.
Single-Step by M1 or any M cycle switch. When this
switch is set to M1 stepping mode, and the Step switch
is pressed, the CPU will be permitted to run until the
next M1 cycle where it will stop. In the any M cycle
stepping mode, the CPU will stop in the next M cycle,
regardless of status. This can be used to view non-M1
memory read, memory write, Input and Output port
operations and Interrupt Acknowledge cycles.
Full or variable speed slow-step selected by switch.
When set to full-speed, the CPU runs at full clock
speed. When set to variable speed, the CPU will
automatically step at a user-selectable rate from as low
as about one cycle every 5 seconds, up to several
hundred cycles per second.
Hardware 16 bit address breakpoint with enable/disable.
A switch controls this feature. When set to disable, no
hardware breakpoints are triggered. When set to enabled,
the hardware will stop the CPU when the address set in
the breakpoint switches is encountered. This feature
will break regardless of the location of the code: any
on-board RAM or ROM. There are several sub-options:
24-bit address breakpoint is a build option.
When installed, a jumper permits selection of
either 16-bit or 24-bit operation.
Three switch options are available during build
for setting the breakpoint address (one must be
Standard 8-position DIP switch.
Rotary hexadecimal switch. These make setting
addresses very easy. They come in active high
and active low versions. Either can be used.
Different pull-up/pull-down resistors are
required and two jumpers must change between the
two options. Details are in the schematics and
the construction notes.
External via headers. The user can use any
available or desirable SPST switches. They can
be wired in either active-high or active low
(but all must be the same), and like the rotary
hex switches, there are resistor and jumper
Range switches - these switches will enable
setting an address range for the breakpoint.
This is done by permitting each and every bit
(16 or 24) to be designated as “don’t care”. For
example, if a 16-bit breakpoint is set to F663,
and the two least significant bits are set to
“don’t care”, then a break will occur on F660,
F661, F662 or F663.
An 8-bit output “test” port that can be written by the
CPU and displayed on the data out bus display. It can be
addressed at any 8-bit I/O port. A switch is used to
select if the output port or the data out bus shows on
An 8-bit input “test” port that reads a set of 8-bit
data input switches on the display board. It uses the
same port address as the output test port. The data
input switches have the same dip-switch, hex rotary or
off-board options as the breakpoint address switches
Memory Examine, Memory Examine Next, Deposit and Deposit
Next pushbutton switches are provided to enable
low-level memory operations when the CPU is stopped.
These switches are available to the user when the Front
Panel enable switch is moved from Normal to Front Panel.
These functions operate directly with the on-board
memory. The breakpoint set switches double as memory
address switches for these functions, and data for the
deposit operations are taken from the 8-bit data input
switches described previously in the “test” input port
All front-panel control switches are self-contained and
easily accessible at the top edge of the main board
using miniature tactile push-button switches and
miniature slide switches. Optionally, they can be
duplicated on an external control pod or with switches
mounted on the front of an enclosure. A header is
provided for this possibility.
For 8080, 8085 and Z80 processors only, a program run
function is provided that can force the CPU to any
location in memory and have it begin execution there.
Existing switches are double-purposed to allow the
operator to set a 16-bit start address, and optionally,
a memory bank. Then the program can be started at that
address in stopped, slow-stepped or full-speed execution
modes. Along with the memory examine and deposit
functions, complete programs can be “toggled” into the
computer at any memory address and run from there, even
without a terminal of any kind or a monitor program ROM.