Board Descriptions, Schematics, Drawings and Gerber Files
This section contains schematics, drawings and Gerber files of all the Mod 8 / Mod 80 boards including the back plane board. Unless otherwise indicated, the boards below are either mine. Boards from the York University Computer Museum's MIL Collection are indicated with (YUCoM) in the caption.
Mod 8-1 8008 CPU Board
This board contains the 8008 CPU, clock generators, state decoding and bus switching control logic
Mod 8-2 Restart TTY I/O Board
This board contains the teletype I/O, paper tape reader control and system restart logic.
The TTY board was modified by users to perform more than just communicate with the ASR 33.
Mod 8-3 8008 Buffer Board
This board contains address latching and the 8-bit bi-directional bus switches which communicates between the low power 8008 8-bit data lines and the 8-bit system bus (DB0 thru DB7).
Mod 8-5 RAM Board
This board holds sixteen 1Kx8 of 2102 type static RAM for a total of 2K of RAM. The RAM board select is performed by a one out of eight decoder (74LS138). This decoder decodes A10-A12. The sixteen RAMS are directly addressed by A0·A9.
Mod 8-7 Output Board
This board provides three 8-bit output channels. Up to 24 8-bit output ports can be
accommodated by the system. They are selected by a one out of eight decoder (74LS138) performed on A9-A11. A12 & A13 can furthermore have
3 states. These additional select signals are decoded and combined with the one out of eight select signals, to yield a total of 24 output port select combinations.
The board below is in the
Mod 8-9 RAM/ROM Board
The Mod 8-9 board is not mentioned in any of the MIL documentation for the Mod 8 or Mod 80.The Mod 8-9 board has a provision for a 2Kx8 ROM (8316) containing the complete Monitor 8 software, four 1702a EPROMs (1Kx8), and eight 256x4 of MIL MF2113-2P RAM (similar but not identical to Intel 2101 RAM).
Mod 80-3 Jumper Board
The slot previously occupied by the Mod 8-1 CPU board is now loaded with a simple jumper Mod 80 System Configuration board. In addition, the Mod 80-1 board goes in the Mod 8-3 slot. The function of this jumper board is to configure the back-plane to the 8080 CPU rather than the 8008 CPU.
The jumper board (also known as the NANO 80-J) was available from Moducomp for $5, however I suspect most people just made there own using the jumper settings shown on the bus diagram in the Mod 80 Supplement.
Mod 8 Cassette Interface Board
An audio cassette interface was designed by MIL, and offered for sale by Mini Micro Mart. An applications note was written by Robert Swartz which included assembly code to add cassette capability to the Monitor 8 software. This was included in the version of Monitor 8 on a 2k 8315 ROM that came with the Mod 8-9 board described above.
I have no pictures of the actual board, however I did draw up my own version using the schematics provided by Robert Swartz in his "Mod 8 Data Package".
Apparently there is a Mark-8 version as well.
If you have any photos of this, please let me know.
Jump on Reset Board
This card was made up based on an article that ran in Kilobaud in November 1980. The purpose of this circuit is to allow one to execute code located at an address other than 000 000 when the computer is reset. In this case, it is setup to execute code (i.e., Monitor-80) at octal address 340 000. This circuit was built on Mod 8-8XA Card (see next entry).
Mod 8-4 ROM Board
This board holds up to eight 2Kx8 of 1702a type EPROM. All eight EPROM's are addressed by A0-A7. Address lines A8-A10 are decoded by a one out eight decoder (74LS138).
The Celetron C-MOD 8-4 and the TSL MAC 88-104-10 are two variations of the Mod 8-4 ROM board.
Mod 8-6 Input Board
This board provides three 8-bit input ports. The input port select is decoded by a one out of eight decoder (74LS138) fed from A9-A11. This allows a system total of eight
possible 8-bit input ports.
Mod 8-8 Back Plane
The back plane designed to interconnect a full set of Mods 8 boards into a microcomputer configuration with PROM programming capability. The chassis has nine PC edge connectors and various receptacles. Four memory slots are allotted for Mod 8-4 & 8-5 boards. Two slots are provided to accept either Mod 8-6, 8-7 I/O boards. The remaining slots are specifically assigned to the Mod 8-1 8-2 and 8-3 boards. The back plane also includes a PROM programmer with 24 ZIF socket.
Mod 80-1 8080 CPU Board
This board contains the 8080 CPU, clock generators, state decoding and bus switching control logic. In a Mod 80 system, the buffer board is not used. Address latching is done by the 8080 CPU, and therefore, the MOD 80-1 board is able to perform the functions of both old Mod 8-1 and Mod 8-3 boards.
Mod 80 Power Supply
The unit was originally intended to be powered from external (bench) supplies through the Molex connector provided. The power requirements are as follows: +5V@3.5A, -9V@1.5A, +75V@750mA (20% duty cycle) & +12V@250mA (for 8080). The power supply below was designed by myself to fulfill those requirements.
Homemade Parallel & Serial Board
This is a custom made board that is in the York University Computer Museum MIL Collection. It is labeled P & S (Parallel and Serial). It appears to have one parallel output port, one parallel input port and one serial port. Likely this card was placed in slot 6 or 7 on the Mod 8-8 back plane. In all probability is is a standard circuit from a magazine or perhaps a Brother McGahee circuit.
Mod 8-8XA Extender Board
Mod 8-8XA extender board for the Mod8 / Mod 80. This one was modified to provide a 90-degree 44-pin edge connector. Pin 2 (WRITE) is connected to the switch. Not sure what this was used for, but I suspect it was for connecting the main board to a second backplane where additional memory could be added. Apparently the original owner had made up his one 19-inch backplane with a ridiculous number of slots just for that purpose. Space Circuits listed these boards as either Mod 8-A or Mod 8-AX.
Last updated on March 15, 2019.