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Preliminary MPM-103 Found!

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  by Bill Degnan - 10/16/2018 20:28
RCA COSMAC Microkit Backplane Module Assignments
Five years searching and finally I found a copy of the Operator's Manual for the Microkit. Pictured is the module assignment chart, vital for operation of the Microkit. I don't believe the person who sold this to me had any idea where to put the cards, he simply inserted them any old way. Fortunately no power cord was included, I assume power was not applied since the original owner used it. Glad I was patient to verify their correct locations. Click image for larger view.


RCA COSMAC Microkit
The complete set of Microkit modules, installed. Finally. Click image for larger view.


RCA COSMAC Microkit
Here is a view from the top, note the card guide numbers. The card guides are completely intact, they have not become brittle like those of the Microkit's contemporary the MITS Altair 8800. The Microkit was an expensive well engineered device that seems to have held up nicely. Click image for larger view.


RCA COSMAC Microkit
Here is a close up of the modules. The far left module (Terminal Interface GHA01) cable attaches to a Teletype and the other cabled module (Clock Control GGB) connects to the front panel. The 5V signal is pulled through the backplane through the clock control cable, otherwise the front panel has no source of power as designed. Click image for larger view.


RCA COSMAC Microkit
Another key page from the manual, the TTY pin assignments. Click image for larger view.


A more detailed checkout to follow...

Card by card I applied power and was able to make it through the general troubleshooting steps without issues. When I apply power to the unit the red Power LED lights and the remaining lights stay dark. Depressing the RUN UTILITY button causes the RUN indicator to light momentarily. If the system is attached to a terminal (in my case a Teletype), depressing the RUN UTILITY button would also cause the system to bootstrap. Striking either the CR or LF key, assuming everything is ok, will cause the system to send an asterisk character to the printer. From the asterisk prompt one can enter U2 monitor commands.

NEXT - Attach to a Teletype
On the Microkit the pins are:
4 (grey) keyboard in (TTY TS 7)
8 (black) keyboard out (TTY TS 6)
3 (orange) printer in (TTY TS 4)
7 (white) printer out. (TTY TS 3)

RESULTS - Current loop is established and the system resets properly. When I run the RUN UTILITY and strike LF or CR on the Teletype nothing happens. The keystroke is not frozen however, it appears to be transmitting, but it appears that nothing is being sent.

NEXT: Put an oscilloscope on the Terminal module, see what's happening.

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