Some calculators from the late 1960s have a connector for an external device. The external device may be a service unit or an external programmer (see Rick Bensene's page about the Sharp Compet 22 for an example of such a programmer). In either case the connector may sometimes be used to control and monitor aspects of the operation of the calculator.
In the absence of the original device it is possible to construct a new 'Controller/Monitor Unit' to plug into the connector of some of these calculators. I have constructed Controller/Monitor Units for the following machines:
The schematics of the CMUs are contained in the schematic of the appropriate calculator.
The Sharp and Facit machines actually share some design lineage as Sharp was an OEM for Facit, as indicated on Rick's page about the Sharp EL-8. The CMUs for these machines are very similar and provide similar capabilities: basically, the ability to 'single-step' through the execution of functions. For example, during a multiplication each shift of a register and each of the many additions can be stepped through one at a time, with the calculator displaying the intermediate results.
For the Sharp and Facit machines the CMU can be as simple as a switch or two and an LED with resistor. A mate for the old, somewhat unusual, multi-pin connectors may be difficult to come by (I had a couple from another dismantled piece of equipment) but it should be possible to make a connector from some suitable gauge wire or pins from some other connector and some insulated base material such as printed circuit board.
Based on the similarity to the above calculators I would anticipate that CMUs could also be constructed for the following calculators:
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