Designing the E50 interface to the 1130

First step was to work out how to use the power supply to drive the 12V for the solenoids, which was a question mark because the power supply board had an IBM MST logic module, multiple mysteriously marked regulators and other circuitry on it, which might have required a specific handshake before delivering full power. Fortunately, I discovered that a simple bridge across two wires was enough to convince the PS to deliver full 5V and 12V power. I intend to use the 5V for the LEDs and other circuit boards that run at TTL levels, as well as the reed switch, photocell and photocell lamp inside the E50.

Having measured the resistance of the solenoid coils, I know what currents and power levels they will require and based on that, designed the driver circuit. The input is 3.3V logic levels from the FPGA, which will drive the solenoids from the 12V supply. I will prototype one of the driver circuits, verify operation and then have a board built for surface mount transistors and resistors to fit compactly into the printer enclosure.

Meanwhile, I used a lab power supply to validate the operation of the solenoids for the basic motor circuit. It was erratic turning on when the power switch was activated, caused by lubricating oil that got into the motor switch and partially burned a contact due to arcing. It was unreliable even after cleaning so I replaced it with a switch from the Selectric II typewriter I originally bought.

At this moment, the carriage is jammed at the left margin and the escapement lead screw is not turning, manually or under motor power. This also needs some investigation but the basic mechanism is clearly suitable for use as the 1130's console printer.

It was my belief that the signal listed as PUC was a switch reporting the shift status, but it is all a bit confusing, since the connector to the carriage where that signal is included does not +5V, the voltage level used with reed switches. The cable only carries ground, +12V and some control lines. My working hypothesis is that the machine does not discrete upshift and downshift solenoids, but further investigation is necessary before moving ahead on the design. The extended theory of operation manual for the E50 explicitly mentions a shift mode switch used for reporting the position of the mechanism. The diagram shows it swinging between 0V and 5V, but indicates the status at the halfway point of the PSCC rotation. Not sure if the switch maintains its 'value' when the shift movement is complete. Commanding the carrier to shift appears to require only the R2 solenoid and PSCC activation. I need to see if downshift is the same code or a different one.

Further web research suggests that the ribbon cartridge on the later Selectrics (and the Electronic xx models) did not support multiple colors, so that the shift to red and shift to black cannot be implemented. I will have those commands seem to work correctly, signalling completion at an appropriate time, to preserve programming compatibility.

I am still unable to identify the purpose of the RSH solenoid in the carriage assembly. It appears I will need to fire the solenoids to see if I can identify what each does. The tear in the ribbon cable affects only the velocity and correction solenoids. This is underway now.

A bit more testing has to be done and I must repair the ribbon cable, then I can begin testing candidate logic for my interface module.

My module has to support the tab set, tab clr, space, return and tab buttons on the console printer, as well as offering a way to set left and right margins. I am also working on the assumption that the printer operates in 10 character per inch mode, thus the scale across the front suggests a 13" wide printable line with one division of the scale covering a single character on the paper at 10 CPI.

The Electronic Model 50 is a wide platen machine, 19", which will cause difficulties fitting it inside an 1130 sized printer cover. I am not sure what I will do to address this particular issue; might have to tolerate some distortion from scale.

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