IBM 1410 FPGA: “Space Cadet”

No, not that kind of cadet. Unlike the IBM 1620, the IBM 1410 does have adder circuits for doing addition (and multiplication and division as well). Instead, this issue had to do with the IBM 1415 space function.

I had thought that the principle instruction diagnostic, CU01, was working perfectly, but it was not quite so. I had noticed that even after finishing the diagnostic, I kept getting space characters sent to the PC support program, but figured that was just a software bug somewhere. However, when I started to run IBM 1410 1401 compatibility diagnostic M011, it stopped typing its instructional message to switch to 1401 mode on the console when it hit the first space character.

After some sleuthing, I discovered that the console typewriter logic never asked the channel for the character after the space – it just kept sending that space to the console typewriter again and again and again.

As one might expect, the issue turned out to be in the console typewriter Selectric emulation VHDL code. I had miss-interpreted the signal -V CON PRINTER SPACE NO to just involve console input via the space bar. a closer look at the related automated logic diagrams (ALDs) made me realize that in fact this was also the Selectric’s way of telling the 1411 that the space process had been completed, and that it really had nothing to do with input from the space bar.

At first I tried to just use the relevant states of the finite state machine (FSM) that controls spacing. That did not work right, however, because that same FSM also controls back-spacing. Qualifying the logic signal to only be active during a space, but not backspace, operation filled the bill.

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