IBM 1410 FPGA: Console Output

Getting console output was going to require writing some VHDL of my own – I don’t have an actual IBM I/O Selectric available! I started out by reading through the material in the IBM 1415 Console CE Instruction manual (S223-2648) to design some finite state machines (FSM) to stand in for the console cams and feedback contacts.

The I/O Selectric uses cam contacts to control the timing of things like when the type ball tilt and rotate has completed, when shifts have completed, when spaces and backspaces have completed and when a carriage return has completed. It uses additional feedback contacts to indicate the parity of the received character, whether the keyboard is locked and the current shift state (upper or lower case).

To save time, particularly during simulation, I sped those timings up by a factor of 100. Mostly this just involved the console implementation module itself, via a VHDL generic parameter, but I also had to adjust the single shot on ALD 45.50.01.1 as well.

The Selectric also uses a set of solenoids to initiate actions such as printing a character, locking/unlocking the keyboard, spacing and backspacing, carriage return and shifting from/to upper and lower case.

After writing some simple state machines for the cam timing, I enabled the normal stop print-out in the CPU via a test bench signal, and then used simple VHDL with “wait” statements to check for the outputs and provide the input signals required by the CPU in order to test my understanding under simulation.

The first thing I came across was a latch on page 45.50.15.1 that did not have a reset. In real world hardware this can work because (unless it enters a meta-stable state on power up) it will be in one state or the other. However, under simulation that does not work – it ends up as an undefined signal – so I added a Program Reset signal to the latch at location 4A on that page to compensate.

The next thing I saw in the traces – which I hadn’t expected, but probably should have – was activation of the carriage return solenoid so I had to add the FSM for that. (Note: The I/O Selectric implements a carriage return by returning the type ball to the home position and initiating a paper (line) feed operation).

This was then followed by an “S” character – and the fun began. The interface feedback signals from the Selectric are full of names that end NC or NO (normally closed, normally open). Now one might expect that these refer to the state of those signals at idle, but the cam contacts aside, that quickly got pretty confusing – more so because most of these signals (except those that drive the solenoids) are active low. In addition, the keyboard lock signal is -W on the CPU diagrams, but +W on the 1415 Console Printer contact diagram page 40.30.01.0, and a couple of the feedback signals from the Selectric to the CPU are marked as -B level on the ALD page — but are actually -V level, as can be determined from page 40.30.01.1 as well.

Eventually I came to the realization that the best way of handling the NC vs. NO +/- confusion was to more or less ignore it, and instead use the signal name and level to determine what state a given signal ought to be in at a given time. That helped a lot. Once I got ordinary characters working right, I proceeded to code the state machines for space/backspace, and shifts.

Shifts were a little problematic at first, because the CPU drops the solenoid drive the instant it sees the feedback signal from the Selectric – which didn’t give me a clock cycle to save the new shift state.

Once I got that working under simulation, I turned my attention to getting that I/O from the Selectric to a PC for display. A few months back I had looked into using a MicroBlaze soft CPU to do that, but looking at how my Digilent Nexys 4 USB to PC connection works, I discovered I can run successfully at speeds of at least 115,200 bits/second, which should be plenty enough to run a simple flag bit/type protocol for not only console output, but also switch inputs, printer output, card input, tape I/O and disk I/O – perhaps adding a silo or two for lower priority devices. This is a huge simplification over what I had been thinking would be necessary.

But at this point there was also a little bit of extra work required. initially I had written the VHDL to use character output for the UART, but I subsequently discovered that VHDL doesn’t have a consistently synthesizable way to convert from character to std_logic_vector, so now that type-ball characters are specified as std_logic_vector bit strings, instead of ASCII characters – which is probably better in the long run, anyway. Mostly I am using the same encoding I used from the IBM 1410 simulator software, which in turn was derived from Joseph Newcomers IBM 1401 emulation software.

Once I had tested the UART VHDL, found at Nandland, I put together a test to combine that with the CPU and generate a bitstream for the FPGA. I had two small problems in the process – trivial ones, really. The first was that I keep forgetting to wire signal “+P SPECIAL +12V POWER FOR OSC” (which then feeds “+P SPECIAL +12V FOR REL DRIVERS”) to logic ONE. The second issue was that while most of the buttons on the Digilent Nexys4 are active high, the Soft CPU reset button – which I feed to the 1410 CPU Computer Reset signal is active LOW – wasted an entire day on that one. 8(.

One I did that, I obtained the following result: A printout of:

S 00009 00007 00007 .c … _ _

This is exactly the expected result (one which I had already observed in simulation). The “c” is a 1410 blank (different from the console space operation), and will print as a “b” using a PC-side application. Also, the three periods after that, which are the contents of the A Data Register, B Channel and Assembly Channel, have wordmarks – which you cannot see in the PuTTY output because they get backspaced over. Finally, the last two characters, the channel unit select and unit number registers for Channel 1 are wiped out by the backspace/underscore combination that occurs because they seem to be uninitialized, and therefore have bad parity.

May be an image of text that says 'COM10-PuTTY'
IBM 1410 CPU Stop Printout


Next on the list will be cleaning up the latches in the VHDL code which are currently tracking upper/lower case, tilt, rotate, space/backspace, and the character to be sent to the UART, and maybe a couple of more, as well as working on the PC side console application – starting with designing the protocol I will use for PC / IBM 1410 communication.

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