PDP-8/sl Update

I added a small front panel to the unit, simply because I wanted some "blinking lights". The lamps represent diag 2,diag3/Paper tape read (see below),diag4, disk read, disk write, memaccess0, memaccess1, and battery backup fail. These lamps were mostly driven from HC04 gates.

Figure 1. PDP-8/sl at work.

I added a paper tape reader RS-232 input that shares the RS-232 port. This is OR'ed via an HC08 gate with regular console RS-232. As long as no one touches the terminal while reading a tape (and tape is not read during console input), this works well.

Figure 2. Paper Tape Reader Reading in FOCAL '69 (With Genuine PDP Tape, of Course)

The paper tape reader I wanted to use with this computer requires 24 volts DC, and I originally designed a small desktop power supply for it consisting of a 30 volt wall wart and a 323K that dropped this to 24 volts. I didn't like this klunky setup, so I took apart the wall wart and mounted the transformer in aluminum brackets (because the transformer had no mounting ears) inside the case. I then made a small filter/regulator board to make the drop to 24 volts. A switch with an LED indicator on the front panel powers the paper tape reader via an RCA jack on the back of the unit. This is very inefficient (never mind that the 24 is dropped to +5 within the paper tape reader with another LM323K), but it works, and I had this available.

The other switch, also indicated with a red LED, grounds a pull-up leading to an unused MAX-232 output driver that signals the paper tape reader to run. Figure three shows that it's getting a little crowded in there.

Figure 3. Inside the PDP-8/SL

I noted that BIN formatted tapes need to stop right on the trailer, and can't be allowed to continue (otherwise the reader sends NULLS to the computer). The reader is very fast (and my others are even faster). So, I modified the source code to light diagnostic lamp 3 when starting a paper tape read, and to turn it off when ^C or the trailer is reached. The inverted output of the HC175 is routed through a diode to the MAX-232 input. Therefore, the paper tape reader will turn on when the LB (load binary paper tape) command is sent, and will shut off when the trailer or ^C is reached. I shortened the time loop on the ^C somewhat. Naturally, it also briefly turns on when booting, but this is of no concern because the paper tape reader power is usually off.

The box is getting pretty jammed now, and I think I'm going to leave well enough alone. A full front panel for a PDP-8/e clone will just have to wait until I build another one! I have enough parts left to build two more.