New, Desktop Model PDP-8 Clone!

This is a SBC6120-based PDP8/e clone complete with paper tape reader. It runs on two wall warts!

The 6120 was modified slightly, to handle even parity and also has two RS-232 inputs that are OR'ed together (the terminal and PTR go into the terminal port). As long as you don't press any keys while reading a tape, this works well. Additionally, a READ_TAPE signal, RS-232 level, is brought out from one of the diagnostic LED's. A small firmware change allows FOCAL to be read this way automatically. It isn't, however, a true "reader run" signal, and is not paced with the 6120's ability to receive characters. For this reason, I don't think FOCAL programs will load. BASIC programs, however, work fine. A 2.5" slim-profile laptop drive holds 8 OS/8 partitions. I have some 16M IDE flash drives, but I like the sound of a disk!

The front panel is just "blinking LED's", and is derived from the data/address I/O connector running to some HC373's; wire wrap and ribbon cable construction was used.

The entire unit is powered by a nice lightweight switching wall wart.

The paper tape reader is based on a mechanism from a Remex Director. An SX-28 microcontroller handles serialization (300,8,e,1 or 1200,8,n,1, rear switch selectable). It also drives the steppers. The unit will only read a maximum of 60 CPS, however, because it is powered by an 18 volt surplus laptop power supply. The mechanism originally ran at 24 volts, so it isn't as powerful or capable of operating as fast as it could at full power. Nonetheless, it works flawlessly at these speeds. I didn't want it to run faster than I could handle the paper, anyway! The unit was built with wire wrap techniques.

The 1950's era radio meters display +5 and +18 voltages; the illuminated front panel switches include a manual read override, and forward/backward controls. They were difficult to install but worth it.

Because of an error in selecting some resistor values, the unit will only read dark paper tape or Mylar; flimsy yellow tape won't work. Fortunately, I have lots of DEC reel and folded tape.

I have another pair of these nearly identical units near completion. I also have a third reader not much bigger than the mechanism itself; hopefully, this one will read cheap paper tape OK with more careful resistor selection.