Internal power / disable switch for the Axiom Parallax printer interface

The Axiom Parallax sidecar Centronics printer interface has only nine chips inside, and they’re all small LS TTL glue.

The console +5VDC supply should be able to handle the additional (minimal) current draw, especially if the console has an F18A and the now-useless 4116s were removed. One less wall wart to get in the way, plus the Axiom runs cooler because the linear regulator isn’t dumping ~+4VDC as waste heat.

So I tried it. It works. Here’s what you have to do:

Ensure that you’ve actually got + 5VDC on pin 1 of the expansion bus (lower left as you’re looking at it). If you have a speech synthesizer, you don’t, because TI didn’t run the line. If that’s the case, crack the synthesizer open and run a wire between pin 1 on the socket and pin 1 on the card edge. Be careful not to oversolder the card edge — solder it to the very edge and clean up with desoldering braid.

Disassemble the Axiom. There are a lot of screws and standoffs. Keep track what goes where .

Desolder and remove the 7805 voltage regulator. Note the orientation of the 7805 — pin 3 is the pin on the right-hand side as you’re facing the regulator, and that’s the one we care about. The board quality is just a tad above crappy; if I had to do it over again, I’d snip the leads and pull what’s left out of the hole rather than do a full desolder, because I lost the through-plating on the (now unused) voltage supply pin.

On the bottom of the board, solder a wire between pin 1 on the socket and the solder pad of where pin 3 of the 7805 used to be. The pad may not have been run through to the bottom of the board ; if so, poke the wire through the hole and solder to the pad on the top of the board.

Remove the old power jack, because you’ll need that hole for the disable switch.

There’s another modification that the Axiom should have if you’re using an UberGROM.

As documented, the UberGROM uses spacebar-at-powerup to bring up the recovery code.

Well, the Axiom does the same thing — it goes into diagnostic mode when spacebar is held at powerup, and keeps outputting chr(0x01) through chr(0xff) until you release it. And the UberGROM never sees the spacebar held down, so it never invokes the recovery code.

And if you’re like me, you probably have your sidecar connectors wired down with something like double-sided tape to keep them from shifting and crashing the console. Thus, unplugging the Axiom is a PITA when building up UberGROMs on real hardware.

Luckily, disabling the Axiom at boot-time is simple. Do this:

  • locate pins 20 (!CE) and 18 (!OE) on the ROM/EPROM,
  • disconnect them. There’s a trace on the bottom of the PCB that wires them together — take a razor knife and break that connection.
  • prepare a single-pole dual-throw switch (i.e., two positions, three wire lugs). Solder a 4.7k resistor and a length of wire to the leftmost lug (which we’ll call 1), and regular wire to the middle and right lug (which we’ll call 2 and 3).
  • Wrap electrical tape or heat shrink tubing around all of the switch solder joins (both sides of the resistor especially). You don’t want anything shorting out inside.
  • feed the three wires in through the top of the PCB. There are a bunch of viable holes: pick one that feels suitable.
  • Connect 1 to +5VDC. There’s a nice big lug to the left of pin 24 on the ROM/EPROM; that’s a safe place to hook up.
  • Connect 2 to 18 (!OE).
  • Connect 3 to 20 (!CE).
  • Test before reassembly. If it all works, reassemble. Put the switch where the power input used to go.

What you’re doing is forcing the EPROM output-enable signal to be logic-high all the time when the switch is engaged, essentially disabling the EPROM. When the switch is disengaged, !OE gets the same signal as !CE (the chip-select line) and the EPROM is enabled when the decode glue kicks in. This method takes advantage of the old ROMs having separate chip-enable and output-enable, and makes it easy to turn off the EPROM without messing with the chip select logic.

(Also of interest: the Axiom can use either a 2716 or a 2732. Pin 21 (A11 on 2732) is grounded on the PCB, so both will work. Just put the firmware in the bottom half of the 2732 and you’re good to go — no need to fill the ROM by double-copying.)

About Chris Kobayashi

I'm a security systems engineer, specializing in UNIX, network, and physical security. I'm in Tokyo, and I'm mostly retired now. I'm well-versed in both electrical and software engineering, with a particular interest in old computers and game consoles. You can contact me here.
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