Diagnosis of the 1802 "Membership Card"

Last updated April 18 2013. Edited by Herb Johnson, (c) Herb Johnson, except for content written by Lee Hart and others.

An owner writes: "I powered up my 1802 Membership Card and all the LED's came on, but it was not responsive to the switches. I disconnected it and found it was sitting on an anti-static bag. I took it off, reconnected power, now it does not work."

Lee Hart responds:

Ouch! Was it a conductive antistatic bag (like those metallic-looking ones that have an aluminum or nickel coating? The common mostly-clear pink or blue or green greasy-feeling polyethylene bags are nonconductive, but won't generate a static charge. The opaque black carbon bags you mentioned are slightly conductive. The dark metallic bags that you can barely see through are nickel coated, and moderately conductive. The opaque silvery bags are aluminized and very conductive. These look like the material potato chips and other snacks come in. You can actually make membrane keyboards with these. :-)

What were you using for a power supply? If it was current limited, the good news is that it's not likely to have damaged anything important.

The Membership Card (and 1802 circuits in general) are fairly easy to troubleshoot. You can slow the clock down, or even stop it completely at any point, and probe things with a multimeter, or even a "logic probe" that is nothing but an LED and series resistor.

A useful logic probe can be built with 3 LEDs and a resistor. Let's say you're running it on a 5v power supply. Connect 1 yellow and two green LEDs all in series (green on the positive end). When connected across the 5v supply, they have a voltage drop of 1.7v + 1.75v + 1.75v = 5.2v, so they will be off (or perhaps just barely glowing). Now connect one end of a 1k to 10k resistor to the junction of the yellow and green LEDs. The other end of this resistor is your logic probe. Connect a piece of wire to it with an alligator clip, or better yet an EZ-hook clip.

 +5v__________
             _|_
  green LED _\_/_  lights if logic
              |    probe is low
             _|_
  green LED _\_/_
              |____/\/\____\ logic probe
             _|_    1k     / alligator or EZ-hook clip
 yellow LED _\_/_
              |    lights if logic
 GND__________|    probe is high

- When this probe is open, the LEDs are off.
- When this probe is low (grounded), the green LEDs light. (Green = Ground)
- When this probe is high (VCC), the yellow LED lights.

Most problems in things you just built are bad connections. A pin will be open (floating) when it should be high or low. Or, you will have two outputs shorted together, producing a faulty voltage somewhere between high and low. This probe can find them.

D11 to the power connection, is a protection diode; it fails if you connect the supply backwards (and it is not fuses or current limited), or if the supply voltage is too high.

Have you checked the supply voltage? Is it between 3v and 5v?

Check to see if the oscillator is running. U4D pin 12 should be high, and U1 pin 39 should be an AC voltage (as indicated on your multimeter.

If the clock is running, then put the 1802 in RUN mode, and see if you have an AC voltage on the various 1802 outputs; TPA, TPB, SC0, and MRD. All of them should have an AC voltage (i.e. are cycling high/low) even if the program that is running is utter nonsense.

The LEDs may be stuck because of some problem in the output port. If you ignore the 8 LEDs, and toggle in a program to blink Q, does the Q LED then go on and off?

Lee Hart

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