A MOnSter Mystery, Solved

10:01 pm MOnSter 6502

I brought up an additional MOnSter6502 board today. At first it failed my basic validation routines, tripping up on the LDA nn,Y instruction ($B9).

The bus diagnostic output showed that LDA nn,Y was trying to read nn+Y+1 instead of nn+Y (in my test, it accessed $0211 instead of $0210). The LEDs showed that the Y register contained the expected value, so I thought the ALU carry in signal may have been loading a ‘1’ instead of a ‘0’, thus causing the incremented value. I scoped it out and it was fine.

Then I noticed that LDA nn,X worked fine which is totally weird because those instructions are just about identical! So I physically inspected the Y register and saw this tomfoolery:
IMG_20160707_123730
Those transistors are in bit zero of the Y register. The one on the left prevents anything but a ‘1’ from being in the LSB. The one on the right was supposed to drive the LED on so that I could tell the bit was stuck, but it decided to cover for its dead buddy. I don’t even know how this happened. It could have been shipping damage, or maybe I fat fingered a screwdriver.

After fixing it, the board worked fine. Two dead transistors and a lying LED. Gotta love transistor level debugging.

2 Responses
  1. Richard :

    Date: July 19, 2016 @ 9:11 am

    Good find! Sometime, you just don’t know your own strength!

  2. Bill Seiler :

    Date: April 10, 2017 @ 7:12 pm

    I would love to get a Monster 6502 to give to Chuck Peddle on his 80th birthday!

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