Scanning Electron Microscope reverse engineering- 8741A programmer/reader

The AMRAY 1845 FE EM we got is proving to be an interesting beast.  I has its own internal signaling network called NibbleNet.  This seems to be some variant on GPIB, but the hardware is slightly different.  We decided the best way to figure out how NibbleNet worked was to dump the program out of one of the NibbleNet control processors.  Since there are two, interchangeable instances of this IC in the microscope we think that they are generic in some way and can be helpful in figuring out the signaling protocol.  Once we’ve got an idea of the protocol we can build a sniffer and start logging data from the machine.


The NibbleNet processors this SEM is an 8741A, and at first it seemed like we would just be able to throw it into one of our many chip readers and read/program it but that was not the case.  The MCS-51 line of processor (the infamous 8051 line) has wide support to this day in all sorts of modern variants and vintage applications.  This is not one of that series.  The precursor to the MCS-51 is, of course, the MCS-48.  the one we have here is the 8741A (EPROM version of 8041) with 1K × 8 EPROM, 64 × 8 RAM.  Now before we get to building a reader, we should have a way to know if it read correctly.  To me, that means writing to a different processor and seeing if the board operates correctly.  We were unable to get an 8741a for this purpose, but we got an 8742 which is the double size EPROM version so it should be pretty compatible.


I started development using the datasheet and a small stack of 8742 chips, which worked out better than you might think.  Using this for a base I built a fairly complete hardware setup that was capable of sending all the right voltages to the right pins.  The code was written from scratch, because after being unable to get the other guy’s code to do a write/verify I didn’t trust it.  Following the datasheet for the 8742 wasn’t hard and eventually I got it to the point where I was confident in the programmer.  At this point we still only have two 8741a chips, both presumably the same and containing code we can’t get back from anywhere else if we accidentally blow it away.  Turns out the 8741a is slightly different in several important ways.


The first thing I wanted to do to ensure the integrity of the data was to install a hardware program/verify switch.  This physically interrupted the programming voltage from being sent to the chip so any rouge arduino couldn’t accidentally overwrite all or part of our precious chips.  I had only added the write functionality so I could validate the integrity of the tester.  The second thing that had to be done is add another switch that selected between the 8741a and the 8742 voltage on the EA pin.  On the 8742 it’s 18v, which is not the highest voltage in the system so I’m creating it with a linear regulator.  On the 8741a it’s 23v which is the highest voltage in the system so it’s coming straight off the input pins (and why I have the board labeled very carefully.  This also means that when going to the 8741a you have to run the whole board at two volts higher than when you run an 8742.


With EA sorted out it took Frank to determine that you have to bring Test0 low to reading the 8741a.  I didn’t get that from the datasheets, you can see that’s not the behavior on the 8742 and it’s not called out on the 8741a. He got it from the 8748 datasheet and it worked beautifully.  We now have the NibbleNet program memory dumped, but there’s an issue.  When we load this program into the top half of memory of the 8742, it doesn’t run correctly.  That being said, my validation of running correctly is that an LED is solid and not blinking so I’m not 100% sure here.  When we put the code into both pages of memory it seemed to work, but it may just be doing enough to keep the watchdog on the other (I’m assuming 80186) processor happy.


The pictures are here and all the rest of the documentation for this project is here.

2 Responses to “Scanning Electron Microscope reverse engineering- 8741A programmer/reader”

  1. Terrence McGuckin Says:

    Do you have a pdf copy of the schematics? I have an old 1850 FE and I am having trouble reading the poorly scanned copy I have.

    • abzman2000 Says:

      Here’s what we’ve got. If you know anything else about working one of these, we’re very interested:

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