DEBUG INFORMATION FOR IOB6120
The SBC6120 computer illuminates three LEDs to give a POST (Power On Self Test) code that indicates where it might have stopped and from that the cause of any problems. With the IOB6120 (IOB) plugged in, it stops with code 3 showing. The manuals for the computer list that as reserved for future use, but after I dug through the firmware of the system I discovered what this phase is attempting.
The firmware reads from a high memory address that is mapped to the flash ROM on the IOB board. If the firmware sees a specific header value and verifies the checksum of the remaining contents matches the expected value, it then activates the hooks that put the IOB into the system.
Since it is stuck in that phase, I am assuming that the memory access is stalling somehow. It gives me something to look for as soon as I have the logic analyzer connected - memory address matching the value from the firmware and no contention from other parts of the IOB.
CONNECTIONS FOR LOGIC ANALYZER REMAIN ELUSIVE
The length of signal travel between the FP6120 front panel and the IOB can't be very long. The original testing of the IOB was apparently directly connected to the SBC6120 without the front panel. This further shortens signal lines such that I might have a timing issue with any sort of access to the expansion slot signals.
What I need does not seem to be manufactured. I need an IDC 50 pin (25 x 2) female socket with long leads, such as those used with a wire wrap system, so that I can use this as an extender between the IOB male pins and the front panel female socket, but have room to hang logic analyzer clips between the boards.
POTENTIAL DESIGN ISSUE FOR THE IOB6120 WHEN HOOKED TO THE FRONT PANEL
On some online post about the IOB6120 there is a mention of an issue using the IOB with the front panel, such that modifications were made to the IOB to fix the issue. These are not documented. The post states that anyone with a board having the 74ACT345 chip, matching the design, needs to contact the designer of the IOB for advice on modifications. Boards with a different version of the 345 chip are okay.
I of course contacted the designer through is web site but have heard nothing at all back from him. Perhaps I will be discovering the issue myself with the debugging I intend to accomplish and thus can develop my own parallel correction to fix the problem.
I thought I might just plug the IOB and SBC6120 boards directly together as a way of testing this theory. If the system completes power up and doesn't hang at POST code 3, then my board is good but the design itself needs a fix. If the same failure occurs, then the problem is defnitely on my board.
TESTING FLASH CHIP LOAD APPARATUS BUT LOAD STILL NOT WORKING PROPERLY
I tried some alternate methods of programming (writing) and reading the flash memory, but rather than making things better, they got worse. I am concerned that I don't understand why the issues are happening, thus it would require dumb luck to get everything working properly.
I was seeing the first byte duplicated as the second byte, with all the data shifted downward one position. I wasn't sure if this was a reading defect or a defect while writing. I put in the code to detect the rising edge of the RD/BY# signal but ran into the logic stalling partway through writing a block of 256 bytes. I went back to simply verifying that the ready signal is high before issuing a new command.
I added some delays of 50 to 100 microseconds, enormous by the standards of the chip I am controlling. The read appeared to work better, with the duplicated data going away. I extended this to the writing process, but then saw that the second half of the block of 256 was untouched at xFF and a few bytes in the first 128 were bad as well. Increasing the delays increased the failure rate, with delays on the order of milliseconds giving me only one byte properly written.
I will pore over the data sheet, look at other manufacturers sheets for the same 29F400 technology, and hopefully figure out the issue. That, plus more experimentation.
RUBBER SLEEVE GLUEDOWN COMPLETE
I completed gluing down all the flaps of the heavy rubber sleeve today. I will do some trimming of the rubber on the outside then assemble the connector onto the chassis. When that is done I can move forward with the checkout of my expander box for the IBM 1130 system.