Working on diode-resistor modifications to IOB6120

TOTAL OF FOUR ENABLE LINES REQUIRE THE DIODE AND PULLUP 

The four chip enable lines that were thought to be draining power during battery only operation are:

•  fpga enable
• compact flash enable
• flash memory enable
• status output enable

I completed the modification to the flash enable line yesterday, with the pullup resistor bridged to a nearby VCC source thus not requiring a jumper wire. I had begun on the fpga enable signal, which is done underneath the board where the traces are long enough to allow for the mod. As I previously documented, the diode and resistor were so close to each other that I kept unsoldering one as I reattached the other.

GREAT LOCATION FOR DIODE AND PULLUP FOR STATUS ENABLE

Because my diodes are much smaller than the ones done with the original rework, I could fit them in little more than the width of a trace. Thus, for the status enable signal, I had a clean location for the diode. The remaining part, a pullup resistor, could be mounted elsewhere as long as one side contacted the signal line that went to the status circuit. 

Fortunately, the enable line for the status circuit went to pin 1 of the chip, with pin 16 being VCC. That naturally supported placing the resistor between those pins on the end of the chip. I did need a short jumper because the distance was longer than an 0805 resistor.

Pullup resistor for Status enable modification

GREAT LOCATION FOR DIODE AND PULLUP FOR COMPACT FLASH ENABLE

I also found a much better location for my rework than the spot chosen by the prior rework technicians. There is a line on the front of the board between the CF socket and the coin battery holders, which is a long stretch but also has a +5V line just above it. It was relatively easy, to the extent any of these ridiculously small component placements could ever be called easy, to solder these on.

CF Enable line diode and pullup

SLIGHT RELOCATION OF PULLUP FOR FPGA ENABLE LETS ME SOLDER BOTH PARTS

The problem I had with the pullup resistor, being so close to the diode location that when I soldered one of the parts the other became loose, was solvable by moving that resistor a bit. I pivoted it around so that it projected from the +5V trace in a direction away from the diode, allowing me the clearance to solder the diode and the resistor without any negative interactions. 

Pivot of pullup away from diode above

BOARD NOW READY FOR CONNECTION, BUT MOD NEEDED TO FP6120 PANEL

The board with its 50 pin connector is ready but the front panel PCB (the FP6120) does not have a header soldered onto it. At the time I built the front panel the IOB6120 appeared to be unobtainium and further I really didn't have a need for the additional peripherals it provided. In retrospect, anything to do with a replica of an old system is by definition something without a need, so the latter factor was irrelevant. 

Now that I built an IOB6120, I disassembled the FP6120 to have access to solder down the header, once the parts arrive in the mail. From there I can put together the entire stack of FP6120, SBC6120 and IOB6120. That will permit some live testing.

POTENTIAL ISSUES WITH MY IOB6120

Since I had to buy the SRAM and flash rom from ebay sellers, there is a more than negligible risk that one or both products are counterfeits. I will only learn that if the parts fail to operate when I begin testing. 

The second issue I may face is the substitution of the 2MB flash chip for the 512KB part for which the project was designed. The protocol to load the flash might be incompatible or require some tweaking, something that has occured to others who had to substitute different parts for the 512KB flash. I will know this early on, when I try to load the firmware for the board.