REVISION B VERSUS REVISION A - BATTERY DRAIN
Apparently it was discovered after the first batch of IOB6120 boards were put into service that the drain on the batteries that provide nonvolatile memory protection was excessive. A change was introduced as revision B which introduced four diodes and pullup resistors that isolated the chip select lines from the TTL drive chips that fed them.
Instead of having the battery power feed back from the SRAM and Flash to the TTL output pin, a diode isolates the two and a pullup resistor ensures that the chip is not selected until the TTL chip is driving a low output during normal powered operation.
I SENT REV A TO THE FAB, NEED TO RETROFIT FIX
The version of the gerber files I transmitted to the factory were for the original revision A boards. The manual with the bill of materials does not list the diodes and resistors from the revision B, thus I never spotted the discrepancy.
After the problem was found, those with rev A boards wanted to retrofit a fix. The method involves breaking the trace on four select lines from the TTL chip to the various battery powered nonvolatile chips, bridging the broken trace with a small surface mount diode, then tacking on a teeny resistor whose other end is routed to VCC with a jumper wire.
This is done in four places, for the FPGA select, Flash select, cf select and status select circuits. The devices to place on the board are pretty small and this requires delicate work, which I will probably do prior to populating the PCB with the other components.
ORDERED RESISTORS AND DIODES
I placed an order with Digikey for 1mm long surface mount diodes, definite overkill with 65V 100 ma ratings. I chose 0603 sized resistors as a compromise between minimizing the risk it will overlap another trace and minimizing the risk that I can't place and solder it due to small size. I felt that 0603 was a reasonably large resistor that would not be too hard to locate somewhere free of interference with other traces.
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