Finalizing changes to IOB6120 board for parts substitution and correction of known defect

 PART SUBSTITUTION REQUIRES A MODIFICATION

I could not find the Flash ROM part that the board was designed to use, I had to improvise with one that was as close to a drop in substitute as I could find. The original ROM is a 512K byte device in a 48 pin TSSOP layout. The substitute I found is a 2M device also 48 pin TSSOP. 

Most importantly, all the signals routed to the original part have exact equivalents on the larger flash except for the two additional address bits needed due to the larger capacity. These added signals are on pads which were unused on the original part and thus unrouted on my PCB. 

Address bit A18 is on pin 16, in between a control signal and bit A17. If I make a solder bridge then this bit will match the value of A17 and will always be a valid logic state. Similarly, new address bit A19 is between bit A8 and a pad that will still be unused with this larger chip. A solder bridge here will make A19 match the value of A8. 

Additional address pins joined to adjacent pins

KNOWN DEFECT OF REV A BOARD DESIGN

The original design of this board was known to have very high drain on the two coin batteries that retain contents in the volatile devices specifically the four static RAM chips. The defect was said to be backflow of current from the protected chips and their battery provided power, through a TTL 3 to 8 demux chip which controls four chip select signals. The chip select from the SRAM and other devices burned power in the TTL chip even when its source of VCC was shut off. 

The fix that was implemented in rev B of the board is to interrupt the chip select traces, insert a diode to block back power flow, and to add a pull up resistor on the selected chip side. This can be retrofitted to rev A boards such as mine by carefully cutting the trace of the four chip select lines, soldering an extremely small SMD diode across the gap and then soldering a tiny SMD resistor somewhere on that line for the pullup. In most cases the other side of the pullup resistor needs a jumper wire to a nearby source of VCC, but in one of the cases a nearby trace is ideally situated to directly mount it. 

Very tiny diodes shown next to a pen tip

I completed the first diode-resistor pair, for the flash chip enable, then went to work on the underside where the fpga enable diode and resistor need to be installed. The space is so tight that each time I solder on the resistor, the diode comes loose, or vice versa, so the progress is slow indeed. 

Diode on the right, 0805 resistor on the left