Even though there is (obviously) separate parallel/serial shift registers inside the chip for each channel, I am assuming that the parallel data bus enters the chip through only one set of pins - it is entirely possible that the input gate, or some internal buffer common to both channels has failed. An easy test would be to trigger on chip select, scope the affected data bit on the bus inputs, and write patterns to the chip with the affected bit in both hign and low positions - it should be very easy to determine if the data bit is making it to the chip or not. If yes, replace the device, if no, move to the next upstream component and repeat the test (all the while keeping triggering on the device chip select). It should be a very easy matter to identify the defective device in this fashion - no need to remove anything other than whatever device is defective. Even if you don't have an extender board, it is far easier to tack a lead on to points you want to test than to be removing and replacing entire devices in the hope of being lucky. -- Just MHO. Regards, Dave -- dave04a (at) Dave Dunfield dunfield (dot) Firmware development services & tools: www.dunfield.com com Collector of vintage computing equipment: http://www.parse.com/~ddunfield/museum/index.html