I originally posted here; then, thinking the issue was solved, moved on. Unfortunately, fate is not so kind and again I face the issue...

When the I2C Clock is connected to my 16F887A it works perfectly. When the port expander, PCF8574, occupies the bus, it too performs as expected... Either, individually, no issue. Together, the clock displays 04:04, 04/04/04... disconnect the PCF and presto... the clock displays correct time and date. Originally, I attributed this to power issues and - temporarily - resolved it by removing the clock backup battery but, as I've begun to experiment with a 3RD I2C on the buss - the problem has recurred.

The bus wires are no more than 12 inches and 24 gauge wire soldered (neat, clean, no residue) to perf board of Radio Shack quality. I've tried I2C resistors from 1K8 to 10K and it makes no difference. I've checked and rechecked everything I can think of and moved between clock modules (2 suppliers) and a socketed chip without effect. The problem is consistent... 04:04 - not random at all - regardless...

I thought perhaps there was some interaction - that perhaps I bunged the address or something; not so. The expander counts merrily from 0 to 256... the clock remains stubbornly 04:04.

Similar issues anyone? Thoughts? Advice? Anything...? If you think it might be code, I'll post it, but I doubt that's an issue as both devices work independently - without reprogramming...