I'll look again at the addresses and try to find some way to provide a drawing or schematic and code (after work tonight). I was hoping it was a simple oversight someone might recognize without, but something more is needed; these are things I should have had ready or posted with my OP.

I tried slowing the I2C to no avail, but I will try again; no need for particularly fast communications anyway. Both addresses are grounded to 000. Power supply is bench type, regulated, well filtered, and stable. In fact, I have another, smaller, PS to dedicate to this project - same issue with it as well.

I noticed this morning that the clock initializes correctly and displays the correct time during the first main loop, then fails on subsequent reads. I know I'm writing to the RTC1307, because the time will display properly if the port expander is unplugged - reading seems to be the issue. The subroutines used are verified - read/ write bit set as it should be - and tested from other projects (and previous versions of this code).

All good suggestions and just having your interest is motivating... Let me do some more work, double check everything suggested so far and get back to you with some additional information.

Thanks again...