Hi all,
I capture 2 waveform with a scope from 84A to 84A which work fine and 683 to 84A (no data) and Alain is correct, 683 waveform is shorter (Faster) then the 84A, after playing with the OSCTUNE, every 10 data send, only 1 data is show on the LCD. Also try change the Baud Rate from 813 to 1646 & 188, no data show.

Looks like device with internal clock is no good for sending data to a device with external clock