The master is an 18F4550 which will do most of the grunt work and talks to the PC host via USB (we use Oshonsoft's excellent USB/HID library for our USB work).
The slave is a 16F877A (no reason for this particular model other than we had quite a few of these lying around). This basic idea is that the 16F reads 28 input pins and writes a four-byte value out to the serial port.
The master periodically polls the slave, reading the serial data. It looks for two specific bytes which signify the start of the message, then reads in the next four bytes that make up the message (the fifth byte is the XOR sum of the previous four bytes so we have some sort of checksum).
The problem is that while we can check the 16F using the PICKit2 UART tool - and it successfully reads the message being "broadcast" by the slave - when we try to read data using the hardware UART port on the 18F master chip, the USB comms lock up and our PC app stops working.
It seems that we aren't the first people to run into this sort of problem and there are lots of queries across the 'net asking for help with USB and serial comms together. Here's just one example:
So it looks like USB and UART together isn't going to work.
Which is a bit of a bummer.
Also, like the author of the article above, we were also hoping to use the timer1 interrupt to keep a simple clock running, which makes software-based bit-banging and no-go either. Getting a PIC to work as an I2C slave looks quite complicated too, so we're running out of ideas.
The only thing left for us to try is getting two PICs talking to either other using SPI hardware on both the master and slave chips.
Hopefully this will work with both USB and timer1 interrupts running on the master chip. But until we try, we're not going to know......