How to identify an unknown protocol with a logic analyzer?
I doubt it's plain SPI, though it's bidirectional with a clock. The way I see it:
GP1 is some kind of clock, that seems obvious.
GP2 acts as a kind of chip select, but not quite. When it goes low, GP1 either has 1 or 2 low-high-low transitions before GP2 goes high again. Question is what the second transition indicates because it has a meaning for sure.
After GP2 went high again, GP2 has one low-high-low transition where GP0 is always low. Then follow two more transitions but for the first one, GP0 is always high. For the second transition, GP0 is mostly high but sometimes low.
I'd guess GP0 is MOSI and that the first two clock cycles after GP2 went high are always 0 and 1, followed either by a read/write indicator flag (with read=high which could explain why it occurs more often) or maybe a few slave address bits.
GP5 looks like MISO but sometimes, it is low for extended periods of time after a packet was transmitted. This could be a &quot;busy&quot; indication mechanism, telling the master to not send anything while GP5 is low.
Unfortunately, you posted nowhere near enough about this device to make an educated guess about anything else, really.
What are brand and model?
What do the devices look like?
How many sensors can be attached to the master?
What do those sensors measure and how?
What information does the master show about the attached sensor(s)?
Since this is a completely custom protocol, maybe reading out the PIC's flash and reverse engineering it is an easier route to go, depending on your skill set.