18F4550 + SPI + RS232

[rwskinner]

I hadn't done this before and I'd like to know if there are any problems with the SPI and Hardware RS232 sharing the RX line (RC7 is RX and SDO), especially since I will be using both. RS232 will be a Modbus Slave and I don't need things causing problems.

Anything to look out for? Suggestions, tips....

Thanks,
Richard

[PCM programmer]

Presumably your Modbus Rx line is coming out of a MAX232-type chip,
so the Rx pin on the PIC is being driven. You can't just disable the UART
in the PIC and enable the hardware SPI, because then you would have
the SDO pin fighting the MAX232 signal. Now, you could get a MAX242
or similar chip, that has tri-statable Rx outputs. Then you could only
enable that output when the PIC's hardware UART is enabled (and the
SPI module is disabled). This would require an extra i/o pin on the PIC
to control the tri-state control input on the MAX242.

[rwskinner]

Thanks, I was afraid of that. I'll use SW SPI on a different set of pins then.