Hi Steve,

what about using CAN drivers ?
The PCA82C251 is good for up to 1MBit. Also it is protected against up to +/-40V and still working on a ground difference of up to +/-5V.
But SPI is not preferable, since you must connect 3 signals. So you need 3 driver circuits on every board.
Better, you use half duplex UART or CAN.
Also other Philips CAN drivers have fault detection and can be switched from differential to single wire on lower speed, if one wire was broken.


Peter