It's O.K. to suggest other solutions for the discussed problem, but we should be nevertheless able to tell under which conditions the circuit in post #5 works.
- Expectable MAX232 propagation delay (transmitter and receiver) is about 2.5 us, so the additional round trip delay (SCK to SDI) is about 5 us. Standard SPI setting tolerates up to 1/(2*fclk) delay, so SPI clock frequency must be clearly below 100 kHz to work with the MAX232 chain. By setting the receiver to late sampling, frequency can be increased to nearly 200 kHz. By changing to fast RS422/485 drivers/receivers, higher clock frequencies up to several MHz are feasible.
- Usually nCS is required for SPI synchronization, unless the slave has other synchronization means. nCS is apparently missing.