// send 00 to MAX3140 "READ DATA" return data tells why we got an interrupt
disabling the CS after the RX

if(HighByte & 0x80) // byte received load into queue (R bit = 1) Lowbyte = Data


if(HighByte & 0x40)// TX buffer empty (T bit = 1)

I think you are required to read the reason for the Interrupt.
I do not have a delay. You do have to wait for the char to leave either by time or interrupt.