THVD4431A provides complete path diagnostic loopback modes for both RS-232 and RS-485. These modes internally short bus outputs to bus inputs. So, if data is toggled from logic input, data reaches bus and is reflected back on logic buffer output. This enables MCU to detect bus side short (due to connector/cable) by comparing logic input and logic output.

In RS-232 hardware loopback mode, L3 reflects on R3/R2/L2; L4 reflects on R4/R5/R8/R1/L5/L8/L1; L6 reflects on R6/R7/L7 enabling to detect short to ground on all bus pins from R1 through R8. RS-232 loopback mode is optimized for -40°C to 85°C ambient temperature. RS-232 diagnostic loopback can be performed on a node (DUT1) even with another node (DUT2) connected via cable, but listening node (DUT2) is not allowed to transmit anything on the RS232 lines while loopback check by DUT1 is ongoing.

In RS-485 loopback mode, internally R2 gets connected to R3 and R1 gets connected to R4. L3 input data is routed to driver output terminals and receiver input terminals and reflects on L2. DIR and TERM are not supported in this mode. Specific to this mode, external termination on R3/R4 is not supported, but R1/R2 can have internal or external termination. This is able to detect short between R1/R2 and between R3/R4 bus terminals. Recommended maximum data rate for RS-485 diagnostic loopback to detect cable or connector shorts is 500kbps with SLR = V_IO.