Here is an explanation of half duplex vs full duplex serial port operation. It is really quite simple. With half duplex operation, only one side of the serial link can transmit data at a time. When a serial port is receiving data, it will not allow the transmitter to transmit. It can only transmit when no data is coning in to the receiver. This mode allows the ECU and a piece of test gear to use one wire to carry information back and forth between each other. Generally, this requires one device to be the master and the other to be the slave, and for the slave to speak only after the master tells it to.

In general, PC serial ports are set up to run full duplex, which means that they can't use the same wire for bi-directional communication. It assumes that the PC's transmitter is hooked up to the ECU's receiver and that the PC's receiver is hooked up to the ECU's trnamitter, and the two are not hooked together. If the PC's transmitter is connected to it's own receiver and the ECU's transmitter, it gets very confused at times. When the PC transmits a command to the ECU, if J12 is jumpered the PC receiver will receive the command as a byte of data coming back to it at virually the same time. The logging software may assume the command is the returned result, and get very confused.

This is why you need to cut J12 in order to do datalogging with a PC.

-- markolson - 24 Oct 2005