It is not as simple as you think. I have seen all sorts of combinations of defects have an effect on the TIU function. In the end the only thing that matters is can the TIU transmit and RECEIVE the signal back. I have seen the TIU transmit the initial watch dog pulse, but not the longer sustained one once the TIU finishes starting up and flash the LED ID #. I have seen a single transmit chip effect other channels, I have seen partial receive chip function, I have seen receiver chip effect Transmit output, and the combination goes on and on.
The MTH guidance in ASC manual was before an actual device to look at output. Techs did not have oscilloscope (except John:-)). So it was a logic chart. Early models more susceptible to receiver failure. Rev L more susceptible to Transmitter failures. BUT I do see both on Rev L and sometimes together.
The simplest is when one channel does not work but all the others do. The worst is none work and LED barely lights or the current draw is high just TIU. This means a short that is effecting the Power Supply. So you can pull all the transmit chips and if still present pull the receiver. If still shorting change the 5V reg, if still shorted, look for round object with lid and deposit TIU in the object. 
