This input will shut the transmitter down if the heat sink temperature exceeds a preset limit. In the event the thermal shut down occurs, a high=True logic level is routed to the Monitor via J1-11.The output from the control logic is routed to the PIN diode modululator driver input transistor base terminals (transistors Q10 CSB and Q2 SBO) and essentially shunts the AM loop amplifier output to ground,which removes the RF drive from the linear amplifiers in both the CSB and SBO channels.