88 Circuit Descriptions TM9100 Service Manual© Tait Electronics Limited August 2005Input Signals(e.g. AUX GPIO 4) An input signal applied to AUX GPIO4 is coupled via R757 to ITF AUX GPI 4 andfed to the digital section. As the input signal may exceed the maximumallowed by the digital section, it is clamped by D711 and a shunt regulator.The shunt regulator consists of Q708, R719 and R720 and begins to turnon at approximately 2.7V. In combination with D711, the input toITF AUX GPI 4 is therefore clamped to 3.3V nominally. The value of R757 ismade large to minimize the loading effect on the output pull-up resistors.Input Signals(AUX GPI 1 to AUX GPI 3) Dedicated inputs are provided on three pins of the auxiliary connector(AUX GPI1 to AUX GPI3). AUX GPI 1 is a general-purpose input with strongprotection of the same type used for AUX GPIO 4. AUX GPI 2 is normally adedicated emergency input but can be made a general-purpose input likeAUX GPI 1 by removing the link LK3 in the power supply area. AUX GPI 3 isnormally a dedicated ignition-sense input but can be made a general-purpose input like AUX GPI 1 by removing the link LK2 in the power supplyarea and fitting the 33kΩ resistor R775.ESD Protection On exposed inputs of the auxiliary and control-head connectors ESD(electrostatic discharge) protection is provided by a 470pF capacitor and byclamping diodes to ground and to 13.8V. For example, on AUX GPIO 4 thiswould consist of D713 and C725. The lines IOP GPIO1 to IOP GPIO 7 areintended for connection to internal digital devices and so these haverelatively light protection.HookswitchDetection Hookswitch detection is performed by Q700, R709, R706 and R712.When the resistance to ground on the PTT line is less than 13.2kΩ, Q700will turn on and drive the ITF CH HOOK line high; this indicates either that themicrophone is on hook or that the PTT (press-to-talk) switch is pressed.3.8 Digital BoardIntroduction For a block diagram of the digital board, refer to Figure 3.10.The digital board is not serviceable at level-2 and is not described in thismanual.
