88 Circuit Descriptions TM9100 Service Manual
© Tait Electronics Limited August 2005
Input Signals
(e.g.
AUX GPIO4)
An input signal applied to AUX GPIO4 is coupled via R757 to ITF AUX GPI4 and
fed to the digital section. As the input signal may exceed the maximum
allowed 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 turn
on at approximately 2.7V. In combination with D711, the input to
ITF AUX GPI4 is therefore clamped to 3.3V nominally. The value of R757 is
made large to minimize the loading effect on the output pull-up resistors.
Input Signals
(AUX GPI1 to AUX GPI3)
Dedicated inputs are provided on three pins of the auxiliary connector
(
AUX GPI1 to AUX GPI3). AUX GPI1 is a general-purpose input with strong
protection of the same type used for
AUX GPIO4. AUX GPI2 is normally a
dedicated emergency input but can be made a general-purpose input like
AUX GPI1 by removing the link LK3 in the power supply area. AUX GPI3 is
normally a dedicated ignition-sense input but can be made a general-
purpose input like
AUX GPI1 by removing the link LK2 in the power supply
area 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 by
clamping diodes to ground and to 13.8V. For example, on
AUX GPIO4 this
would consist of D713 and C725. The lines
IOP GPIO1 to IOP GPIO7 are
intended for connection to internal digital devices and so these have
relatively light protection.
Hookswitch
Detection
Hookswitch detection is performed by Q700, R709, R706 and R712.
When the resistance to ground on the PTT line is less than 13.2kΩ, Q700
will turn on and drive the
ITF CH HOOK line high; this indicates either that the
microphone is on hook or that the PTT (press-to-talk) switch is pressed.
3.8 Digital Board
Introduction 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 this
manual.
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