Tait TM8235, Руководство по обслуживанию

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TM8100/TM8200 Service Manual Circuit Descriptions 89
© Tait Electronics Limited June 2006
Input Signals
(e.g.
AUX GPIO
4 )
An input signal applied to
AUX GPIO
4 is coupled via R757 to
ITF AUX GPI
4 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 GPI
4
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 GPI
1
to
AUX GPI
3
)
Dedicated inputs are provided on three pins of the auxiliary connector
(
AUX GPI
1 to
AUX GPI
3 ).
AUX GPI
1 is a general-purpose input with strong
protection of the same type used for
AUX GPIO
4
.
AUX GPI
2
is normally a
dedicated emergency input but can be made a general-purpose input like
AUX GPI
1
by removing the link LK3 in the power supply area.
AUX GPI
3
is
normally 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 supply
area and fitting the 33k
Ω
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 GPIO
4
this
consists of D713 and C725. The lines
IOP GPIO
1 to
IOP GPIO
7 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 13.2k
Ω,
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
Different digital boards are used for the TM8200 and TM8100 radios.
For a block diagram of the digital board of the TM8200 radios, refer to
Figure 3.12 . For a block diagram of the digital board of the TM8100 radios,
refer to Figure 3.13 .
The digital board is not serviceable at level-2 and is not described in this
manual.