Tait TB7100, Руководство по обслуживанию

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58 Circuit Descriptions TB7100 Service Manual
© Tait Electronics Limited October 2005
4.7 Interface Circuitry
Introduction
For a block diagram of the interfaces circuitry, refer to Figure 4.7 .
For more on the connector pinouts, refer to “Connections” on page 63 .
Bi-directional Lines
Bi-directional lines are provided on four pins of the auxiliary connector,
(
AUX GPIO
4 to
AUX GPIO
7 ) one on the control-head connector (
CH GPIO
1 ), and
seven on the internal options connector (
IOP GPIO
1
to
IOP GPIO
7
). Those on
the auxiliary and control-head connectors are formed by combining two
uni-directional lines. For example, the line
AUX GPIO
4
at pin 10 of the
auxiliary connector is formed from
ITF AUX GPI
4 and
DIG AUX GPO
4 .
The circuitry is the same in all five cases and is explained below for the case
of
AUX GPIO
4 .
Output Signals
(e.g.
AUX GPIO
4
)
An output on the line
AUX GPIO
4
originates as the 3.3 V signal
DIG AUX GPO
4
from the digital section. The signal is first inverted by Q703 (pins 3 to 5) and
the output divided down to 1.6V by R748 and R753 to drive the base of
Q703 (pins 1, 2 and 6). When the latter’s collector current is low, the base
current is a maximum and creates a small voltage drop across R761, causing
the collector emitter to saturate. As the collector current increases, the base
current decreases proportionally until the voltage across R761 reaches 1 V.
At this point the base-emitter begins to turn off and the base current
diminishes rapidly. The net effect is a current-limiting action. The current
limit value is approximately 18mA (the inverse of the value of R761).
The output configuration is open-collector with a pull-up to 3.3 V by
default. Pull-up options to 5V and 13.8 V are also available. On
AUX GPIO
4
only, the optional MOSFET Q707, which has a high current drive, may be
fitted. If Q707 is fitted, R768 must be removed.
5-Volt Regulator
The 5 V supply mentioned above is provided by a simple buffered zener
regulator formed by Q702, D721, R721 and R722. The resistor R722
limits the current to about 25mA under short-circuit conditions.
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 2.7 V. In combination with D711, the input to
ITF AUX GPI
4
is therefore clamped to 3.3 V 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.