TM8100/TM8200 Service Manual Circuit Descriptions 91
© Tait Electronics Limited November 2007
Bi-directional Lines
Bi-directional lines are provided on four pins of the auxiliary connector,
(
AUX GPIO4 to AUX GPIO7) one on the control-head connector (CH GPIO1), and
seven on the internal options connector (
IOP GPIO1 to IOP GPIO7). Those on
the auxiliary and control-head connectors are formed by combining two
uni-directional lines. For example, the line
AUX GPIO4 at pin 10 of the
auxiliary connector is formed from
ITF AUX GPI4 and DIG AUX GPO4.
The circuitry is the same in all five cases and is explained below for the case
of
AUX GPIO4.
Output Signals
(e.g. AUX GPIO4)
An output on the line AUX GPIO4 originates as the 3.3V signal DIG AUX GPO4
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 1V.
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 3.3V by
default. Pull-up options to 5V and 13.8V are also available. On
AUX GPIO4
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 5V 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 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.
Table 3.4 Connector power supply options
Link state Connector power state
LK5 LK6
LK7
(R787)
LK8
(R786)
Auxiliary
Internal
options
Control head Microphone
out out out out switched no power no power no power
in out in out switched switched switched switched
out in in out switched switched unswitched unswitched
in out out in switched unswitched switched switched
out in out in switched unswitched unswitched unswitched
in in/out in in unswitched unswitched unswitched unswitched
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