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Network Circuitry
TB9100 Reciter Service Manual
© Tait Electronics Limited January 2006
The RS-232 level signals, RS232_TXD and RS232_RXD, are converted
to LVTTL levels using dual RS-232 transceiver U600 (ADM3202).
This device incorporates an internal charge pump to generate the positive
and negative supply voltages required for RS-232 compatibility. Clamping
diodes D603 and D604 protect against excessive voltages for short periods;
prolonged application of voltages in excess of ±15V may blow fuse F601 or
resistor R620. EMC filtering components are provided on both RS-232
lines.
5.5.2
General-purpose Digital Inputs
Four general-purpose digital inputs, DIG0_IN through DIG3_IN, are also
provided for external status signals, or configuration and mode setting.
These are also connected through the DB9 connector, with their pin
allocations being such that they do not conflict with the operation of the
RS-232 interface. The description below covers only one input circuit, as
the others are identical
Considering input DIG0_IN, the input signal passes through EMC filtering
components E602 and C605, before passing through a noise filter, R637 and
C649. This filter, with a nominal time constant of 270us, provides some
degree of noise rejection, but this is generally not sufficient to debounce
switch or relay contact inputs, which require further software debouncing.
The filtered output passes through inverting Schmitt trigger U602, which
squares up the signal and converts it to the LVTTL levels required by the
MPC. U602 has LVTTL compatible input threshold levels, but can
withstand input voltages to +5.5V.
A switch, or relay contact to ground, may be used on the input as a suitable
pull-up resistor (R603), is provided on board. This pull-up resistor is
sourced from a +5V supply and supplies approximately 1mA to the switch
or relay contact. Generally, switches or relays with gold-plated contacts are
recommended for reliable operation with this relatively low current and
voltage.
Alternatively, the input can be directly connected to the outputs of most
logic families running from 3.3V supplies or above. The exceptions are low
voltage (<5V) CMOS logic that does not have 5V compatible outputs and
4000 series CMOS logic running from 5V or lower supplies, which does not
have enough drive capability to sink the pull-up current. In such cases, the
input pull-up resistor should be removed from circuit.
The input is protected for momentary application of ESD voltages or surge
voltages of up to ±50V, but prolonged application of excessive voltage may
fuse the pull-up resistor R630 or series resistor R637. The input voltage to
U602 is clamped by diode array D605 to ground or the +5V supply. In the
event that the input voltage is forced above +5V, excess current will flow
through resistor R630 and diode D605; Zener diode D610 diverts this
excess current to ground, maintaining the efficacy of the protection clamp.
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