The 74HC240’s, U205 and U206, provide the output high drive
current necessary to maintain a TTL high output level (
≥ 2.4 Vdc)
under load.
At instrument turn-on, following a reset, and whenever the data lines
are being read, the MOSFETs are in the passive high state, and the
high output drivers are disabled. The resistor connected between the
MOSFET’s gate and ZGND holds the gate near ground potential
when the module is initially turned-on to ensure that the MOSFET is
in the passive high state.
The comparators U301, U302, U401 and U402 maintain correct
TTL high and low levels by shifting the voltages from the input to
compensate for the forward voltage drop of the blocking diode.
A reference voltage of +2.1 Vdc (TTL_REF) is applied to the inverting
input of the comparator. When the input voltage is in the range of
0 Vdc to +4.3 Vdc the blocking diode is forward biased and its forward
voltage drop is added to the applied voltage. For example, when 0 Vdc
is applied to the data line, +0.7 Vdc is present on the non-inverting
input of the comparator and the comparator output is low. When the
input signal level is above 1.4 Vdc, a voltage greater then +2.1 Vdc is
applied to the non-inverting input of the comparator causing its output
to go high. When the input signal is less than 1.4 Vdc, a voltage less
than 2.1 Vdc is applied to the comparator’s non-inverting input
causing its output to go low. This ensures an input voltage < 1.4 Vdc
is interpreted as a TTL low level and an input > 1.4 Vdc is
interpreted as a TTL high level.
The pull-up resistor (connected to the comparator’s non-inverting
input) allows external ground connections and open circuits to be
detected. When the data line is grounded, the blocking diode is
forward biased applying a +0.7 Vdc level to the comparator, a TTL
low. When the data line is allowed to float, the non-inverting input of
the comparator pulls up to +5 Vdc, a TTL high.
The blocking diode on the output is used for circuit protection.
The diode reverse biases when the applied voltage exceeds +4.3 Vdc
preventing externally supplied current from being injected into the
module’s +5V supply line.
The MOSFETs have a built in zener diode that conducts at any voltage
of approximately 75 Vdc or greater. The zener diode provides protection
from external over voltage situations including static electricity.
Chapter 5 Theory of Operation
Multifunction Module
156
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