Passport 2®/Passport 2 LT™ Service Manual 0070-10-0441 2 - 39
Theory of Operation Communication Isolation
2.11 Communication Isolation
The communication between the Front End Module and the host must be isolated to the same
degree as the power supply. The communications consist of an asynchronous bidirectional
serial data stream at 500K baud. Since these signals are all digital, the isolation is
performed with optocouplers. All these devices are a special type, which feature very high
isolation voltage.
Data is sent from the main part of the Passport 2 CPU to the Front End Module. It is
inverted twice by U100, and then buffered by Q2, so that it is capable of driving the LED
portion of optocoupler U231. The current applied to U231 is set by R47. Note that the
power for these devices is obtained from the non-isolated main +5V. The output side of U231
is connected to the isolated circuitry. Data at a +5.5V pullup level (IRXD) is obtained by the
use of pullup resistor R346. This data is connected to the UART receive data input of the
microcontroller, through a translation buffer.
The communication from the Front End Module to the CPU is very similar. The
microcontroller's UART transmit data output switches driver transistor Q204, which performs
an inversion while driving the LED portion of optocoupler U230, with the current being set by
R328. These components operate from the isolated +5.5V rails. Note that this supply is used
to power the LED, so that the current pulses do not create noise on the +3.3VD and +5V rails.
The output of the optocoupler is connected to the non-isolated circuits, where, with the
assistance of pullup resistor R48, a 5V CMOS level signal is obtained.
A transmit data enable signal is applied to Q206 so that it may drive the LED portion of
optocoupler U232, with the current being set by R354. The output of the optocoupler is
furnished with pullup resistor R58 to the non-isolated +5V supply, and drives the enable pin
of transceiver U28 through U100.
The data communication consists of brief data packets, which occur at low duty cycle.
Therefore, the communications lines spend much of the time in the idle state. Further, the
ETRIG signal is also active (only on) for very short periods. The ETRIG signal is applied
through a series resistor R864 to Q207 so that it may drive (on or off) the LED portion of
optocoupler U233, with the current being set by R65. The output of the optocoupler is
furnished with pullup resistor R72 to the non-isolated +5V supply, and directly drives two
inverters U100.
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