User's Guide ________________________________________________________
30 _____________________________________________________ M210283en-A
Communication Options
Communication to the outside world occurs via an RS-232
maintenance line or an RS-485 data line. The RS-485 bus is
electrically isolated with a transformer in the power supply
section and two optocouplers for the receiver and
transmitter sides. Both communication interfaces are
protected against ESD spikes and surge voltages.
Additional interfaces include the +12 V 10mA frequency
output for FD12P and MITRAS, and the I
2
C interface for
the external calibrator. The frequency output is generated
with a single comparator circuit and the I
2
C bus interface is
driven directly by the microcontroller.
Microcontroller
The microcontroller is a 16-bit Hitachi H8 board running
with a 16 MHz clock rate and an internal flash memory. The
microcontroller includes an external SRAM chip attached
for the data memory. The microcontroller also contains an
external watchdog circuit to generate a processor reset when
the board is powered or the watchdog has not been updated
during the time-out period. The board includes a small
amount of non-volatile EEPROM memory attached to the
I
2
C bus.
Input Protection
The LM21 serial interfaces are protected against
overvoltage with transildiodes and voltage dependent
resistors (VDR). The RS-485 communication interface is
also galvanically isolated. The calibrator interface is
protected against overvoltage and short-circuits with
transildiodes and a PTC resistor. The frequency output and
power supply input connections have short-circuit
protection with PTC resistors. The power supply
connections are all equipped with EMI filters.
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