Some of the parameters described above are password protected and thus require the
password to be entered at the Password register before they can be changed. The default
password is 0000. When the password has been entered it will timeout in one minute unless
the Password or Password Lock register is read to reset the timeout timer. Once the required
changes have been made to the protected parameters the password lock should be reapplied
by
a) allowing the password to timeout, or
b) writing any value to the Password Lock register, or
c) power cycling the instrument.
2 RS485 General Information
Some of the information in this section relates to other Eastron Digital meter product families,
and is included to assist where a mixed network is implemented.RS485 or EIA (Electronic
Industries Association) RS485 is a balanced line, half-duplex transmission system allowing
transmission distances of up to 1.2 km. The following table summarizes the RS-485 Standard:
PARAMETER
Mode of Operation
Differential
Number of Drivers and Receivers
32 Drivers, 32 Receivers
Maximum Cable Length
1200 m
Maximum Data Rate
10 M baud
Maximum Common Mode Voltage
12 V to
–7 V
Minimum Driver Output Levels (Loaded)
+/
– 1.5 V
Minimum Driver Output Levels (Unloaded)
+/
– 6 V
Drive Load
Minimum 60 ohms
Driver Output Short Circuit Current Limit
150 mA to Gnd,
250 mA to 12 V
250 mA to
–7 V
Minimum Receiver Input Resistance
12 kohms
Receiver Sensitivity
+/
– 200 mV
Further information relating to RS485 may be obtained from either the EIA or the various
RS485 device manufacturers, for example Texas Instruments or Maxim Semiconductors. This
list is not exhaustive.
2.1 Half Duplex
Half duplex is a system in which one or more transmitters (talkers) can communicate with one
or more receivers (listeners) with only one transmitter being active at any one time. For
example, a “conversation” is started by asking a question, the person who has asked the
question will then listen until he gets an answer or until he decides that the individual who was
asked the question is not going to reply.
In a 485 network the “master” will start the “conversation” with a “query” addressed to a
specific “slave”, the “master” will then listen for the “slave’s” response. If the “slave” does not
respond within a pre-defined period, (set by control software
in the “master”), the “master” will
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