Endress+Hauser EngyVolt RV15, Руководство по эксплуатации

Страница: 43 / 64

EngyVolt RV15 Appendix
43
Password
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
• allowing the password to timeout, or
• writing any value to the Password Lock register, or
• power cycling the instrument.
11.3 RS485 general information
Some of the information in this section relates to other 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
200 m (3 900 ft). The following table summarises
the RS-485 Standard:
PARAMETER Value
Mode of Operation Differential
Number of Drivers and Receivers 32 Drivers, 32 Receivers
Maximum Cable Length 1 200 m (3 900 ft)
Maximum Data Rate 10 Mbaud
Maximum Common Mode Voltage 12 to –7 V
Minimum Driver Output Levels (Loaded) ±1.5 V
Minimum Driver Output Levels (Unloaded) ±6 V
Drive Load
Minimum 60
W
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 k
W
Receiver Sensitivity ±200 mV
Further information relating to RS485 may be obtained from either the EIA or the various RS485
device manufacturers.
11.3.1 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
abandon the “conversation”.
11.3.2 Connecting the instruments
Screened twisted pair cable should be used. For longer cable runs or noisier environments, use of a
cable specifically designed for RS485 may be necessary to achieve optimum performance. All “A”
terminals should be connected together using one conductor of the twisted pair cable, all “B”