Operator Manual
Page
23
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40
P/N 1580281
SEC3120 Digital Transmitter
Sensor Electronics Corporation
Rev 10, 20150707
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8.7.
ModBus Settings
The SEC3120 Digital Transmitter is capable of speaking to various devices that communicate using
standard 16-bit Modbus and every version of the SEC3500 HMI using its various proprietary protocols.
(For more details, see the SEC3120 16-bit Modbus Technical Sheet, SEC P/N 1580282) Based on the
Modbus register address and function code combinations provided in queries to the Transmitter by a
Modbus Master, it is able to determine whether it is communicating with a standard Modbus device or an
SEC3500 HMI and further distinguish which HMI protocol is appropriate. The SEC3120 communicates over
RS485 as a Modbus RTU Slave (compatible with Modicon Modbus Specification PI-MBUS-300 Rev. J),
and does not perform any bus-management functions.
Configuring SEC3120 RS485 Bus Parameters
Since the SEC3120 implements Modbus RTU over RS485, bus communication parameters can be
changed to accommodate various line configurations for other Modbus compliant Masters (compatible with
Modicon Modbus Specification PI-MBUS-300 Rev. J). SEC3500 HMI Panels communicate using SEC
standard RS485 (or default) configuration parameters, as shown below, since they provide the most reliable
compromise for speed, distance and error tolerance:
Baud Rate:
9600
Parity:
None
Stop Bits:
2
Data Bits:
8
The default parameters (485 Bus Settings Menu Item SEC-DEFAULT) shown above are mandatory for the
SEC3500 and are the recommended parameters. However, a different Modbus compliant Master used in
place of the SEC3500, such as a Modbus Ethernet Gateway, may choose to use a different configuration.
The Most common alternative configuration is as follows (485 Bus Settings Menu Item SEC-ALTERNATE):
Baud Rate:
9600
Parity:
None
Stop Bits:
1
Data Bits:
8
The operator may choose to completely configure the communication configuration by setting custom
settings (485 Bus Settings Menu Item SEC-CUSTOM):
Baud Rates:
1200, 2400, 4800, 9600, 19200, 38400
Parity:
None, Odd, or Even
Stop Bits:
1 or 2
Data Bits:
8 or 9
Though the parameters are customizable, the total frame of bits cannot exceed ten (not including the start
bit), hence 8 data bits with 2 stop bits will be valid, but parity cannot be used. Likewise, 8 data bits with
parity is valid with only 1 stop bit. Nine data bits can only be valid with one stop bit and no parity. Though
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