LAUREL LTS, Owner'S Manual

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11. LTS COMMAND MODE WITH MODBUS RTU PROTOCOL
Laurel’s implementation of the Modbus RTU protocol allows the LTS to receive data in the
form of Modbus commands. As for the customer ASCII Protocol in Command Mode, these are
used to write values to a variable called “Item 3”, which will be used for analog output inter-
polations and for relay setpoint comparisons for relay action. Modbus commands are also
available to overwrite setup parameters entered with IS Software “on the fly,” including the
following:
1. Change scale and offset or coordinates of 2 points parameters to generate the analog
output derived from Item 3.
2. Change setpoints to control relay action derived from Item 3.
3. Force relay action regardless of Item 3
4. Reset latched alarms by forcing a Function Reset.
The Modbus RTU protocol for Counters, including model LTS, is described in the user manual
at
https://www.laurels.com/downloadfiles/serialcom2.pdf .
See our Modbus Manual for Counters at https://www.laurels.com/downloadfiles/Modbus-
Manual-CTR.pdf for a Modbus RTU overview and for all available Modbus commands, not just
those for model LTS. In particular, see the long section entitled, “FC03 (READ) & FC10 (WRITE)
HOLDING REGISTER ADDRESSES.” Use Hi Word starting Register Addresses and an even
number of Registers. The Register Addresses shown in this section apply to both FC03 and
FC10. They are for Modbus numbering systems that start at 00 01 (Base 1). Subtract 1 from
each Register Address for Modbus numbering systems that start at 00 00 (Base 0).
To write values to Item 3 with Base 1 addressing, write the Hi Word applied to Item 3 to hex
register address 006
006 006 006C C C C, and the Lo Word applied to Item3 to hex register address 006 006 006 006D D D D. With
Base 0 addressing, the Hi Word address becomes 006B
006B 006B
006B, and the Lo Word address becomes
006C
006C 006C
006C. The qModMaster example on the next page illustrates how to use Base 0 addressing to
generate 4, 12 and 20 mA analog outputs.
Example on how to send the value 999999 to Item #3 of an LTS using the Modbus protocol:
1 10 00 6B 00 02 04 00 0F 42 3F F4 87 1 10 00 6B 00 02 04 00 0F 42 3F F4 87 1 10 00 6B 00 02 04 00 0F 42 3F F4 87 1 10 00 6B 00 02 04 00 0F 42 3F F4 87
Address: 01 01 01 01
Function Code: FC10
FC10 FC10
FC10
Holding Register: 006C
006C 006C
006C
Number of Registers Sent: 0002 0002 0002 0002
Number of Bytes Sent: 04 04 04 04
Register Data: 000F 423F 000F 423F 000F 423F 000F 423F (999999 in decimal)
CRC: F487 F487 F487 F487