Automationdirect.com Stellar SR44, User Manual

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Chapter 6: Serial Communications
6.5.3 – Writing Single Register (Modbus Function 06)
This function is used to set the value of a single parameter. It differs from the Modbus protocol in
that only the data low byte of the query is written to the parameter address. The data high byte is
ignored. (The normal response for correct operation is an echo of the data.)
If the PLC uses Function 06 to write one word of data into the SR44, only the low byte of the PLC
data is used. The PLC low byte is written into the SR44 parameter, and the PLC high byte is
ignored.
Example – Modbus Function 06:
PLC writes “Enable” to SR44 (send a value of 2 to P-124):
• Start Slave Address (SR44): 40125 *
• Start Master Address (PLC): internal PLC register #1 (value = 2)
Modbus Function 06 Transaction Table
Query Response
Field Hex Byte Field Hex Byte
Slave Address 01 Slave Address 01
Function 06 Function 06
Address High 00 Address High 00
Address Low 7D (decimal = 125) * Address Low 7D (P-124) *
Force Data High n/a (doesn’t matter) Force Data High n/a (ignored)
Force Data Low 02 (Enable Starter) Force Data Low 02 (Enable Starter)
CRC Low
PLC & SR44 automatically
calculate CRC checksum values
CRC Low
PLC & SR44 automatically
calculate CRC checksum values
CRC High CRC High
* SR44 has a Modbus offset of one address for registers & bits; PLC targets one address higher.
SR44
씯
PLC
Parameter Address / Byte *
Data Value
Hex
Data**
Address ***
Decimal Hex
P-124 (OffLine Command) 40125* / high 02 02** 00 02**
internal PLC
register #1
* SR44 Modbus addresses have an offset of one address from the parameter #.
The PLC needs to target slave address 40125 in order to change P-124.
** For Function 06 single write instructions, the SR44 accepts only the low byte of the PLC word;
the high byte is ignored. In this example, only P-124 is changed by the write instruction.
*** PLC addresses are any valid user-assigned memory addresses.
SR44 Series Soft Starter User Manual
1st Ed, Rev B 07/31/2019
6–16