Automationdirect.com Stellar SR44, User Manual

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Chapter 6: Serial Communications
6.5.1 – Reading Holding Registers (Modbus Function 03)
This function is used to read a group of up to eight parameters from the SR44. The allowable
address range for this function is 0-127 for working parameters and 1000–1127 for offline EEROM
parameters.
Example – Modbus Function 03:
Read the fault history (P-73 – P-77) from SR44 slave #1 into a PLC
• Start Slave Address (SR44): 40074 *
• Start Master Address (PLC): internal PLC register 1
• Number of Elements: 3
Modbus Function 03 Transaction Table
Query Response
Field Hex Byte Field Hex Byte
Slave Address 01 Slave Address 01
Function 03 Function 03
Start Address High 00 Byte Count 04
Start Address Low * 4A (decimal = 74) * Data High Byte 00 (P-73)
No. of Registers High 00 Data Low Byte 02 (P-74)
No. of Registers Low 02 Data High Byte 0D (P-75)
CRC Low
PLC & SR44 automatically
calculate CRC checksum values
Data Low Byte 0E (P-76)
CRC High Data High Byte 10 (P-77)
Data Low Byte C0 (P-78)
CRC Low Byte
PLC & SR44 automatically
calculate CRC checksum values
CRC High Byte
* The SR44 has a Modbus offset of one address for registers & bits; PLC targets one address higher.
SR44
씮
PLC
Parameter Address / Byte *
Data Value
Hex
Data
Address ***
Decimal Hex
P-73 (Last Trip) 40073 / high 0 (no trip) 00
00 02
internal PLC
register #1
P-74 (2nd Last Trip) 40073 / low 2 (too hot) 02
P-75 (3rd Last Trip) 40074 / high 13 (overload) 0D
0D 0E
internal PLC
register #2
P-76 (4th Last Trip) 40074 / low 14 (shearpin) 0E
P-77 (5th Last Trip) 40075 / high 16 (external trip) 10
10 C0
internal PLC
register #3
P-78 (Protection2) ** 40075 / low ** 192 (full+optimize) C0
* SR44 Modbus addresses are variable.
Only the 1st address of the Function 03 read instruction ‘matches’ the parameter number, but
there is an offset of one address from the parameter#. If the PLC targets 40074, the PLC will
return data starting with P-73. P-73 will be in the high byte, and P-74 will be in the low byte.
** The Function 03 mutliple read instruction gets data from an even number of parameters.
Unwanted data can be discarded by the PLC.
*** PLC addresses are any valid user-assigned memory addresses.
SR44 Series Soft Starter User Manual
1st Ed, Rev B 07/31/2019
6–14