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1st Ed, Rev B 07/31/2019
SR44 Series Soft Starter User Manual
Chapter 6: Serial Communications
6.2 – Quick Start (continued)
5) Function 06 (Write Single Register)
Parameters in the SR44 are 8-bit bytes which are written to by 16-bit Modbus words. Only the
low byte of the Modbus word in the PLC gets sent to the SR44 parameter.
6) Function 05 (Write Single Coil)
This function is used to set or clear a single bit within the Read/Write parameters between 0
through 127. (Only the bits within parameters designated as Read/Write can be preset with this
function; Parameters 18, 51, 52, 53, 54, 86, 95, 96, 121, & 127)
7) Function 03 (Read Holding Registers)
This function reads 8-bit bytes back to the PLC. If more than one byte is requested from the
SR44, the PLC program will have to separate 16-bit Modbus words into the 8-bit SR44 bytes.
8) SR44 Status Bits (User Flags) are read as holding registers, i.e., read register 9 to read Status2
bits. For status bit writes, bits are mapped one-to-one, e.g., the Kickstart flag bit 0 of User Flags
1 (P51) is bit no 51 * 8 + 0 = bit 408.
(Refer to the “Specifications” section of this chapter for supported Modbus function codes.)
6.3 – Specifications
1. Refer to Modicon Modbus Protocol Reference Guide PI-MBUS-300 at www.modbus.org for full
protocol specification.
2. Baud rate is 9600.
3. 8 data bits, no parity, 1 stop (default) or 8 data bits, no parity, two stop bits.
4. Modbus timeout is 3.5 character times. The in message timeout is also 3.5 character times,
unlike the standard which is 1.5 character times. Further, it is possible that the SR44 will be
delayed in responding to a request if it is checking a fault condition.
5. Broadcast is not supported.
6. SR44 parameters retain their normal numbers (0-127).
7. Status bits (flags) are bit positions starting at parameter 0 bit 0.
8. Register numbers are 0…127.
9. Coils (flags) 0…1024.
10. Offline EEROM Registers are 1000…1127.
(Non-volatile registers where the “Permanent Store” parameters are saved.)
11. Supports the following Modbus functions:
Function 03 read holding registers up to 4 words (8 parameters) in number.
Function 05 write single coil.
Function 06 write single register (byte).
Function 07 exception status.
• Error 01 illegal function (write to read-only parameter).
• Error 02 illegal data address (bad parameter number).
• Error 03 illegal data (trying to read or write too much data).
• Error 06 slave busy.
• Error 07 negative acknowledge.
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