Automationdirect.com Stellar SR44, User Manual

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6–13
6.5 – Serial Communication to SR44
SR44 parameters are stored in 8-bit bytes. Modbus reads the values from the slave as 16-bit
words. Therefore two SR44 parameter bytes are packed into each Modbus word. The lowest
parameter is in the data High of the word, and the next parameter is in the data Low. This also
means that even when the read request is for one register, two SR44 parameters are read. Up to
four contiguous words (8 parameters) may be read with one query.
6.5.0 – Parameter Scaling
Certain parameters have non-unity scaling factors which must be applied when using serial
communication to read and write the parameters. (Parameter scaling factors are shown in Chapter
5: Keypad Menu Items and Parameters.)
• Scaling applies only when using serial communication to read and write parameters.
• Scaling Factor = Maximum Parameter Value ÷ 255
• When READING from parameters, MULTIPLY the serial comm value by the scaling factor.
• When WRITING to parameters, DIVIDE your desired value by the scaling factor (multiply by the
inverse of the scaling factor).
Parameter Scaling Example:
P-71 = Contactor Delay; Default value = 160 ms; Scaling Factor = 4
For default value = 160 ms:
Keypad shows “160 ms”; Comms data reads “40” [multiply by 4 to get actual value of 160]
To change value to 200 ms:
Enter “200” through keypad, or
Write “50” through serial comm [divide by 4 to determine value to write]
Keypad display will show “200 ms”; serial comm will read “50” [multiply by 4 to get actual value]
To change value to 240 ms:
Enter “240” through keypad, or
Write “60” through serial comm [divide by 4 to determine value to write]
Keypad display will show “240 ms”; serial comm will read “60” [multiply by 4 to get actual value]
Parameters referenced in the following examples:
Modbus Function 03 example 6.5.1:
• P-73 = Last Trip
• P-74 = 2nd Last Trip
• P-75 = 3rd Last Trip
• P-76 = 4th Last Trip
• P-77 = 5th Last Trip
• P-78 = Protection 2
Modbus Function 05 example 6.5.2:
• P-52 / B-6 = User Flags 2 / 2nd parameter set
Modbus Function 06 example 6.5.3:
• P-124 = OffLine Command
The minimum value that a scaled parameter can be changed is equal to that parameter’s scaling factor.
SR44 has a Modbus offset of one address for registers and bits. PLC programs should target one address
higher than desired, e.g., PLC targets SR44 address 40019 to read/write to/from address 40018 (P-18).
Chapter 6: Serial Communications
SR44 Series Soft Starter User Manual
1st Ed, Rev B 07/31/2019