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20 DDA LPM Modbus Implementation

The MTS DDA LPM Modbus implementation conforms to the “Modicon Modbus Protocol

Reference Guide, PI-MBUS-300 Rev. G” available from Modicon, Inc. The information provided

below assumes familiarity with the Modbus protocol as outlined in this reference guide. All infor-

mation provided applies to both Modbus RTU and Modbus ASCII protocol versions.

Modbus Function Codes

The following Modbus function codes are supported:

Function 03 - Read Holding Registers

Function 04 - Read Input Registers

Function 08 - Diagnostics (Subfunction 00, Return Query Data)

Function 08 - Diagnostics (Subfunction 01, Restart Communications Option)

Function 08 - Diagnostics (Subfunction 04, Force Listen Only Mode)

Function 17 - Report Slave ID

Function 03 - Read Holding Registers

The LPM responds to this message by returning the contents of the requested data regis-

ter(s) (see Appendix F, LPM Modbus Register Maps). The following implementation-spe-

cific considerations apply:

• If an unsupported or reserved register is requested, a maximum negative value (8000H

or 80000000H for paired-registers) is returned (see Appendix F, LPM Modbus Register

Maps for unsupported/reserved registers).

• If a register contains an LPM error (displayed on the LPM’s LCD screen as a '*' character

followed by an error message), a maximum negative value is returned.

• If a register is blank, indicating that the desired function is not enabled (e.g., volume

calculations), a value of 0000H is returned.

• If the alarm data register is requested, and either a 'GAUGE level' error or 'GAUGE

temperature' error exists, both the SCERR (Sensor Communication Error, bit D12) and the

SOERR (Sensor Operation Error, bit D13) bits will be set. Unsupported or reserved bits

will always be set to 0. See Appendix F, LPM Modbus Register Maps, for alarm bit defini-

tions.

Function 04 - Read Input Registers

This function is handled exactly the same as Function 03. Note that all registers are read-

only in this implementation.

Function 08 - Diagnostics (Subfunction 00, Return Query Data)

The LPM responds to this request with the following data:

Slave address: echoed

Function: 08H

Subfunction high: 00H

Subfunction low: 00H

Query data (16-bit): echoed

Error check: 16-bit CRC/8-bit LRC

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