I-GARD DSP-OHMNI Instruction Manual
26
DSP MODBUS output structure
Two functions are supported
Read Holding Register (03)
Set Coil (05)
There are 154 registers available, which can be accessed by an external host system. This
document summarizes the format and function of these registers.
The request from the master is always 8 bytes long and are as shown in Table 13.1.
TABLE 13.1 MODBUS RTU STANDARD 8 BYTE HOLDING REGISTER READ FUNCTION (03)
Unit I/D Function Starting Address No. of Registers requested CRC
High Low High Low High Low
01 03 00 01 00 02 nn nn
All bytes are in hexadecimal. Numbers above are, for example, a request for 2 registers only,
starting from address 01. CRC checksum is 16 bit CRC as described in MODBUS information.
The high bytes are not used in any requests.
If successful the DSP will return the message shown in Table 13.2.
TABLE 13.2 RETURNED INFORMATION STRUCTURE FOR HOLDING REGISTER REQUEST
Unit I/D Function No. of bytes Data 1 Data 2 CRC
High Low High Low High Low High Low
0103 00 02 00 02 00 01 nn nn
Register contents are shown in Tables 1 to 3 as follows.
NOTE: Register number is shown in decimal but must be sent in hexadecimal form in the request.
The only write functions presently supported in the DSP system are RESET control and PULSE
ON/OFF which will require the use of Set Coil function in MODBUS. The format for Setting is as
shown in Figure 13.3
TABLE 13.3 REQUEST TO WRITE TO SET A BIT ON A REGISTER (MODBUS FORCE COIL)
Unit I/D Function Coil Address Force Coil CRC
High Low High Low High Low
01 05 00 00 ff 00 nn nn
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