I-Gard DSP-OHMNI, Manual

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UNIT I/D FUNTION STARTING ADDRESS 3 OF REGISTERED REG. CRC
01 03
High Low High Low High Low
00 01 00 02 nn nn
Table 8: Modbus RTU Standard 8 Byte Holding Register Read Function (o3)
DSP MODBUS Output Structure
Two functions are supported: Read Holding Register (03) and Set Coil (05)
There are 156 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 8
below.
All bytes are in hexadecimal. Numbers above are, for example, a request for 2 registers only, starting from address 01.
CRC check sum 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 9 below.
UNIT I/D FUNTION # OF
BYTES
Data 1 DATA 2 CRC
01 03 04
High Low High Low Low High Low
00 nn 00 nn nn nn nn
Table 9: Returned Information Structure for Holding Register Request
Register contents are shown in Tables 13 to 16.
Note: Register number is shown in decimal but must be sent in hexadecimal form in the request. The only write func-
tions 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 Table 8.
UNIT I/D FUNTION COIL ADDRESS FORCE COIL CRC
01 05
High Low High Low High Low
00 00 ff 00 nn nn
Table 10: Request to Write to Set a Bit on a Register (MODBUS Force Coil)
In this case there will only be two ‘coils’ (actually DSP functions).
COIL NO NAME FORCE COIL DATA
0 Reset ff00=on 0000=off
1 Reset ff00=on 0000=off
Table 11: Write Register Information