PACiS GTW GTW/EN T/C40, Technical Manual

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GTW/EN CT/C40 Communication
Page 34/64 PACiS Gateway
4.4.2 General format of MODBUS functions
A query message typically contains some of the following 8-bit fields:
−
Starting Address Hi
−
Starting Address Lo
−
No. of Registers / Points Hi
−
No. of Registers / Points Lo
−
Force / Preset Data Hi
−
Force / Preset Data Lo
A response message typically contains some of the following fields:
−
Byte Count *
−
Data
−
Data Hi
−
Data Lo
−
No. of Registers / Points Hi
−
No. of Registers / Points Lo
−
Force / Preset Data Hi
−
Force / Preset Data Lo
( * ) Byte Count Field:
A Byte Count Field is used in slave responses to indicate the number of 8-bit bytes in the
Data field; the value is exclusive of all other field contents, including the Byte Count field.
4.4.3 MODBUS Data Addresses
Point addresses in the data field are 16-bit values in the range of 0 – 65535 for each item
type.
That’s means that you can have a coil item configured at address 100 for example, and a
holding register item configured at address 100 too.
Starting addresses in queries from SCADA may begin from address 0.
Example: request to read the 16 first coils on device 17.
Slave address 11
Function 01
Starting Address HI 00
Starting address LO 00
Number of points HI 00
Number of points LO 10
CRC XX