Modbus TCP server
41
COM465IP-COM465DP_D00216_02_M_XXEN/03.2017
6.2 Modbus process image in the memory of the COM465…
The device holds a process image in memory. This image represents the current states and values of
all devices that are in the same system as the COM465…
6.2.1 Querying data
6.2.1.1 Modbus function code FC03
The parameters and measured values of all devices in the subsystem can be read using the Modbus
function 0x03 "Read Holding Registers". This is only possible on the subsystem level, not in the entire
system. The unit ID refers to the respective device address.
6.2.1.2 Modbus function code FC04
The process image in the memory of the COM465… can be read using the Modbus function 4 "Read
Input Registers". Device name, device channel states and alarm and operating messages can be ac-
cessed via this function. This is possible for all devices in the system. Here, the unit ID refers to the
subsystem address.
The volume of the queried data depends on the number of bytes selected in the Modbus client used.
Up to 125 words (0x7D) can be read with a single query.
An individual word can also be read, for example, to detect the set bit for a saved common alarm.
6.2.1.3 How are the memory areas organised?
The assignment of the memory addresses and the associated memory content for one subsystem is
described below. Please refer also to the "BCOM" manual, which provides information about the en-
tire addressable system.
Memory utilisation Start address
End of memory
area
Size of memory
area
Reference values
for test purposes
0x0000 0x00FF 0x0100
Process image 0x0100 0x95FF 0x9500
Not used 0x96FF 0xFFFF 0x6900
For some Modbus clients an offset of 1 must be added to the register addresses.
Example: process image start address = 0x0101.
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