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5. CONFIGURATION

Figure 70: MODBUS TCP Communication Network

The association of MODBUS variables with CPU symbolic variables is made by the user through relations deﬁnition via

MasterTool IEC XE conﬁguration tool. It’s possible to conﬁgure up to 32 relations for the server mode and up to 128 relations

for the client mode. The relations in client mode, on the other hand, must respect the data maximum size of a MODBUS

function: 125 registers (input registers or holding registers) or 2000 bits (coils or input status). This information is detailed in

the description of each protocol.

All relations, in client mode or server mode, can be disabled through direct representation variables (%Q) identiﬁed as

Disabling Variables by MasterTool IEC XE. The disabling may occur through general bits which affect all relations of an

operation mode, or through speciﬁc bits, affecting speciﬁc relations.

For the server mode relations, IP addresses clusters can be deﬁned with writing and reading allowance, called ﬁlters. This

is made through the deﬁnition of an IP network address and of a subnet mask, resulting in a group of client IPs which can

read and write in the relation variables. Reading/writing functions are ﬁltered, in other words, they cannot be requested by any

client, independent from the IP address. This information is detailed in the MODBUS Ethernet Server protocol.

When the MODBUS TCP protocol is used in the client mode, it’s possible to use the multiple requests feature, with the

same TCP connection to accelerate the communication with the servers. When this feature isn’t desired or isn’t supported by

the server, it can be disabled (relation level action). It is important to emphasize that the maximum number of TCP connections

between the client and server is 63. If some parameters are changed, inactive communications can be closed, which allows the

opening of new connections.

The tables below bring, respectively, the complete list of data and MODBUS functions supported by the Nexto CPUs.

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