MDM - WinCC: General information System Manual, 11/2008

Communication is routed via the standard Ethernet port of the computer.

The maximum number of connections is restricted by the available system resources and their performance data, particularly in terms of the CPU, RAM and the Ethernet connection.

I have a customer that has over 100 devices connected. Be sure you addressing is correct for the memory areas:

Here are some notes:

Remember, the Modbus TCPIP channel is specifically designed for the Modicon PLC protocols, not just any Modbus device. Here are some general information points on using the Modbus TCP protocol to non-Modicon devices. The most common mistakes in communicating to a non-Modicon device using the Modbus protocol are addressing mistakes. For instance..

A Modicon PLC has 5 main memory areas…

Discrete COIL – 0x --- 000001 to however many are configured in the PLC

Discrete Inputs – 1x --- 100001 to however many are configured in the PLC

IREG – 3x --- 300001 to however many are configured in the PLC

HREG – 4x --- 400001 to however many are configured in the PLC.

XMEMnn – 6x --- 600001 to however many are configured in the PLC

When defining an address for float or longana data types, these tag types are addressed in pairs. The first register in the pair contains the high-order word; the second register contains the low-order word. Enter the address for the starting register in a register pair; the companion register is implied. Try to maintain consistency when using register addresses for float and longana data types. You could adopt either an odd or even starting register sequence and maintain that sequence. (Like: 1, 3, 5, 7, 9, ... or 2, 4, 6, 8, 10, ...)

Sometimes when using a 3rd party device that supports the Modbus protocol, you will need to offset the addresses to get them to match up in the 3rd party PLC or device.

That is all I know!
Good luck
billwest