I have a need to configure a Modbus/RTU master connected to one Lantronix IAP device server running Modbus firmware. The master IAP device server communicates via the ethernet to other Lantronix IAP device servers. What is the maximum number of Modbus/TCP slave devices that can be connected to the remote Lantronix IAP device servers? How do I configure the Lantronix IAP device servers connected to the master?

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The master IAP device server with "Master Attached" has a lookup table with 8 entries - meaning each "Master" can connect to at most 8 remote "Slave' IAP device servers directly by Ethernet.

However, each of these slaves can have from 1 to 31 Modbus-RTU serial slaves attached via RS-485 multidrop. If RS-485 repeaters are used, you could (in theory) attached 255 RTU slaves to each IAP slave for a grand total of 8x255 or 2040 slaves.

However, Modbus only has an 8-bit address, so one master can only talk to 255 slaves via 1 port.

So for example we can easily have a system with:
1) 1 IAP Device Server in "Master-Attached" mode
2) 8 IAP Device Servers in "Slave-Attached" mode
3) 151 Modbus-RTU slaves

For each entry you can define a range of Modbus-RTU slaves to be at that IP:
Slave 001..020 at IP: 192.168.0.20 (20 slaves)
Slave 021..030 at IP: 192.168.0.21 (10 slaves)
Slave 031..031 at IP: 192.168.0.22 ( 1 slave)
Slave 041..070 at IP: 192.168.0.23 (30 slaves)
Slave 071..100 at IP: 192.168.0.24 (30 slaves)
Slave 101..120 at IP: 192.168.0.25 (20 slaves)
Slave 121..130 at IP: 192.168.0.26 (10 slaves)
Slave 131..160 at IP: 192.168.0.27 (30 slaves)

With the above configuration, if the Modbus serial Master sends a poll to slave #57, the IAP Device Server with "Master-Attached" set goes thru the lookup table, sees that this slave is one of 30 at IP 192.168.0.23, and forwards ("routes") the Modbus/TCP request to this IP.

The IAP Device Server at 192.168.0.23receives it and forwards the reconstituted Modbus/RTU message out of it's serial port.

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[Originally Published On: 07/21/2000 11:59 AM]