Owners manual

1: Introduction
Modbus Protocol User Guide 7
It is revolutionary for such a simple and flexible protocol as Modbus to offer such
functionality. Therefore, Modbus/TCP offers exciting new design options for industrial
users, which the Lantronix IAP Device Servers extend to traditional Modbus/RTU
serial devices.
Modbus/TCP Master Talking to Modbus/RTU Serial Slave
Devices D, G, and H are traditional Modbus/RTU slave devices. Device D uses a
point-to-point electrical interface like RS232. This allows only a single Modbus/RTU
master to talk to device D. However, the IAP Device Server makes device D appear
on the Modbus/TCP network as a full Modbus/TCP slave device. All Modbus/TCP
enabled devices, A, B, E, and F, can actively share access to slave device D. A
limitation in traditional Modbus/RTU implementation expects devices to be dedicated
as either master or slave devices, so device D can only act as a Modbus slave.
Devices G and H are different from device D. They share a single RS485 “multi-drop”
line that strictly limits them to act as slaves to a single Modbus/RTU master.
However, a little of the new Modbus/TCP and IAP Device Server magic still
applies⎯all Modbus/TCP enabled devices A, B, E, and F can actively share access
to both slave devices G and H. IAP Device Server manages and coordinates the
shared access. In fact, the IAP Device Server allows up to eight concurrent Modbus
masters to share access to the slaves.
Modbus/RTU Serial Master Talking to Modbus/TCP Slave
Device C is a traditional Modbus/RTU master device. Yet the IAP Device Server
makes device C appear to the TCP/IP network as a Modbus/TCP master⎯plus all of
the Modbus/TCP slaves on the TCP/IP network (A, B, D, E, F, G, and H) appear as
traditional Modbus/RTU slave devices. The only limitation is the traditional
Modbus/RTU assumption that device C is dedicated as a master only. Therefore
Modbus/TCP master devices A, B, E, and F cannot treat device C as a Modbus/TCP
slave.
Modbus/RTU Serial Master Talking to Modbus/RTU Serial Slave
Finally, master device C can poll traditional Modbus/RTU slave devices D, G, and H
as if they were directly multi-dropped on an attached RS485 line. The IAP Device
Server transparently bridges traditional Modbus/RTU devices across any TCP/IP
network. This means users can start implementing for Modbus/TCP long before all of
their required products exist with Modbus/TCP and network interfaces.