Modbus; Network Topologies; Serial Protocol Variants - GE Multinet 4 Instruction Manual

OPERATIONAL GUIDE
6.7

Modbus

6.7.1

Network Topologies

6.7.2

Serial Protocol Variants

6–24
Multinet4 supports client (master) and server (slave) modes of operation for the Modbus/
TCP protocol as per the March 29, 1999 (Release 1.0) Open Modbus/TCP Specification
written by Andy Swales of Schneider Electric.
Figure 6–3: depicts an example Modbus/TCP network. Modbus devices (masters and
slaves) are connected to Multinet4s at the edge of the network. In addition, Modbus/TCP
clients and servers may connected directly to the IP network over an Ethernet link. The
Modbus serial devices are connected to the Multinet4 unit via RS-232 and/or RS-485 single
or multidrop interfaces. The serial Modbus masters initiate requests to the slaves. These
requests are encapsulated and forwarded by the Modbus/TCP client software to the
appropriate Modbus/TCP server. At the server, the request is de-encapsulated, analyzed,
and sent over the appropriate serial port to the serial Modbus slave. When the slave device
responds, the response is encapsulated and sent back to the Modbus/TCP client that in
turn de-encapsulates and forwards the response to the Modbus master. Device tables are
kept on each Multinet4 that describe the locally connected Modbus serial devices as well
as how to reach each remote device.
FIGURE 6–3: Example MODBUS/TCP Network
For serial data both the Modbus ASCII and the Modbus RTU protocol variants are
supported.
Modbus ASCII (depicted in the figure below) uses ASCII message encoding with a
longitudinal redundancy check (LRC). Each message begins with a ':' character and ends
with a CRLF character sequence.
MULTINET4 MULTI-PORT SERIAL SERVER & MANAGED SWITCH – INSTRUCTION MANUAL
CHAPTER 6: OPERATIONAL GUIDE