TPU2000/2000R Modbus/Modbus Plus/ Modbus TCP/IP Automation Guide
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Section 5 - Modbus
Modbus is capable of being transported over three different types of physical interfaces. The port emulation types
are:
Serial
Modbus Plus
Ethernet TCP/IP
Modbus Serial is a traditional mode of providing protocol connectivity. There are two different emulations of Serial
Modbus. The two emulations are, Modbus RTU or Modbus ASCII. All Modbus ( Serial, Plus, or TCP/IP) contain
the common command set. Only the protocol presentation differs between them. Section 5 contains an
explanation of each protocol emulation theory so that the implementor may understand the differing technology
and its impact on the configuration, performance, and capability of the selected emulation.
Serial Modbus is available in two emulation’s, Modbus RTU and Modbus ASCII. Modbus RTU is a bit oriented
protocol (normally referred to as Synchronous), and Modbus ASCII is a byte oriented protocol (normally referred
to as Asynchronous). Both emulations support the same command set.
Networked nodes cannot
communicate unless the same emulation of the Modbus protocol is interpreted. This is an extremely
important issue. The TPU2000, and TPU2000R, support the Modbus ASCII and RTU protocol emulations.
Modbus Protocol
Modbus operates in the following fashion. A host device transmits a command, and one of the attached device(s)
respond. Each device has a unique address assigned to it. Each device is configured for the same protocol
emulation of Modbus. Figure 5-1 illustrates the polling sequence.
Master/Slave Example
The Master node (Circle) contains
a polling list. The master transmits
its request and waits for a response.
The Slave responds with the
information. If the slave data cannot
be transmitted immediately, a not
ready response is generated and
the Master must poll the Slave again.
Figure 5-1. Modbus Polling Sequence
The TPU2000, and TPU2000R, are designed as Modbus slave emulation devices. That is, a device, a host,
(illustrated in Figure 5-1) must be able to generate Master Requests in a Modbus format so that the slave,
(TPU2000 or TPU2000R is able to receive the commands.
Modbus ASCII Emulation
An ASCII character is defined as 7 data bits. A character is represented as a number from 00 HEX to 7F HEX.
Appendix B contains an ASCII character conversion chart. If a 0 is transmitted, it must be decoded to an ASCII
representation to be interpreted by the receiving device. 0 decimal is 30 hex for an ASCII representation. The
frame format for Modbus is represented in Figure 5-2. The device address, function code and checksum is part of
the transmitted frame. The Checksum is a Longitudinal Redundancy Check (LRC). Its calculation shall be
described later in this guide.
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