NMBA-01 Installation and Start-up Guide
B-1
Appendix B – Modbus Protocol
This chapter describes the serial Modbus protocol. This chapter is
intended for people who need to program a serial Modbus master for
their own control system.
The material of this chapter is copyrighted by Modicon, and is used by
permission of AEG Schneider Automation (Modicon). This material is
included in the Modicon publication
Modicon Modbus Protocol Refer-
ence Guide (PI-MBUS-300 Rev. E). The latest revision is available at
Modicon internet homepage
http://www.modicon.com.
Introducing Modbus
Protocol
The Modbus protocol is a serial master–slave protocol. This appendix
describes the Modbus protocol only to the level which is required to
fully access the ABB drives. The Modbus protocol defines what is seri-
ally transmitted on the communication link. The physical interface to the
NMBA-01 is half-duplex RS-485.
Transactions on
Modbus Networks
Standard Modbus ports on Modicon controllers use an RS-232C com-
patible serial interface that defines connector pinouts, cabling, signal
levels, transmission baud rates, and parity checking. Controllers can be
networked directly or via modems.
Controllers communicate using a master – slave technique, in which
only one device (the master) can initiate transactions (called ‘queries’).
The other devices (the slaves) respond by supplying the requested
data to the master, or by taking the action requested in the query. Typi-
cal master devices include host processors and programming panels.
Typical slaves include programmable controllers.
The master can address individual slaves, or can initiate a broadcast
message to all slaves. Slaves return a message (called a ‘response’) to
queries that are addressed to them individually. Responses are not
returned to broadcast queries from the master.
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