ElmoMC Multi-Axis Motion Controller-Maestro - Manual (Page 221)

Maestro Software Manual
MAN-MASSW (Ver. Q)
12-1
Chapter 12: MODBUS Implementation
MODBUS Application Protocol Specification V1.1b (http://www.MODBUS-IDA.org)
defines an application layer messaging protocol providing client/server communications
between devices connected on different types of buses or networks (Fig.12-1).
Additionally, it standardizes a specific protocol on a serial line to exchange a MODBUS
request between a master and one or more slaves.
Figure 12-1: MODBUS “client/server” model
MODBUS data functions are based on a register model. A register is the smallest
addressable entity with MODBUS. The register model is based on a series of tables which
have the following characteristics (Table 12-1).
Table Classic Terminology
Modicon ®
Register Table
Characteristics
Discrete outputs Coils 0:00000
Single bit, alterable by
an application
program, read-write
Discrete inputs Inputs 1:00000
Single bit, provided
by an I/O system,
read-only
Input registers Input registers 3:00000
16-bit quantity,
provided by an I/O
system, read-only
Output registers Holding registers 4:00000
16-bit quantity,
alterable by an
application program,
read-write
Table 12-1: MODBUS register model