Modbus - Emerson NGA2000 Reference Manual

Reference Manual
748384-C
September 2003

1-21 MODBUS

Modbus is a protocol developed by Gould Modi-
con, who provide documentation for it. The pro-
tocol is in the public domain. It operates over
RS232 or RS485 links. It is a master - slave
protocol - a master requests or writes informa-
tion, and the slave responds. In the NGA case,
the NGA control module is a slave.
In Modbus, all information is defined as existing
in "registers", accessible by their address.
These registers may contain integer, floating
point (in our implementation) or string informa-
tion. Registers are addressed by a scheme that
determines first the analyzer module (or sub-
1-38 Introduction
NGA2000 Reference
node) with which it is desired to communicate,
then the variable address within the analyzer.
All NGA variables are available as registers.
These may be read or written to, though output
variable registers will ignore anything written
into them.
The host may access individual registers, or
may define a list of variables that can be ad-
dressed as one. This definition may be per-
formed either as a Modbus command or via the
control module.
A list of available variables and their corre-
sponding registers is available from the Modbus
connection directly as an ASCII file.
Rosemount Analytical Inc. A Division of Emerson Process Management