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QUANTUM™ LX COMPRESSOR CONTROL PANEL
COMMUNICATIONS SETUP
090.020-CS (APRIL 13)
Page 45
SECTION 4
MODBUS
®
PROTOCOL
MODBUS® Protocol
General Description
MODBUS® Protocol is a messaging structure
devel-oped by Modicon in 1979, used to establish
leader-follower
/client-server
communication
between intel-ligent devices. It is a standard,
truly open and the most widely used network
protocol
in
the
industrial
manufacturing
environment. The MODBUS® proto-col provides
an industry standard method that MOD-BUS®
devices use for parsing messages.
Since MODBUS® protocol is a messaging
structure, it is independent of the underlying
physical layer. It is traditionally implemented using
RS-232, RS-422, or RS-485 communications
hardware. With the Quan-tum™ LX, MODBUS®
TCP can also be utilized as it applies to Ethernet
networks.
The
Quantum™
controller
is
setup
to
communicate on standard MODBUS® networks
using either ASCII (American Standard Code for
Information Inter-change), RTU or TCP/IP.
The Quantum™ provides the capability to
interface with other devices that support serial
data com-munications using the MODBUS®
protocol. This is a
Leader
/
Follower
multi-drop
communication method whereby the Quantum™ is
setup to be a MODBUS®
Follower
. The
customer’s PLC (Programmable Logic Controller)
or DCS (Data Communications System, such as a
desktop or laptop computer) must be set-up as a
MODBUS®
Leader
. The
Leader
initiates the reading
and writing of data (queries) to a Quantum™. The
Quantum™ does not generate its own data, it will
only reply to a request by the
Leader
.
The Quantum™ ID number is used as the MODBUS®
Follower
address (for ASCII and RTU). The
Leader
uses Function Code 3 (Read Holding Registers) to
send a request to read data from the Quantum™.
The
Leader
uses Function Code 6 (Load Register) to
request to change a setpoint or to send a command
such as starting the compressor. Up to one hundred
and twenty-
fi
ve (125) data elements can be read
with one read request.
The address references are numbered relative to
the Frick® addresses in the Quantum™ Data Table
(see MODBUS Addressing Note in the Quantum™
Data Table section of this manual for additional
informa-tion). The Quantum™ only accepts one
value with a Load Register request. Changing a
setpoint causes the Quantum™ to save the new
setpoint to nonvola-
tile memory. Be careful not to continuously request a
setpoint change. Keeping the Quantum™ busy writing
to memory will interfere with the Quantum™ com-
municating to its I/O boards. A communication failure
to an I/O board will cause the compressor to shut-
down. For more detail and a list of the data, reference
the Quantum™ Data Table section of this manual. For
details about the actual protocol, reference the Modi-
con website at http://www.modbus.com.
MODBUS® TCP/IP (Ethernet)
TCP/IP is the common transport protocol of the Inter-
net and is actually a set of layered protocols, provid-
ing a reliable data transport highway between Quan-
tum™ LX panels and an Ethernet network. Ethernet
has become the standard for factory networking,
replacing many of the data-bus systems used in the
past.
MODBUS® TCP/IP simply takes the MODBUS® in-
struction set and transparently wraps TCP/IP around
it. Unlike MODBUS® ASCII and RTU, there no param-
eters that need to be set (such as baud rate, data
bits, etc.).
NOTE:
When using Modicon Setup Software, ensure
that:
•
Head number = Rack Position (position of
Ethernet card in its rack)
•
Socket # = 502
Frick® Controls utilizes function codes 3 (Read), 6
(Write) and 16 (Multiple Write) of the TCP/IP proto-
col. Use port 502 for Modbus TCP/IP communications.
The value used for the Modbus ID must be greater
than zero.
MODBUS® TCP LOG
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