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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™ HD, 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 re-
quest to change a setpoint or to send a command
such as starting the compressor. Up to one hundred
and twenty-five (125) data elements can be read with
one read request.
QUANTUM™ HD COMPRESSOR CONTROL PANEL
COMMUNICATIONS SETUP
SECTION 4
MODBUS
PROTOCOL
®
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™ HD 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.
090.040-CS (JAN 14)
Page 43
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