FRICK
®
QUANTUM™ COMPRESSOR CONTROL PANEL S90-010 CS (APR 08)
COMMUNICATIONS SETUP Page 31
MODBUS Protocol
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.
The Quantum™ controller is setup to communicate on
standard Modbus networks using ASCII (American
Standard Code for Information Interchange).
NOTE:
With the Quantum™ Controller,
ONLY
Modbus
ASCII (7 or 8 data bits) is recognized, and all references to
Modbus protocol in this document will be as they relate to
ASCII. The mode and serial parameters must be the same
for all devices on a Modbus network, therefore, ensure
that your network is utilizing the Modbus ASCII protocol
before attempting to try to communicate to the Quantum™
portion of the network. Additionally, typical Modbus
protocols allow for network broadcasting, whereby a single
message can be sent to all devices simultaneously. This
Broadcasting is
NOT
allowed or supported by the
Quantum™ Controller.
The Quantum™ provides the capability to interface with
other devices that support serial data communications
using the Modbus ASCII protocol. This is a Master / Slave
multi-drop communication method whereby the
Quantum™ is setup to be a Modbus ASCII Slave. The
customer’s PLC (Programmable Logic Controller) or DCS
(Data Communications System, such as a desktop or
laptop computer) must be setup as a Modbus ASCII
Master. The Master initiates the reading and writing of
data (queries) to a Quantum™. The Quantum™ does not
generate its own data, it will only reply from a request by
the Master.
The Quantum™ ID number is used as the Modbus Slave
address. The Master uses Function Code 3 (Read Holding
Registers) to send a request to read data from the
Quantum™. The Master uses Function Code 6 (Load
Register) to request to change a setpoint or to send a
command such as starting the compressor. Up to fifty (50)
data elements can be read with one read request.
The most desired data (information on the
Operating
Status
screen) exists in a 50 (fifty) element data area. 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 information). The Quantum™
only accepts one value with a Load Register request.
Changing a setpoint causes the Quantum™ to save the
new setpoint to nonvolatile memory. Be careful not to
continuously request a setpoint change. Keeping the
Quantum™ busy writing to memory will interfere with the
Quantum™ communicating to its I/O boards. A
communication failure to an I/O board will cause the
compressor to shutdown. 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
Modicon
website at http://www.modicon.com.
Port Configuration of The Master
7 or 8 Bits per Character (Data Bits)
Odd, Even or No Parity
1 or 2 Stop Bits
No Handshake
Data Packet
The Modbus protocol establishes the format for the
Master's query by creating a message (data packet) as
follows:
•
Assign the device address (Quantum™ panel ID
#). The address field of a message frame
contains two characters (ASCII). Valid
Quantum™ device addresses are in the range of
01 – 99 decimal. A master addresses a
Quantum™ by placing the Quantum™ address in
the address field of the message. When the
Quantum™ sends its response, it places its own
address in this address field of the response to
let the Master know which Quantum™ is
responding.
•
A function code defining the requested action
(Query):
•
Function Code 3 - to read holding registers
(sends a request to read data from the
Quantum™).
-
OR –
•
Function Code 6 to load a register (to request to
change a setpoint or to send a command such as
starting the compressor).
•
Any data to be sent (Response). The data field is
constructed using sets of two hexadecimal digits,
in the range of 00 to FF hexadecimal. These are
to be made from a pair of ASCII characters. The
data field of messages sent from a Master to the
Quantum™ devices contains additional
information which the Quantum™ must use to
take the action defined by the function code. This
can include items like discrete and register
addresses, the quantity of items to be handled,
and the count of actual data bytes in the field. If
no error occurs, the data field of a response from
a Quantum™ to a Master contains the data
requested. If an error occurs, the field contains
an exception code that the Master application
can use to determine the next action to be taken.
•
An error-checking field.
