
FRICK
®
QUANTUM™ LX AcuAir
®
CONTROL PANEL
S90-510 CS (MAR 07)
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 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. Broadcasting is NOT
allowed or supported by the Quantum™
software.
The Quantum™ provides the capability to interface
with other devices that support serial data
communications using the MODBUS
®
ASCII protocol.
This
is
a
Leader
/
Follower
multi-drop
communication method whereby the Quantum™ is
setup to be a MODBUS
®
ASCII
Follower
. The
customer’s
PLC
(Programmable
Logic
Controller) or DCS (Data Communications
System, such as a personal computer) must
be setup as a MODBUS
®
ASCII
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 from a
request by the
Leader
.
The Quantum™ ID number is used as the MODBUS
®
Follower
address. 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 Air Handler. Up to
fifty (50) data elements can be read with one read
request.
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 and information
about the actual protocol, reference the Modicon
website at http://www.modicon.com.
The read (query) and write examples on the following
pages are executed using a terminal emulation
package known as Hyperterminal (for more
information, refer to the Hyperterminal section in this
manual). When using Hyperterminal, use the Frick
addresses listed in the address tables, rather than the
Modbus addresses. This is because Hyperterminal
does not use a Modbus driver as a protocol, but
rather a pure ASCII data packet. The Quantum™
however, does need to be set to MODBUS
®
protocol
to properly interpret the ASCII data.
Port Configuration of The
Leader
7 Bits per Character (Data Bits)
No Parity
1 Stop Bit
No Handshake
Data Packet
The MODBUS
®
protocol establishes the format
for the
Leader'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
leader
addresses a Quantum™ by placing the
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
Leader
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 digits,
in the range of 00 to FF hexadecimal. These
are made from a pair of ASCII characters.
The data field of sent from a
Leader
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
Leader
contains the data requested. If an
error occurs, the field contains an exception
code that the
Leader
application can use to
determine the next action to be taken.
•
An error-checking field.