User`s manual

Watlow Controls Communications Guide 11
are more accurate, because they are less
prone to both operator and noise errors.
Protocol maintains system integrity by
requiring a response to each message. It’s
like registered mail — you know that your
letter has been received because the post
office sends you a signed receipt.
In Watlow data communications, a dialog
will continue successfully as long as the
messages are in the correct form and
responses are returned to the protocol
leader. If the operator enters an incorrect
message, or interference comes on to the
data line, there will be no response. In
that case the master must retransmit the
message or go to a recovery procedure. If
an operator continues to enter an
incorrect message or interference
continues on the data line, the system will
halt until the problem is resolved.
Watlow provides you with three protocol
choices: XON/XOFF, ANSI x3.28, Modbus
RTU
XON/XOFF
The first protocol is conventional
XON/XOFF protocol. It works very well
for systems that do not require a message
response and for those with only two
devices (one PC and one controller).
Sending of the "XON" (0x11) and "XOFF"
(0x13) controls information. This is the
simplest protocol offered by Watlow
Controls.
ANSI x3.28
The second, called “Full Protocol,” is
based on the American National Standard
Institute’s standard ANSI X3.28-1976,
Subcategory 2.2. The Full Protocol is
required for systems that need a response
to every message, and for any “multi-
drop” network, which has more than two
communication devices. Addressing a
certain controller on the bus, then
enclosing all messages with a <STX> start
of text (0x02) and <ETX> end of text
(0x03) controls information exchange.
The characters <ENQ>, <EOT>, <DLE>
are used to determine who has control of
the bus. Lastly, all messages are <ACK>
acknowledged (0x06) or <NAK> negative
acknowledged (0x15) to ensure messages
where received correctly.
Modbus Remote Terminal Unit
(RTU)
Gould Modicon, now called AEG
Schneider, created the third protocol for
process control systems called "Modbus".
This protocol is the most complex of the
three. It has the advantage of being
extremely reliable in exchanging
information, a highly desirable feature for
industrial data communications. This
protocol works on the principle of packet
exchanges. The packet contains the
address of the controller to receive the
information, a command field that says
what is to be done with the information
and several fields of data. Reading from
these registers retrieves all information in
the controller. These registers are listed in
your user’s manual. You will need this list
to determine where the data is located.
The last item sent in the packet is a field
to ensure the data is received intact. This
is called a cyclic redundancy check-sum.
All information exchanged is in hex
numbers. Watlow only supports the binary
version of Modbus, referenced as RTU.
The ASCII version is less efficient and is
not supported.