Watlow PPC-2000 - Manual (Page 297)

Doc.# 30002-00 Rev 2.3 Watlow Anafaze 269
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Appendix A: Modbus Protocol
Watlow Anafaze offers a modbus driver (.DLL) for use with
user-written Windows-based software applications that
communicate with the PPC-2000. Using that driver makes it
unnecessary for the programmer to understand and implement
the modbus protocol.
PPC-2000 serial communications use the Modbus RTU
protocol. This protocol defines the message structure for all
communication packets. The protocol is the same for both RS-
232 and RS-485 serial interfaces. Modbus ASCII is not
supported.
Up to 32 PPCs may be connected on a network.
Controllers communicate using a master-slave model, in which
only one device (the master) can initiate transactions (called
“queries”). The other devices (slaves) respond by supplying the
requested data to the master, or by taking the action requested
in the query. Typical master devices include host PCs and
operator panels. The PPC-2000 is a slave device.
The master can address individual slaves, or initiate a
broadcast message to all slaves. Slaves return a message
(called a “response”) to queries that are addressed to them
individually. Responses are not returned to broadcast queries
from the master.
The Modbus protocol establishes the format for the master’s
query by placing into it the device (or broadcast) address, a
function code defining the requested action, any data to be
sent, and an error-checking field. The slave’s response message
is also constructed using Modbus protocol. It contains fields
confirming the action taken, any data to be returned, and an
error-checking field. If an error occurred in receipt of the
message, or if the slave is unable to perform the requested
action, the slave will construct an error message and send it as
its response.