Watlow CPC400 - Manual (Page 197)

Doc.0600-2900-2000 Watlow Anafaze 183
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Appendix A: Modbus Protocol
The serial communications port on the CPC400 supports
the Modbus RTU protocol. This protocol defines the mes-
sage structure for all communication packets. The protocol
is the same for both EIA/TIA-232 and EIA/TIA-485 serial
interfaces. Modbus ASCII is not supported. Up to 32
CPC400 controllers may be connected on a network.
Watlow Anafaze offers a Modbus driver for use with Win-
dows-based software applications that communicate with
the CPC400. Using that driver makes it unnecessary for
the programmer to understand and implement the Modbus
protocol.
Master-Slave Model
Controllers communicate using a master-slave model, in
which only one device (the master) can initiate transac-
tions (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 CPC400 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 que-
ries from the master.
The Modbus protocol establishes the format for the mas-
ter’s query by placing into it the device (or broadcast) ad-
dress, a function code defining the requested action, any
data to be sent, and an error-checking field. The slave’s re-
sponse 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 oc-