User`s manual

Watlow Controls Communications Guide 13
they're not considered cause for a
shutdown of the unit itself. There is
always a communications error code
generated when the <NAK> character
(0x15) is sent under the ANSI x3.28
protocol
User Responsibility
Users must refrain from altering prompts
that do not appear on the controller’s
front panel or are not included on the
specific model. For example, do not send
an A2LO command to a unit not equipped
with an alarm for output 2.
Most of Watlow controllers contain a
register to disable saving of the current set
point to EEPROM. When using your PC
or PLC to control the set point in the
Watlow controller, the EEPROM may be
prematurely damaged. There is a limit to
the number of times you can store
information in the EPPROM. PCs can
quickly reach this limit if the set point is
continually changed, as in a ramping
controller. Disabling this feature will
prevent this damage.
Care must also be taken that the process
can not cause damage to property or
injury to personnel if the wrong
commands are sent due to operator error
or equipment malfunction. Be sure to use
limit devices on the equipment to prevent
system runaway.
Because of the wide array of choices
available for setting up a Watlow
controller, only a subset of the prompts
contains parameters in a given situation.
The user's manual explains the
interrelations between prompts.
Writing a Modbus Application
Modbus RTU enables a computer or PLC
to read and write directly to registers
containing the controller’s parameters.
With it you could read all 141 of the
controller’s parameters with five read
commands.
Because of the wide array of choices
available for setting up a controller
utilizing Modbus, only a subset of the
prompts contain parameters in a given
situation. This document explains the
interrelations between prompts.
If you already have a software application
that uses Modbus, you can simply skip to
the Temperature/process Controller
Prompt Table or the Modbus RTU Address
Table in the user’s manual for the address
information your program will need. The
rest of this section on the Modbus
provides information for writing a
software application that uses Modbus.
1. You need to code messages in eight-bit
bytes, with no parity bit, one stop bit
(8, n, 1).
2. Negative parameter values must be
written in twos' complement format.
Parameters are stored in two-byte
registers accessed with read and write
commands to a relative address.
3. Messages are sent in packets that are
delimited by a pause at least as long as
the time it takes to send 30 bits. To
determine this time in seconds, divide
30 by your baud rate.
4. Because changing some parameters
automatically changes or defaults
other parameters, use the Complete
Parameter Download Sequence table
listed in the user's manual to order
write commands.
Using a controller address of 0x00 for a
write command, will send that
command to all the controllers in the
network. This is a powerful feature if all
the controllers on a network use all or
most of the same parameters.