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RTU protocol to read or write to registers in the control-
ler. See the parameter tables for information about the
Modbus registers. These registers contain the parameter
values that determine how the controller will function
and current input and output values of the system. The
address in the tables have been offset by subtracting
40,001 from each one.
Two consecutive registers are addressed for 32-bit
data types. The first word, or lower register number, con-
tains the two higher bytes. The second word, or higher
register number, contains the two lower bytes of the four
byte integer value.
Note: All temperature related values accessed via Modbus are in
degrees F.
Setting Up a Modbus Network
1. Wire the controllers.
The Series SD uses an EIA/TIA-485 serial port, which
is not typically found in a PC, but can be found on many
PLC's. The type of port found in a typical PC is an EIA-
232 port. Internal EIA/TIA-485 PC ports are available,
but the most common way for a PC to communicate using
a EIA-485 port is with an EIA/TIA-232 to EIA/TIA-485
converter. See "Selecting an EIA/TIA-232 to EIA/TIA-485
converter" in the Install and Wire chapter.
The advantages of EIA/TIA-485 are that it is less
susceptible to noise and it allows a PC or PLC to commu-
nicate with multiple controllers on the same port to form
a network. It is important when using EIA/TIA-485, to
install the termination resistors along with pull-up and
pull-down resistors to ensure reliable communications.
Some newer PCs may only have a USB port. USB-to-
serial adapters (usually EIA/TIA 232) are available from
a variety of different PC vendors. Some companies offer
adapters to convert from USB to EIA/TIA-485 directly.
2. Configure each controller's communications pa-
rameters in the Setup Menu using the front panel.
Only a couple of communications parameters need to
be configured on the controller, Baud Rate and Modbus
Device Address. The choices for Baud Rate are 9600 bps,
19200 bps or 38400 bps. 38400 baud allows for the fastest
communication. For compatibility with other devices, re-
ducing noise susceptibility, or increasing communications
distance, 9600 bps could be chosen. When using EIA/TIA-
485, all devices connected to that port must use the same
Baud Rate. The Modbus address is used to identify each
controller on the network. With EIA/TIA-485, every con-
troller on the network must have a unique address.
3. Choose a device to communicate with the con-
troller.
The controller can communicate with devices, such
as a computer running a software program, a PLC (Pro-
grammable Logic Controller) or an OIT (Operator Inter-
face Terminal). Whichever device is chosen, it needs to
be able to communicate using the Modbus RTU Protocol.
OITs would need to be ordered with Modbus RTU sup-
port. PLCs would either have Modbus RTU as a standard
feature or it can be made available with an I/O module.
Wat lo w Se rie s S D
On a computer, the software package to be used would
need to have the Modbus RTU capability.
4. Select a software package for the computer.
Select the software package based on what is required
from the application. For basic communications (such
as reading the process value or setting the set point),
Watlow has the Comm7 software package. This is mainly
used for diagnostics and basic communications.
The WATVIEW™ software package offers more ad-
vanced features. WATVIEW™ is available in three edi-
tions, each offering increasing levels of functionality. If
you need functionality beyond WATVIEW™ or need to
interface with an existing software package, many other
third party software packages can interface with the Se-
ries SD.
When purchasing a third-party software package, be
sure to look for a package that is Modbus RTU compat-
ible or has Modbus RTU drivers. Most third-party pack-
ages require you to specify the Modbus registers of the
controller to setup the package.
Another option is to custom-create a software pack-
age. Using the Modbus register and data information in
this user's manual, a software package can be created
and tailored to the desired application. To assist in ap-
plication development, Watlow offers WATCONNECT™,
which is a Windows-based software library for Modbus
RTU communications. For further information on WAT-
VIEW™ software packages, the WATCONNECT™ soft-
ware library, or to download the Comm7 software, go to
the Watlow web site at http://www.watlow.com.
5. Configure the software's communications param-
eters.
A software package, (be it software for a Computer,
a PLC or an OIT) will need to be configured just as the
controller was configured, setting the Baud Rate and Ad-
dress to match. The software package may have addition-
al parameters to set, such as number of data bits, parity
and stop bits. For Watlow controllers using modbus, these
should always be set at 8 data bits, no parity, and 1 stop
bit. This is often written as "8N1". Some software pack-
ages may give the option to control the activity of the
RTS, CTS and DTR lines, which are sometimes used by
EIA-232 to EIA-485 converters. On packages where the
Modbus registers for the controller need to be defined,
these values can be entered at this time. Be sure to ac-
count for offsets.
6. Test the communications.
Once communications is configured, test the link to
the controller for verification that everything is wired
and configured properly. Check the wiring and configura-
tions if things aren't working. One misplaced wire or one
incorrect setting will keep communications from working.
When using an EIA-232 to EIA-485 converter, be sure to
follow the configuration instructions provided with the
converter, as some may require special jumper/switch
settings, external power supply requirements or special
signals from the software. Some software packages have
built-in routines for testing the communications or use
Comm7 to help diagnose problems.
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Chapter 1 2 F e a tur e s
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