Modbus Rtu Serial Communications User Manual
Overview
8/01 Modbus® RTU Serial Communications iii Configuration/Remote Calibration Interfaces for 10260S About This Document Abstract This document provides configuration information specific to Honeywell’s 10260S Smart Actuators and should be used in conjunction with Modbus® RTU Serial Communications User Manual, document number 51-52-25-66. MELSEC iQ-F FX5 User's Manual (Serial Communication) Describes N:N network, MELSEC Communication protocol, inverter communication, non-protocol communication, and predefined protocol support. MELSEC iQ-F FX5 User's Manual (MODBUS Communication) (This manual) Describes MODBUS serial communication. 2 Modbus RTU Master Communications for Series 90 and VersaMax PLCs User's Manual GFK-2220F 1.3 COMMREQs The application program running in the PLC CPU controls the timing and content of each query by sending a COMMREQ message. The COMMREQ must be addressed to the CPU serial port that is connected to the Modbus RTU serial network.
A Series SD controller can also be programmed and
monitored by connecting it with a personal computer or
programmable logic controller (PLC) via serial commu-
nications. To do this it must be equipped with an
EIA/TIA 485 (SD_ _ - _ _ U_ - _ _ _ _) communications
option for Output 2. Your PC or PLC must have avail-
able an EIA/TIA-485 interface or use an EIA/TIA-232 to
EIA/TIA-485 converter. See “Selecting an EIA/TIA-232
to EIA/TIA-485 converter” in Chapter 2. The EIA/TIA-
485 option directly supports communication with up to
32 devices on a network or up to 247 devices using a
485 repeater.
To view or change controller settings with a personal
computer, you need to run software that uses the Mod-
bus RTU protocol to read or write to registers in the
controller. 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 sys-
tem. The address in the tables have been offset by sub-
tracting 40,001 from each one.
Basic communications settings must first be config-
ured on the controller in the Setup Page. Match the
Baud Rate
[baUd] to that of the computer and select a
unique Address
[Addr] for each Series SD.
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 com-
municate using a EIA-485 port is with an EIA/TIA-232
to EIA/TIA-485 converter.
The advantages of EIA/TIA-485 are that it is less
susceptible to noise and it allows a PC or PLC to com-
municate 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 communi-
cations.
Some newer PCs may only have a USB port. USB-to-
Modbus Rtu Serial Communications User Manual
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
Address. The choices for Baud Rate are 9600 bps, 19200
bps or 38400 bps. 38400 baud allows for the fastest com-
munication. For compatibility with other devices, reduc-
ing 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 controller 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 support. PLCs would either have Modbus RTU as
a standard feature or it can be made available with an
I/O module. 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 re-
quired from the application. For basic communications
(such as reading the process value or setting the set
point), Watlow has the Comm6 software package. This
is mainly used for diagnostics and basic communica-
tions.
When purchasing a third-party software package, be
sure to look for a package that is Modbus RTU compati-
ble 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 the
WATCONNECT™ software library, or to download the
Comm6 software, go to the Watlow web site at
http://www.watlow.com.
5. Configure the software’s communications
parameters.
A software package, (be it software for a computer, a
Modbus Rtu Communication
PLC or an OIT) will need to be configured just as the
controller was configured, setting the Baud Rate and
Address to match. The software package may have ad-
ditional parameters to set, such as number of data bits,
parity and stop bits. For Watlow controllers using mod-
bus, these should always be set at 8 data bits, no parity,
and 1 stop bit. This is often written as “8N1”. Some soft-
ware packages may give the option to control the activi-
ty 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 account for offsets.
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C h a p t e r 9 F e a t u r e s
