Allen RS-232 Diagram

201 Publication 1747-UM011G-EN-P - June 2008

Appendix

C

RS-232 Communication Interface

This appendix provides an overview of the RS-232 communication
interface and explains how the SLC 5/03, SLC 5/04, and SLC 5/05
processors support it. This appendix also provides information on:

• RS-232 and SCADA applications.

• RS-232 communication interface overview.

• SLC 5/03, SLC 5/04, and SLC 5/05 processors and RS-232

communication.

• SLC 500 devices that support RS-232 communication.

• DF1 protocol and the SLC 5/03, SLC 5/04, and SLC 5/05 processors.

• Modbus RTU Master communication.

• ASCII communication.

• DF1 communication protocol modems overview.

• wiring connectors for RS-232 communication.

• applications for the RS-232 communication interface.

For online configuration procedures of the SLC 5/03, SLC 5/04, and
SLC 5/05 processors for DF1 protocol, see your programming software
user manual.

RS-232 and SCADA
Applications

RS-232 is a communication interface included under SCADA
(Supervisory Control and Data Acquisition) applications. SCADA is a
term that refers to control applications that require communication
over long distances. For more information about the use of
Allen-Bradley equipment in SCADA applications, refer to the SCADA
System Applications Guide, publication AG-UM008.

RS-232 Communication
Interface Overview

RS-232 is an Electronics Industries Association (EIA) standard that
specifies the electrical, mechanical, and functional characteristics for
serial binary communication.

One of the benefits of RS-232 communication is that it lets you
integrate telephone and radio modems into your control system. The
distance over which you are able to communicate with certain system
devices is virtually limitless.

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The RS-232 channel on the SLC 5/03, SLC 5/04, and SLC 5/05
processors supports various protocols:

• Full-duplex DF1 (default)

• Half-duplex DF1 (SCADA)

• DH-485

• ASCII communication

• DF1 radio modem

• Modbus RTU Master

The SLC and PLC products detailed in this appendix that communicate
over the RS-232 communication interface also use the DF1 serial
communication protocol. DF1 protocol delimits messages, controls
message flow, detects and signals errors, and retries after errors are
detected.

SLC 5/03, SLC 5/04, and SLC
5/05 processors and RS-232
Communication

The SLC 5/03, SLC 5/04, and SLC 5/05 processors can communicate by
means of the RS-232 communication port, channel 0. Channel 0 supports DF1
full-duplex protocol, DF1 half-duplex master and slave protocol, DH485
protocol, Modbus RTU Master communication, ASCII communication, and
DF1 radio modem protocol. Refer to your programming software user manual
for information on configuring the RS-232 communication port, channel 0.

The details of the DF1 protocols can be found in the DF1 Protocol and
Command Set Reference Manual, Publication Number 1770-6.5.16.

Channel 0 provides a minimum of 500V dc isolation between the I/O
signals and the logic ground of the SLC 5/03, SLC 5/04, and SLC 5/05
processors. The channel is a 9-pin D-shell. The table below provides a
description of each of the pins.

The D-shell is the bottom port on the SLC 5/03, SLC 5/04, and SLC 5/05
processors.

Pin Pin Name

1 DCD (Data Carrier Detect)

2 RXD (Receive Data)

3 TXD (Transmit Data)

4 DTR (Data Terminal Ready)

5 COM (Common Return [Signal Ground])

6 DSR (Data Set Ready)

7 RTS (Request to Send)

8 CTS (Clear to Send)

9 NC (No Connection)

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SLC 500 Devices that
Support RS-232
Communication

The SLC 500 product line has two other modules, aside from the SLC 5/03,
SLC 5/04, and SLC 5/05 processors, that support the RS-232 communication
interface. They are the 1746-BAS BASIC module and the 1747-KE
DH-485/RS-232C interface. Both of these modules can be used with either
the SLC 5/01 or SLC 5/02 processor.

1747-KE Module

The 1747-KE module is a communication interface module that acts as a
bridge between DH-485 networks and devices requiring DF1 protocol. You
can configure the DF1 port on the 1747-KE module for RS-232/423, RS-422,
or RS-485 devices. Residing in an SLC 500 chassis, the 1747-KE module is
ideally used as an interface module, linking remote DH-485 networks via a
modem to a central host.

For more information on the 1747-KE module, see the DH-485/RS-232
Interface Module User Manual, publication 1747-IN006.

1746-BAS and 1746-BAS-T Modules

The 1746-BAS and 1746-BAS-T modules, which are programmed using the
BASIC language, have two configurable serial ports for interfacing to
computers, modems, serial printers, and other RS-232 compatible devices.
You can also use them for off-loading complex math routines from an SLC
500 processor, thereby conserving ladder logic memory.

For more information on the 1746-BAS module, see the SLC 500 BASIC and
BASIC-T Module User Manual, publication 1746-UM004.

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DF1 Protocol and the SLC
5/03, SLC 5/04, and SLC 5/05
Processors

DF1 protocol combines data transparency (ANSI - American National
Standards Institute - specification subcategory D1) and 2-way
simultaneous transmission with embedded responses (F1). It is also a
peer-to-peer, link-layer protocol. This means that system devices have
equal access to messages being sent over the RS-232 communication
interface.

DF1 protocol provides two modes of communication: full-duplex and
half-duplex.

DF1 Full-duplex Protocol

DF1 full-duplex protocol (also referred to as DF1 point-to-point
protocol) lets you use RS-232 point-to-point communication in
applications that require it. This type of protocol supports
simultaneous transmissions between two devices in both directions.
You can use channel 0 as a programming port, or as a peer-to-peer
port using the MSG instruction.

In full-duplex mode, the SLC 5/03, SLC 5/04, and SLC 5/05 processors
can send and receive messages. When the SLC 5/03, SLC 5/04, and
SLC 5/05 processors receive messages, they act as an end device, or

final destination for the data packets

(1)

. The processor ignores the destination
and source addresses received in the data packets. However, the processor
swaps these addresses in the reply that it transmits in response to any
command data packet that it has received.

By setting a parameter with your programming software, you can also
make the processor verify that the host computer can receive
embedded responses. To do this, the processor waits to receive an
embedded response from the host computer, before sending one of its
own. A host computer that can send embedded responses should also
be able to receive them.

If you use modems with DF1 full-duplex protocol, make sure that they
are capable of simultaneous bidirectional communication. Typically,
dial-up modems designed to be connected to standard telephone lines
can support full-duplex.

(1)

The exceptions to this are SLC 5/04 and SLC 5/05 processors that have the DH+ to DF1 or Ethernet to DF1
full-duplex passthru bit enabled. In the case of the SLC 5/04, the processor checks the destination address in
the packet and if it does not match the configured DH+ address of the processor, the packet is forwarded onto
the DH+ network to the destination address DH+ node. In the case of the SLC 5/05, the processor checks the
destination address in the packet. If the routing table exists and an IP address is in the routing table for that
DF1 address, the packet is forwarded out to the Ethernet network to that IP address.

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Full-duplex (Point-to-Point)

DF1 Half-duplex Protocol

DF1 half-duplex protocol provides a multi-drop single master/multiple
slave network. In contrast to the DF1 full-duplex protocol,
communication takes place in one direction at a time. You can use
channel 0 as a programming port, or as a peer-to-peer port using the
MSG instruction.

In half-duplex mode, the SLC 5/03, SLC 5/04, and SLC 5/05 processors
can be either master or slave devices. As a master device, the
processor polls each slave on the network on a regular and sequential
basis. The master also supports routing of data packets from one slave
to another, or slave-to-slave communication. As a slave device, the
processor can send data packets when polled by the master device,
which initiates all communication with slave devices.

If the master device has no data to send, it can still receive data from
the slave device. To do this, the master sends out a poll packet
addressed to the slave. If the slave has data to send, it does so in
response to the poll packet. Otherwise, the slave sends a simple
two-byte response, so that the master knows that it is active.

Several Allen-Bradley products support half-duplex master protocol.
They include the Enhanced PLC-5 processors, and SLC 5/03, SLC 5/04,
and SLC 5/05 processors. RSLinx (2.0 or later) software also supports
half-duplex master protocol.

DF1 Half-duplex supports up to 255 slave devices (addresses 0...254)
with address 255 reserved for master broadcasts. The SLC 5/03,
SLC 5/04, and SLC 5/05 processor support broadcast reception. SLC
5/03, SLC 5/04, and SLC 5/05 processors with operating system FRN
C/6 can also initiate broadcast write commands via the MSG

Modem

SLC 5/03
Processor
(1747-L532)

1747-CP3 Cable

Modem

SLC 5/03
Processor
(1747-L532)

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instruction for all channel 0 system mode drivers and for channel 1
(DH-485) on the SLC 5/03 processor.

Either half-duplex or full-duplex modem types can be used for the
master, but half-duplex modems must be used for the slaves
(assuming there is more than one on a multi-drop network).

TIP

Broadcast is not supported for read commands or any remote
messages. Broadcast is also not supported by the SLC 5/04
channel 1 DH+ network or the SLC 5/05 channel 1 Ethernet
network.

Modular Controller with SLC
5/02 Processor and 1747-KE
Interface Module (slave)

Modular Controller with
SLC 5/03 Processor (slave)

Modular Controller with SLC
5/01 Processor and 1747-KE
Interface Module (slave)

Fixed Controller with 1747-KE
Interface Module (slave)

Personal Computer
Running RSLinx with DF1
Half-duplex Protocol
(master)

RS-232 (DF1 protocol)

Modem Modem Modem Modem

Modem

Modular Controller with SLC
5/02 Processor and 1747-KE
Interface Module (slave)

Modular Controller with
SLC 5/03 Processor (slave)

Modular Controller with SLC
5/01 Processor and 1747-KE
Interface Module (slave)

Fixed Controller with 1747-KE
Interface Module (slave)

Modular Controller with
SLC 5/03 Processor
(master)

RS-232 (DF1 protocol)

Modem Modem Modem Modem

Modem

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DF1 Radio Modem Channel 0 Driver

Series C FRN 6 (FRN C/6) and later versions of operating systems
OS302 (SLC 5/03 processor), OS401 (SLC 5/04 processor) and OS501
(SLC 5/05 processor) include a channel 0 system mode driver called
DF1 radio modem. This driver implements a protocol, optimized for
use with radio modem networks, that is a hybrid between DF1
full-duplex protocol and DF1 half-duplex protocol, and therefore is
not compatible with either of these protocols.

Like DF1 full-duplex protocol, DF1 radio modem allows any node to
initiate to any other node at any time (if the radio modem network
supports full-duplex data port buffering and radio transmission
collision avoidance). Like DF1 half-duplex protocol, a node ignores
any packets received that have a destination address other than its
own, with the exception of broadcast packets and passthru packets.

Unlike either DF1 full-duplex or DF1 half-duplex protocols, DF1 radio
modem protocol does not include ACKs, NAKs, ENQs, or poll
packets. Data integrity is ensured by the CRC checksum.

Using the DF1 Radio Modem

The DF1 radio modem driver can be configured as the system mode
driver for channel 0 using RSLogix 500 version 5.50 or later software.

IMPORTANT

The DF1 radio modem driver should only be used among
devices that support and are configured for the DF1 radio
modem protocol. Only SLC 5/03, SLC 5/04 and SLC 5/05
processors with operating systems FRN C/6 support DF1 radio
modem protocol.

IMPORTANT

There are some radio modem network configurations that will
not work with the DF1 radio modem driver. (See DF1 Radio
Modem System Limitations on page 209.) In these
configurations, continue to use DF1 half-duplex protocol.

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Channel O Configuration

The Baud, Parity, Stop Bits and Error Detection selections are identical
to the other DF1 drivers. Valid Node Addresses are 0...254, just like the
DF1 half-duplex drivers.

The primary advantage of using DF1 radio modem protocol for radio
modem networks is in transmission efficiency. Each read/write
transaction (command and reply) requires only one transmission by
the initiator (to send the command) and one transmission by the
responder (to return the reply). This minimizes the number of times
the radios need to key-up to transmit, which maximizes radio life and
minimizes radio power consumption. In contrast, DF1 half-duplex
protocol requires five transmissions for the DF1 master to complete a
read/write transaction with a DF1 slave - three by the master and two
by the slave.

The DF1 radio modem driver can be used in a pseudo master/slave
mode with any radio modems, as long as the designated master node
is the only node initiating MSG instructions, and as long as only one
MSG instruction is triggered at a time.

For modern serial radio modems that support full-duplex data port
buffering and radio transmission collision avoidance, the DF1 radio
modem driver can be used to set up a masterless peer-to-peer radio
network, where any node can initiate communication to any other
node at any time, as long as all of the nodes are within radio range so
that they receive each other’s transmissions.