176461 User Manual.book

MicroLogix 1500 Programmable Controllers User Manual

DF1 Half-Duplex Protocol

DF1 Half-Duplex protocol provides a multi-drop single master/multiple slave network. DF1 Half-Duplex protocol supports data transparency (American National

Standards Institute ANSI - X3.28-1976 specification subcategory D1). In contrast to

DF1 Full-Duplex, communication takes place in one direction at a time. You can use the RS-232 port on the MicroLogix 1500 as both a Half-Duplex programming port, and a Half-Duplex peer-to-peer messaging port.

DF1 Half-Duplex Operation

The master device initiates all communication by “polling” each slave device. The slave device may only transmit message packets when it is polled by the master. It is the master’s responsibility to poll each slave on a regular and sequential basis to allow slave devices an opportunity to communicate. During a polling sequence, the master polls a slave either repeatedly until the slave indicates that it has no more message packets to transmit or just one time per polling sequence, depending on how the master is configured.

An additional feature of the DF1 Half-Duplex protocol is that it is possible for a slave device to enable a MSG instruction in its ladder program to send or request data to/ from another slave. When the initiating slave is polled, the MSG instruction is sent to the master. The master recognizes that the message is not intended for it, but for another slave, so the master immediately forwards the message to the intended slave.

This slave-to-slave transfer is a function of the master device and is also used by programming software to upload and download programs to processors on the DF1

Half-Duplex link.

The MicroLogix 1500 can only act as a slave device. A device that can act as a master is required. Several Allen-Bradley products support DF1 Half-Duplex master protocol. They include the SLC 5/03™ and higher, and enhanced PLC-5

®

processors.

Rockwell Software WINtelligent LINX™ and RSLinx (version 2.x and higher) also support DF1 Half-Duplex master protocol.

DF1 Half-Duplex supports up to 255 devices (address 0 to 254) with address 255 reserved for master broadcasts. The MicroLogix 1500 supports broadcast reception but cannot initiate a broadcast command. The MicroLogix 1500 supports Half-

Duplex modems using RTS/CTS hardware handshaking.

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Understanding the Communication Protocols

When the system driver is DF1 Half-Duplex Slave, the following parameters can be changed:

Table 25-2: DF1 Half-Duplex Configuration Parameters

Parameter

Baud Rate

Options

300, 600, 1200, 2400, 4800, 9600, 19.2K, 38.4K

Parity

Source ID (Node

Address)

Control Line none, even

0 to 254 decimal no handshaking, Half-Duplex modem handshaking

1200 none

1

Default

Error Detection

EOT Suppression

Duplicate Packet

(Message) Detect

CRC, BCC enabled, disabled

When EOT Suppression is enabled, the slave does not respond when polled if no message is queued. This saves modem transmission power when there is no message to transmit.

enabled, disabled

Detects and eliminates duplicate responses to a message. Duplicate packets may be sent under noisy communication conditions if the sender’s Message Retries are not set to 0.

Poll Timeout (x20 ms) 0 to 65535 (can be set in 20 ms increments)

Poll Timeout only applies when a slave device initiates a MSG instruction. It is the amount of time that the slave device waits for a poll from the master device. If the slave device does not receive a poll within the Poll Timeout, a MSG instruction error is generated, and the ladder program needs to requeue the MSG instruction. If you are using a MSG instruction, it is recommended that a Poll Timeout value of zero not be used. Poll Timeout is disabled when set to zero.

RTS Off Delay (x20 ms)

RTS Send Delay (x20 ms)

0 to 65535 (can be set in 20 ms increments)

Specifies the delay time between when the last serial character is sent to the modem and when RTS is deactivated. Gives the modem extra time to transmit the last character of a packet.

0 to 65535 (can be set in 20 ms increments)

Specifies the time delay between setting RTS until checking for the CTS response. For use with modems that are not ready to respond with CTS immediately upon receipt of RTS.

Message Retries

Pre Transmit Delay

(x1 ms)

0 to 255

Specifies the number of times a slave device attempts to resend a message packet when it does not receive an ACK from the master device. For use in noisy environments where message packets may become corrupted in transmission.

0 to 65535 (can be set in 1 ms increments)

• When the Control Line is set to no handshaking, this is the delay time before transmission. Required for 1761-NET-AIC physical Half-Duplex networks. The 1761-

NET-AIC needs delay time to change from transmit to receive mode.

• When the Control Line is set to DF1 Half-Duplex Modem, this is the minimum time delay between receiving the last character of a packet and the RTS assertion.

no handshaking

CRC disabled enabled

50

0

0

3

0

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MicroLogix 1500 Programmable Controllers User Manual

Rockwell Software WINtelligent LINX,

RSLinx 2.0 (or higher), SLC 5/03, SLC

5/04, and SLC 5/05, or PLC-5 processors configured for DF1 Half-

Duplex Master.

RS-232 (DF1 Protocol)

MicroLogix 1500

Programmable

Controller

SLC 5/03 Processor

Modular Controller

MicroLogix 1500

Programmable Controllers

SLC 500 Fixed I/O

Controller with 1747-KE

Interface Module

Note:

It is recommended that isolation (1761-NET-AIC) be provided between the

MicroLogix 1500 and the modem.

Considerations When Communicating as a DF1 Slave on a Multi-drop Link

When communication is between either your programming software and a

MicroLogix 1500 Programmable Controller or between two MicroLogix 1500

Programmable Controllers via a slave-to-slave connection on a larger multi-drop link, the devices depend on a DF1 Master to give each of them polling permission to transmit in a timely manner. As the number of slaves increases on the link (up to 254), the time between when your programming software or the MicroLogix 1500

Controller is polled also increases. This increase in time may become larger if you are using low baud rates.

As these time periods grow, the following values may need to be changed to avoid loss of communication:

• programming software: increase poll timeout and reply timeout values

• MicroLogix 1500 Programmable Controller: increase poll timeout

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Understanding the Communication Protocols

Ownership Timeout

When a program download sequence is started by a software package to download a ladder logic program to a MicroLogix 1500 controller, the software takes “program ownership” of the processor. Program ownership prevents other devices from reading from or writing to the processor while the download is in process. Once the download is completed, the programming software returns the program ownership to the controller, so other devices can communicate with it again.

The controller clears the program ownership if no supported commands are received from the owner within the timeout period. If the program ownership were not cleared after a download sequence interruption, the processor would not accept commands from any other device because it would assume another device still had program ownership.

Important:

If a download sequence is interrupted, due to electromagnetic interference or other events, discontinue communications to the controller for the ownership timeout period and then restart the program download. The ownership timeout period is 60 seconds.

After the timeout, you can re-establish communications with the processor and try the program download again. The only other way to remove program ownership is to cycle power on the processor.

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MicroLogix 1500 Programmable Controllers User Manual

Using Modems with MicroLogix 1500 Programmable Controllers

The types of modems that you can use with MicroLogix 1500 controllers include dialup phone modems, leased-line modems, radio modems and line drivers.

For point-to-point Full-Duplex modem connections that do not require any modem handshaking signals to operate, use DF1 Full-Duplex protocol. For point-to-point

Full-Duplex modem connections that require RTS/CTS handshaking, use DF1 Full-

Duplex protocol.

For multi-drop modem connections, or for point-to-point modem connections that require RTS/CTS handshaking, use DF1 Half-Duplex slave protocol. In this case, one

(and only one) of the other devices must be configured for DF1 Half-Duplex master protocol.

Important:

Never attempt to use DH485 protocol through modems under any circumstance.

Note:

All MicroLogix 1500 controllers support RTS/CTS modem handshaking when configured for DF1 Full-Duplex protocol with the control line parameter set to Full-Duplex Modem Handshaking or DF1 Half-Duplex slave protocol with the control line parameter set to “Half-Duplex Modem”.

No other modem handshaking lines (i.e. Data Set Ready, Carrier Detect and

Data Terminal Ready) are supported by any MicroLogix 1500 controllers.

Dial-Up Phone Modems

Dial-up phone line modems support point-to-point Full-Duplex communications.

Normally a MicroLogix 1500 controller, on the receiving end of the dial-up connection, will be configured for DF1 Full-Duplex protocol with the control line parameter set for Full-Duplex modem. The modem connected to the MicroLogix

1500 controller must support auto-answer. The MicroLogix 1500 has no means to cause its modem to initiate or disconnect a phone call, so this must be done from the site of the remote modem.

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Understanding the Communication Protocols

Leased-Line Modems

Leased-line modems are used with dedicated phone lines that are typically leased from the local phone company. The dedicated lines may be in a point-to-point topology supporting Full-Duplex communications between two modems or in a multi-drop topology supporting Half-Duplex communications between three or more modems. In the point-to-point topology, configure the MicroLogix 1500 controllers for DF1 Full-Duplex protocol. In the multi-drop topology, configure the MicroLogix

1500 controllers for DF1 Half-Duplex slave protocol with the control line parameter set to “Half-Duplex Modem”.

Radio Modems

Radio modems may be implemented in a point-to-point topology supporting either

Half-Duplex or Full-Duplex communications, or in a multi-drop topology supporting

Half-Duplex communications between three or more modems. In the point-to-point topology using Full-Duplex radio modems, configure the MicroLogix 1500 controllers for DF1 Full-Duplex protocol. In the point-to-point topology using Half-

Duplex radio modems, or multi-drop topology using Half-Duplex radio modems, configure the MicroLogix 1500 controllers for DF1 Half-Duplex slave protocol. If these radio modems require RTS/CTS handshaking, configure the control line parameter to “Half-Duplex Modem”.

Line Drivers

Line drivers, also called short-haul “modems”, do not actually modulate the serial data, but rather condition the electrical signals to operate reliably over long transmission distances (up to several miles). Allen-Bradley’s AIC+ Advanced

Interface Converter is a line driver that converts an RS-232 electrical signal into an

RS485 electrical signal, increasing the signal transmission distance from 50 to 4000 feet.

In a point-to-point line driver topology, configure the MicroLogix 1500 controller for

DF1 Full-Duplex protocol. In a multi-drop line driver topology, configure the

MicroLogix 1500 controllers for DF1 Half-Duplex slave protocol. If the line drivers that are used require RTS/CTS handshaking, configure the Control Line parameter to

Half-Duplex Modem.

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