Rockwell-automation 1775-KA PLC-3 Communication Adapter Module User Ma Manual de usuario descargar pdf (Pagina 129)

Half-Duplex Protocol

Chapter 11

112

You must program a computer to serve as a master that controls which

station has access to the link. All other stations are slaves and must wait

for permission from the master before transmitting. Each slave station has

a unique station number from 0 to 376 octal. The number 377 is a

broadcast address. When the master sends a message addressed to 377,

all slaves receive it.

The master can send and receive messages to and from each station on the

multidrop link. If the master is programmed to relay messages, then slave

stations on the multidrop link can engage in peer–to–peer communication.

Your multidrop link may be either a two–circuit system (master sends and

slaves receive on one circuit, slaves send and master receives on the

other), or a one–circuit system (master and slaves send and receive on the

same circuit).

You may use a half–duplex, dial–up modem to connect the 1775–KA

module to the multidrop link. The modem must signal data–carrier–detect

at least once every 8 seconds. If it does not, the module will hang up. On

a dedicated line, you can jumper lines 6,8, and 11 at the 1775–KA module

to prevent the module from hanging up.

You cannot use multiple masters unless one master is limited to acting as

a backup to the other, and does not communicate until the primary is shut

down.

Half–duplex protocol is a character oriented protocol that uses the

following ASCII control characters:

Control

character

Hexdecimal Code

SOH (Start of Header)

STX (Start of T

ext) 02

ETX (End of T

ext) 03

EOT (End of T

ransmission) 04

ENQ (Enquiry)

ACK (Acknowledge)

DLE (Data Link Escape)

NAK (Negative Acknowledge)

Transmission Codes

1 2 ... 124 125 126 127 128 129 130 131 132 133 134 ... 238 239

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Rockwell-automation 1775-KA PLC-3 Communication Adapter Module User Ma Manual de usuario Pagina 129