Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 129)

Half-Duplex Protocol
Chapter 11
11Ć2
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) 01
STX (Start of Text) 02
ETX (End of Text) 03
EOT (End of Transmission) 04
ENQ (Enquiry) 05
ACK (Acknowledge) 06
DLE (Data Link Escape) 10
NAK (Negative Acknowledge) 15
Transmission Codes