Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual User Manual
Page 65
Asynchronous Link Protocols
Chapter 4
4-22
One station is designated as master, and it controls which station has
access to the link. You can use a 1771-KGM PLC-2 Family module as
the master, or this function can be performed by your own programmed
intelligent device. All other stations are called slaves and must wait for
permission from the master before transmitting. Each slave station has a
unique station number from 0 to 254.
The master can send and receive messages to and from each station on the
multi-drop. The master can send and receive messages to and from every
station on highways that are connected to the multi-drop via a KF2
module. If the master is programmed to relay messages, then stations on
the multi-drop can engage in peer-to-peer communications. The
multi-drop will not easily provide flexible peer-to-peer communication
between the multi-drop and connected highways, or between different
highways.
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) can be
used.
Half-duplex dial-up modems can be used as long as a carrier is detected
before the carrier timeout (about 8 seconds). If a carrier is not sensed
before the timeout, the module will hang up the phone. Carrier must be
sensed at least every 8 seconds to maintain the connection.
Multiple masters are not allowed, except when one acts as a backup to the
other and does not communicate unless the primary is shut down.
Transmission Codes
Half-duplex protocol is a character-oriented protocol that uses the
following ASCII control characters extended to 8 bits by adding a zero for
Bit 7. See ANSI X3.4, CCITT V.3, or ISO 646 for the standard definition
of these characters.
Control Character
Hexadecimal Code
SOH (Start of Header)
STX (Start of Text)
ETX (End of Text)
EOT (End of Transmission)
ENQ (Enquiry)
ACK (Acknowledge)
DLE (Data Link Escape)
NAK (Negative Acknowledge)
01
02
03
04
05
06
10
15