Rockwell Automation 1775-S5_SR5,D17756.5.5 User Manual PLC-3 FAMILY I/0 - Manual (Page 72)

DH and DH+ Communication
Chapter 4
4-12
PLC-4 Stations
PLC-4 stations can only communicate on a DH. To read or write to a
PLC-4 station, you can send either protected or unprotected commands.
Switches: at the 1773ĆKA module specify whether the PLCĆ4 station
accepts unprotected or protected commands, respectively
through the:
2 and 3 (on the second
row of switches)
DH port of the 1773ĆKA module.
1 and 3 (on the third row
of switches)
RSĆ232ĆC port of the 1773ĆKA module.
In all cases, if the switch is set to the closed position, the module accepts
that type of command.
PLC-5 Stations
The PLC-3 processor can read/write to a PLC-5 family processor in
two ways:
as if the PLC-5 processor was a PLC-2 family processor. See “PLC-2
Stations,” page 4-10 for detailed information.
using logical ASCII data type addressing (See chapter 5).
The difference between the communication links is the means of
communication. DH link uses a floating-master function to handle
communication between stations. This function allows a station to access
the DH based on 3 factors:
station’s readiness to transmit a message
message priority
station number
In this way, the floating-master function prevents any single station from
“hogging” the DH link, and it enables the stations to become disabled.
The DH+ protocol uses a token-passing function to handle communication
between stations. The token is constantly passed from station to station
even if no messages are sent. Each station obtains time to send a message.
A station becomes master when it obtains the token. Then it can send a
message to another station. When a station is done communicating, the
token automatically passes to the next higher station number on the link.
DH+ communication is faster than DH communication for configurations
of 16 stations or less. The delay time for link access increases as you
increase the number of stations on the DH+ link.
Choosing Between DH or
DH+ Communication