User Manual PLC-3 FAMILY I/0 Instruction Manual

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 1773KA module specify whether the PLC4 station
accepts unprotected or protected commands, respectively
through the:
2 and 3 (on the second
row of switches)
DH port of the 1773KA module.
1 and 3 (on the third row
of switches)
RS232C port of the 1773KA 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