Programming the Local Host
Chapter 5
524
Normal Operation
Command execution begins when the ladder-diagram program turns on a
start bit, normally with a latch instruction. The PLC-2/DHII interface
detects the ON state of this bit and then begins the operation necessary to
format and transmit a command message.
When the remote node interface receives the command message, it
acknowledges it. Then, while normal operation continues, the remote
node interface reacts to the command and prepares a reply message
(except for a write with no reply). The remote node interface waits until it
receives the token, then transmits its reply message to the local (calling)
station.
When the local PLC-2/DHII interface receives the reply, it turns on the
done bit at the host processor. The done bit, in turn, is examined in the
ladder-diagram program to turn off the start bit.
After the start bit has been turned off, the PLC-2/DHII interface resets the
done bit.
Timing of start and done bits for a command is shown in Figure 5.14. The
significance of start/done bit status is summarized in Table 5.A.
Figure 5.14
Start/Done Bit Timing Normal Operation
ÉÉÉÉÉÉÉÉÉÉÉÉÉ
ÉÉÉÉÉÉÉÉÉÉÉÉÉ
ÉÉÉÉÉÉÉÉÉÉÉÉÉ
Start Bit ON
OFF
Host Proc PLC2/DHII
Interface Scan
essor Scan
Command/
Response
Time
Done Bit ON
OFF
ÉÉÉÉÉÉÉÉÉÉÉ
ÉÉÉÉÉÉÉÉÉÉÉ
LEGEND: Start Bit turned ON by the program.
Done Bit set ON by the PLC2/DHII interface to indicate
that a command has been completed.
Start Bit turned OFF by the program.
Done Bit set OFF by the PLC2/DHII interface after it senses
that the Start Bit has been set OFF.
A
B
C
D
A
B
C
D
