Programming the Local Host
Chapter 5
536
interface to monitor command execution time without using timer (TON)
instructions in the program.
Figure 5.23 shows the significance of the timeout preset interval. From
the time the start bit is set ON, the PLC-2/DHII interface must set either a
done bit or a remote or local fault bit within the timeout preset interval.
Should the PLC-2/DHII interface detect no done or fault bit response
within the preset interval, a fault is assumed. As a response to this type of
timeout situation, the PLC-2/DHII interface turns on the local fault bit and
enters the values B8 or 37 in the lower byte of the error word defined by
the header rung.
Figure 5.23
Timeout Preset Significance
ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ
ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ
ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ
Start Bit ON
OFF
Header Rung
(Representation)
Time
Local Fault Bit
Set if Timeout Occurs
ON
OFF
Remote
Fault Bit
Done Bit ON
OFF
L
XBF
G
G
QQQ
G
Code
Optional GET
Local
ON
OFF
Fault Bit
Code BF" Entered
if Timeout Occurs
Time Preset Interval
Within This Period
Done or Fault Bit Expected
ÉÉÉÉ
ÉÉÉÉ
Timeout preset monitoring is intended as a backup for the other
communication monitoring functions of the PLC-2/DHII interface. It is
designed to signal any condition where the PLC-2/DHII interface has not
completed its communication with another node or detected some fault
condition within a short time. Because this timer is primarily intended as
a backup for some local fault type of situation, its preset value is not
critical.
