Multipoint Alarm Mode Response; Watchdog Timer For Communication - Omega DP41-B Series User Manual

9. Meter Bus Response

9.3 MULTIPOINT ALARM MODE RESPONSE

The meter is commanded into the ALARM mode by detecting the alarm code–E03 after
its own address or the common 00 address. It stays silent unless an alarm already
exists. If an alarm is detected, it immediately transmits its own address with its alarm
status characters. No turnaround time delay is used. All devices on the bus which are
in the ALARM mode drop out of that mode if any character is detected on the bus, so
that bus contention is minimized.
The program for the controlling device (computer) is usually written to poll the alarm
status of all devices once bus activity is detected after an alarm command, so that
nearly-simultaneous alarm conditions are properly detected. Such a meter poll by the
computer after an alarm response usually starts at the detected device address, or at
device 01 in case of a garbled address (if two or more devices alarm at once).
If there is adequate time allowance in the data acquisition program, the computer may
poll the alarm status of all meters before commanding the ALARM mode to reduce the
probability of bus contention and garbled transmissions.
a) Slow mode = 0 to 300 ms
b) fast mode = 0 to 100 ms
Although transmission's program delay seems longer than chunk
transmission, in most cases total transmission time will be shorter, specially
using 19200 baud rate.

9.4 WATCHDOG TIMER FOR COMMUNICATION

There is an eight second watchdog timer available in the communication portion of the
meter. When the computer starts sending a command to the meter, meter enables its
receive mode and starts getting the incoming characters, until a carriage return
completes the command string. Then the meter disables its receive mode and enables
its transmission mode to transmit or act according to the command information.
Receive mode time is the period that the meter is in receive mode. Receive time
should always be less than eight seconds, otherwise meter will ignore the command
and will finish the receive mode (there will be no affect on the performance of the
meter). In this case command should be executed again.
Watchdog timer is disabled when meter is in continuous communication
mode.
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