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Publication S130E
Issue 1.0
Pakscan IIS Technical Manual
The master station has the facility to double the loop Baud rate. If this facility is enabled, via the Keypad, the
master station will issue a broadcast command to all the field units to get them to double the baud rate they are
using, once it has detected that the loop is complete.
6.2.2 Normal Operation
In normal operation, the master station polls each field unit for any changed data it has to report. Most of the
time, field units have no changes to report, and traffic on the loop is kept to a minimum. Field units are polled
in address order. Data which might be reported includes changes to digital inputs, alarm changes, or
significant changes to an analogue input.
If a field unit which responded on the last scan fails to respond correctly, the master station will retry twice to
get a reply. On subsequent scans only a single attempt is made to get a response.
All field units up to the address preset in the master station database are polled. Thus new field units which
are powered-up on a working loop will be 'found' by the master station provided they are within the selected
address range.
Commands to field units (open, close, etc.) are interleaved between polls.
A check is made at the end of each scan to see if a loop fault has occurred. For a complete loop (no
loopbacks), a test message is sent round the loop; for a broken loop (loopbacks in use), communications
failure to the last field units is taken as an indication of a fault. In the event of a fault, loop re-configuration is
initiated.
6.3 Modbus Serial Interfaces
The Pakscan IIS master station may be configured with two Modbus serial interfaces, or one Modbus interface
and one serial printer interface. In the former case, the two Modbus interfaces perform similarly.
The master station's use of the Modbus protocol is described in detail in the protocol documents listed in
section 1 (Related Documents). The master station may operate differing protocols from each of its two ports.
The protocols available are indicated on the set up screen display, (see screen displays section).
The two serial ports can both issue commands to field units via the master station, and to the master station
itself. Three separate registers of field unit alarms are maintained by the master station, one for each serial
port and one for the front panel display. This ensures that alarms cannot be missed by any of the
interfaces.
If the host system is reading alarms from the data bases it must consider each one separately
and perform the read and accept routines for each link. Accepting an alarm on the serial
interface does not accept the alarm on the display, or vice versa.
6.4 Local User Interface (LCD and Keypad)
The Local User Interface allows the master station to be interrogated directly for its own status and diagnostic
information, and for field unit status. Various parameters may also be configured from the front panel and
commands sent to field units.
