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metasys n2
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7
7.
METASYS N2
7.1
Metasys N2 Protocol Introduction
The N2 communications protocol is used by Johnson Controls and others to connect terminal unit
controllers to supervisory controllers. It is open to any manufacturer and based upon a simple ASCII
protocol widely used in the process control industry.
The physical characteristics of the N2 bus are three wire RS-485 with a maximum of 100 devices over
a 4,000 foot distance running at 9,600 bps. Logically, the N2 is a master-slave protocol, the supervi-
sory controller normally being the master. Data is partitioned into common HVAC control objects,
such as analogue input, analogue output, binary input and binary output. N2 messaging supports the
reading, writing and overriding of these points. Additionally, there are messages defined to perform
uploads and downloads of devices as well as direct memory reads and writes.
7.2
Metasys N2 interface
Features of the N2 Interface:
•
Direct control of Drive ( e.g. Run, Stop, Direction, Speed reference, Fault reset)
•
Full access to necessary parameters
•
Monitor Drive status (e.g. Output frequency, Output current, Fault code )
•
In standalone operation, or should the polling stop, the overridden values are released after a
specified period (about 10 minutes).
7.2.1
Analogue Input (AI)
All Analogue Input (AI) points have the following features:
•
Support Change of State (COS) reporting based on high and low warning limits.
•
Support Change of State (COS) reporting based on high and low alarm limits.
•
Support Change of State (COS) reporting based on override status.
•
Always considered reliable and never out of range.
•
Writing of alarm and warning limit values beyond the range that can be held by the drive’s in-
ternal variable will result in having that limit replaced by the “Invalid Float” value even though
the message is acknowledged. The net result will be the inactivation of the alarm or warning
(the same as if the original out of range value was used).
•
Overriding is supported from the standpoint that the “Override Active” bit will be set and the
value reported to the N2 network will be the overridden value. However, the value in the drive
remains unchanged. Therefore, the N2 system should be set up to disallow overriding AI
points or have an alarm condition activated when an AI point is overridden.
•
Overriding an AI point with a value beyond the limit allowed by the drive’s internal variable will
result in an “Invalid Data” error response and the override status and value will remain un-
changed.
7.2.2
Binary Input (BI)
All Binary Input (BI) points have the following features:
•
Support Change of State (COS) reporting based on current state.
•
Support Change of State (COS) reporting based on alarm condition.
•
Support Change of State (COS) reporting based on override status.
•
Always considered reliable.
Overriding is supported from the standpoint that the “Override Active” bit will be set and the value re-
ported to the N2 network will be the overridden value. However, the value in the drive remains un-
