Application Note

Application Note
F r o m t h e F l u k e D i g i t a l L i b r a r y @ w w w . f l u k e . c o m / l i b r a r y
Breaking new
ground without
breaking the loop
Communications
measurement
A controls technician receives a
too-hot complaint from a room
on the third floor of a building
on campus. A quick check of the
BAS front-end computer reveals
that the variable air volume
(VAV) box controller is offline,
not communicating. The techni-
cian grabs his new Fluke 773
and heads out to the building.
•
After reaching the room, a
quick check of the VAV box
in the ceiling reveals that the
VAV damper is in the closed
position. The VAV control-
ler normally has an LED that
blinks when the controller is
online. That LED is off.
•
The technician quickly
attaches his Fluke meter to
the RS-485 Communication
bus wiring terminals and
measures the proper voltage,
reading approximately +2.5 V
dc between the three termi-
nals, labeled +, –, and REF.
•
A quick check of the wiring
shows that the communica-
tions connector was not fully
seated, causing a communica-
tions loss.
•
After seating the connector
properly, the VAV box control-
ler comes back online, the
communications LED resumes
blinking, and the VAV damper
begins to stroke open.
•
A call is then placed to the
operator in the control room
and the controller is now
online. The room immediately
begins to cool down.
Troubleshooting HVAC control
systems with the new Fluke
772 and 773 Milliamp Process
Clamp Meters
The new Fluke 772 and 773 Milliamp Process
Clamp Meters are setting new benchmarks
for ease of use. The meters’ feature sets are
designed to make troubleshooting the most
common problems in modern control systems
quick and efficient.
Common applications for these meters
include communications measurement, milliamp
loop sensor measurement, damper and valve
actuator troubleshooting, and troubleshoot-
ing the operation of variable frequency drives
(VFDs). Let’s look at some typical scenarios for
each of these examples.