COMPACT VAV CONTROLLER WEB-VA423B24N – INSTALLATION INSTRUCTIONS
Sylk™ Bus capable wall modules such asTR40x /
T42x can be connected to the controller's Sylk™
Interface (terminals 7 and 8).
The Sylk™ Bus is single pair, and polarity-
Maximum current provided at the Sylk™
Bus interface: 96 mA.
The maximum number of wall modules
depends on the following wall module
1. Sylk™ bus power consumption
2. Number of parameters used
3. Total config file size
The IRM NX tool has an inbuilt resource calculator
to calculate the amount of Sylk™ wall modules.
The Sylk™ devices supported by the WEB-
VA423B24N are; TR40, TR40-H, TR40-CO2, TR40-
H-CO2, TR42, TR42-H, TR42-CO2, TR42-H-CO2,
TR71, TR71-H, TR75, TR75-H, TR120, TR120H wall
modules and C7400S Sylk™ sensor.
Single twisted pair, Non-
Shielded, Stranded or Solid
18 - 22 AWG
(0.33 to 0.82 mm
As a rule of thumb, single twisted pair (two wires
per cable, only), thicker gauge, non-shielded cable
yields the best results for longer runs.
The 100 ft (30 m) distance for standard
thermostat wire is conservative but is meant to
reduce the impact of any sources of electrical noise
(incl. but not limited to VFDs, electronic ballasts,
etc.). Shielded cable recommended only if there is a
need to reduce the effect of electrical noise.
These distances apply also for shielded twisted
The RS485 Standard
According to the RS485 standard (TIA/EIA-485:
"Electrical Characteristics of Generators and
Receivers for Use in Balanced Digital Multipoint
Systems"), only one driver communicating via an
RS485 interface may transmit data at a time.
Further, according to U.L. requirements, each
RS485 interface may be loaded with a max. of 32
unit loads. For example, if a controller utilizes as
unit load each, up to 256 devices can be
wire shielded or Non-
18 - 24 AWG
(0.20 to 0.82 mm
BACnet connections to the RS485 interfaces must
comply with the RS485 standard. Thus, it is
recommended that each end of every bus should
be equipped with a termination resistor (not
included in shipment) having a resistance equal to
the cable impedance (120 Ω; the wattage should
be in the range of 0.25 – 0.5 W).
RS485 systems frequently lack a separate signal
ground wire. However, the laws of physics still
require that a solid ground connection be provided
for in order to ensure error-free communication
between drivers and receivers – unless all of the
devices are electrically isolated, and no earth
A separate signal ground wire must be used.
Failing to obey this requirement can lead to
unpredictable behavior if other electrically
non-isolated devices are connected and the
potential difference is too high.
EIA 485 Cable Specifications
The following cable specification is valid for BACnet
MS/TP EIA 485 buses.
Table 6. EIA 485 cable specifications
The following cables fulfill this requirement:
Shielded, twisted pair cable J-Y-(St)-Y 4 x 2
x 0.8 mm
CAT 5,6,7 cable - use only one single pair
for one bus
Belden 9842 or 9842NH.
4000 ft (9.6–78.8 kbps) or
2600 ft (115.2 kbps).
Twisted pair shielded (foil
or braided shields are
Less than 100 pF per
meter (30 pF per foot)
Less than 200 pF per
meter (60 pF per foot)