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Rs485 Standard - Honeywell Centra Line EAGLE Installation & Commissioning Instructions

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EAGLE CONTROLLER – INSTALLATION & COMMISSIONING INSTRUCTIONS
RS485-2 (+)
RS485-2 (-)
Fig. 14. RS485-2 bias and termination resistors
NOTE:
GND-2 is internally connected with 24V-0 (terminal
1) and system GND (terminals 19+37)

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 com-
municating 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. E.g.,
CentraLine devices have as little as ¼ unit load each, so that
up to 128 devices can be connected.
BACnet MS/TP connections to the RS485 interfaces must
comply with the aforementioned RS485 standard. Thus, it is
recommended that each end of every connection be equipped
with one termination resistor having a resistance equal to the
cable impedance (120 Ω / 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
EN1Z-0970GE51 R1112
+5V
ISO
550 OHM
30
130 OHM
31
550 OHM
GND-2
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 grounding
exists.
IMPORTANT
In the case of new EAGLE controller installations, we
strongly recommend using a separate signal ground
wire. Doing otherwise may possibly lead to
unpredictable behavior if other electrically non-isolated
devices are connected and the potential difference is
too high.
In the case of the installation of EAGLE controllers in
already-existent RS485 two-wire systems (e.g., when
replacing PANTHER or LION controllers with EAGLE
controllers), not using a separate signal ground wire
will probably have no undesirable effects.
The cable length affects the baudrate. The following table
provides a few examples.
Table 6. Baud rate vs. max. cable length for RS485
Baud rate
9.6 - 76.8 kbps
*115.2 kbps
** In the case of configuration of RS485-2 for Panel Bus, the
communication rate is set to 115.2 kbps.
For information on wire gauge, max. permissible cable length,
possible shielding and grounding requirements, and the max.
number of devices which can be connected to a bus, refer to
standard EIA-485.
8
Max. cable length (L)
1200 m
800 m

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