EAGLEHAWK CONTROLLER – INSTALLATION & COMMISSIONING INSTRUCTIONS
EN1Z-1005GE51 R0216
8
RS485-2 Bias and Termination Resistors
The RS485-2 interface is not affected by the aforementioned
three-position slide switch. The 550
Ω
bias resistors and 130
Ω
termination resistor of the RS485-2 are thus always ON.
550 OHM
130 OHM
550 OHM
+5V
GND-2
RS485-2 (+)
RS485-2 (-)
30
31
Fig. 14. RS485-2 bias and termination resistors
NOTE:
GND-2 is internally connected with 24V-0
(terminal 1)
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
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 EAGLEHAWK 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 EAGLEHAWK
controllers in already-existent RS485 two-wire systems
(e.g., when replacing PANTHER or LION controllers
with EAGLEHAWK controllers), not using a separate
signal ground wire will probably have no undesirable
effects.
The cable length affects the baud rate. Table 6 provides a few
examples.
Table 6. Baud rate vs. max. cable length for RS485
Baud rate
Max. cable length (L)
9.6 - 76.8 kbps
1200 m
*115.2 kbps
800 m
** 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.
Modbus Connection
The EAGLEHAWK controller can function as a Modbus
Master or Slave. In general, the RS485 wiring rules must be
followed.
Wiring Topology
Only daisy-chain wiring topology is allowed.
MODBUS
MASTER
MODBUS
SLAVE
MODBUS
SLAVE
MODBUS
SLAVE
MODBUS
SLAVE
Fig. 15. Allowed Modbus wiring topology
Other wiring topologies (e.g., star wiring, or mixed star wiring
and daisy chain wiring) are prohibited; this is to avoid com-
munication problems of the physical layer.
MODBUS
MASTER
MODBUS
SLAVE
MODBUS
SLAVE
MODBUS
SLAVE
MODBUS
SLAVE
MODBUS
SLAVE
MODBUS
SLAVE
Fig. 16. Prohibited Modbus wiring topology (example)
Cables
See also section "EIA 485 Cable Specifications" on pg. 18.
Use shielded twisted pair cable J-Y-(St)-Y 2 x 2 x 0,8.
You
must
use three wires:
One wire for D1 =
One wire for D0 = Modbus –
One wire for the signal common
When using one pair for D1 and D0 and one wire of another
pair for the signal common, CAT5 cable may also be used.
