Rs485-2 Bias And Termination Resistors; Rs485 Standard - Honeywell CIPer 50 Installation & Commissioning Instructions

CIPer MODEL 50 CONTROLLER – INSTALLATION AND COMMISSIONING INSTRUCTIONS
TM

RS485-2 Bias and Termination Resistors

The RS485-2 interface is not affected by the aforementioned three-position slide switch. The 550Ω bias
resistors and 130Ω termination resistor of the RS485-2 are thus always ON.
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 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. E.g., WEBs 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 CIPer Model 50 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 CIPer Model 50 controllers in already-existent RS485 two-wire
systems (e.g., when replacing Spyder and Stryker controllers with CIPer Model 50 controllers),
not using a separate signal ground wire will probably have no undesirable effects.
The cable length affects the communication rate. Table 6 provides a few examples.
* 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.
31-00233-02
Figure 18: RS485-2 bias and termination resistors
Table 6. Bit rate vs. max. cable length for RS485
Bit rate
9.6 - 76.8 kbps
*115.2 kbps
+5V
550 OHM
30
RS485-2 (+)
130 OHM
31
RS485-2 (-)
550 OHM
GND-2
Max. cable length (L)
3600 ft (1200 m)
2400 ft (800 m)
20