3.7. RS485 Network
The following simplified rules apply to a given system consisting of master and slave devices. For
more complex configurations, please refer to standard wiring specification rules for the RS485
network.
The maximum system capacity is 4,000 ft. (1219.20 m) of communication cable and 32 devices.
Make sure the twisted pair wire is recommended for RS485 network use. For reference, review
the RS485 wiring spec for wiring specifications.
Tie the communication cable shield to ground only once at the RS485 master device.
Table 1. Cable Specifications.
Terminal Port Name
Port
Type
Port
Connection
Cable Length Cable Type
Max
Baud
Rate
Max
Number
of
Devices
End of Line
Termination
Star Tap Twisted Pair
Sensitivity
CM5
COM0RS485 RS485 Terminal
Plug
2,000 ft / 610 m Shielded Twisted-
Pair
115 K 16
120 ohms NO NO YES
CM3
COM1RS485 RS485 Terminal
Plug
**4,000 ft /
1,219.20 m
Shielded Twisted-
Pair
115 K 32
120 ohms NO NO YES
CE3
COM2RS485 RS485 Terminal
Plug
**4,000 ft /
1,219.20 m
Shielded Twisted-
Pair
115 K 32
120 ohms NO NO YES
CE4
COM3RS232 RS232
DB9
50 ft / 15 m
Shielded Cable
115 K 2(1:1) na
NO NO YES
CE1
LAN/WAN
100Fx ST
1312.33 ft /
400 m
Multimode 62.5 m 100 M 2(1:1) na
NO NO NO
CE2
LAN/WAN
10/100 T RJ45
328 - 492 ft /
100-150 m
CAT5 Shielded
Twisted Pair / UTP*
100 M 2(1:1) na
NO NO NO
CM1
Local Con-
figuration
10/100 T RJ45
98 ft / 30 m
CAT5 Shielded
Twisted Pair / UTP
100 M 2(1:1) na
NO NO NO
**RS485 maximum cable length is dictated by baud rate. Above 38.4 K baud, the length is restricted to 2,000 ft (609.6 m).
3.8. RS485 Cable Characteristics
Make sure the twisted pair wire is recommended for RS485 applications:
• Single twisted pair for data 18-24 AWG
• Shield consiting of aluminized Mylar & partial coverage braid with drain wire
• Characteristic impedance ~ 120 ohms
• Data L-L capacitance ~12pf/Ft.
• Typical ratings are 75C and 300V NEC CM
• Belden Data Tray series 3074F is 600 V NEC TC
3.9. RS485 Wiring Basics
The maximum system capacity is 4000 ft. (1219.2 m) of cable and 32 devices. The baud rate can
restrict the maximum cable length.
• Wiring is done in a strict daisy chain without taps or stars.
• 120 ohm 1/4W end of Line Termination Resistors (EOLTR) are required at each end of
the cable.
• Attach like terminals to like terminal types for each RS485 port being networked.
• Tie the communication cable shield wire to ground at one location only.
• Data line definition - as per the RS485 standard - in an idle marked state (logic 1) Data
B (+) will be electrically > than Data A (-).
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3. Mounting and Wiring