Engineering Documentation

Table Of Contents
Chapter 2 Network Electrical Systems
RS-485 MS/TP Communications
36
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125-3002
3-Wire Interface Nodes
NOTE:
The wiring method for devices with a 3-wire interface is the same whether they are
on a BACnet ALN or FLN.
The following table outlines the Siemens Industry devices that were re-released with
3-wire RS-485 network interfaces.
3-Wire RS-485 Network Interface Terminal Wiring (Using 1.5-Twisted Shielded Pair Cable).
Product Name Network
Protocol
2)
Terminal Usage
3)
Terminal for 2-Wire
4)
Network Electrical
Loading
1)
DXR2.M BACnet MS/TP
- +
--- 1/8
BACnet Actuator (550-430, 550-431) BACnet MS/TP
- +
E 1/8
BACnet Short Platform (550-432, 550-433) BACnet MS/TP
- +
E 1/8
BACnet Long Platform (Updated Version)
(550-490, 550-491, and 550-492)
BACnet MS/TP
- +
E 1/8
MSTP-BIM (TXB3.M) BACnet MS/TP
- +
-- 1/8
PXC Compact ALN/FLN (P2,
P1, MS/TP)
- +
1/8
PXC Modular ALN/FLN (P2,
P1, MS/TP)
S - +
1/8
P1-BIM (TXB1.P1, TXB1.P1-4) FLN (P1)
S - + --- 1/8
1)
RS-485 spec allows for 32 electrical loads on a section of network cabling (a network repeater allows for more devices). Electrically 32
full loads (factor 1) have same resistance as 256 x 1/8 load devices.
2)
RS-485 communication traffic and speed will limit number of MSTP devices per ALN/FLN, refer to BACnet Application Guide. Typical limit
is 10 devices for MSTP ALN while limit is 50 devices per MSTP FLN at 76800 bps.
3)
RS-485 network common may be marked with S, but functions as .
4)
Terminal must be connected to earth ground for compatibility with 2-wire (1-Twisted Shielded Pair) cable.
1.5-Pair Network Cable
The network cable recommended for use with the 3-wire (isolated RS-485 common) is
a single pair cable with third wire (1.5-pair) that is used to tie the RS-485 reference
(communication common) of all the nodes on the network together.
All the Siemens Industry products listed in the Table 3-Wire RS-485 Network Interface
Terminal Wiring (Using 1.5-Pair Cable) use the 3-wire interface.
By providing the RS-485 ground signal of the interface to the network termination
plug, all node communication ports can be referenced together providing a high
degree of noise immunity.
The RS-485 common reference wire is terminated at one point (and only one
point) to earth ground.
An overall foil shield and drain wire provide additional noise protection.