Connecting Multiple Rs485 Devices - Siemens RUGGEDCOM RS416 Installation Manual

Chapter 3
Communication Ports
NOTE
Pins 7 and 8 are connected internally. In RS232 mode, these pins enter a high impedance state. A
DTE that asserts RTS will see CTS asserted. However, the device will not perform hardware flow
control.
Section 3.4

Connecting Multiple RS485 Devices

Each RS485 port can communicate with multiple RS485 devices by wiring devices together in sequence over a
single twisted pair with transmit and receive signals on the same two wires (half duplex). For reliable, continuous
communication, adhere to the following guidelines:
• To minimize the effects of ambient electrical noise, use shielded cabling.
• The correct polarity must be observed throughout a single sequence or ring.
• The number of devices wired should not exceed 32, and total distance should be less than 1219 m (4000 ft) at
100 kbps.
• The Common terminals should be connected to the common wire inside the shield.
• The shield should be connected to earth ground at a single point to avoid loop currents.
• The twisted pair should be terminated at each end of the chain.
The following shows the recommended RS485 wiring.
2
1
Figure 26: Recommended RS485 Wiring
1. RS416 Device 2. Common (Isolated Ground)
Devices (32 Total)
22
3 5
4
120Ω 10nF
3. Negative
5
120Ω 10nF
4. Positive 5. Shield to Earth (Connected At a Single Point)
RUGGEDCOM RS416
Installation Guide
6
Connecting Multiple RS485 Devices
6. RS485