RS485 (Multidrop communication for up to 32 devices)
RS485 communication is defined as multidrop network of up to 32 stations connected together using two wires or four wires, plus ground. An RS485 network can be up to 4,000 feet or more in length (see below). Typically one station acts as the master station, sending commands to one slave station at a time. (Only one station at a time can transmit data on an RS485 network.) RS485 is effectively a "partyline" network. All slave stations will hear the commands from the master station. Since each command contains a station address, only the correct slave station will respond to the command.
On a two-wire RS485 network, messages are sent and received alternately on the network (half duplex communication). On a four-wire network, messages can be sent and received simultaneously on the network (full duplex communication). Each station turns off (tri-states) its transmitter when it is not sending data, effectively "disconnecting" its transmitter from the network so another station can send its data. RS485 mode does not use the CTS and RTS signals for handshaking.
RS422 (Communication between two devices only)
RS422 is an alternative to RS232 communications. RS422 offers greater noise immunity and greater communication distance than RS232, at any given baud rate. Two RS422 devices can be interconnected with or without handshaking. RS422 wiring can be up to 4,000 feet in length using 24 AWG cabling.
SIXNET Windows I/O devices with RS422 ports support both RS422 and RS485 communications.
RS485 /RS422 cable distance
SIXNET stations and I/O modules with an RS485 or RS485 / RS422 port can communicate over distances of up to 4,000 feet, depending on the product. The maximum distance increases with wire size and a corresponding reduction of baud rate. Here are the limits:
For EtherTRAK I/O modules and all IPm-based products:
0.5 mile (0.8 km) using 24 AWG wire & maximum 57,600 baud
For all RemoteTRAK products, the ET-GT-485 converter and most legacy SixTRAK and VersaTRAK products:
1 km (approx. 4,000 feet) using 24 AWG wire & maximum 57,600 baud
2 km (approx. 8,000 feet) using 20 AWG wire & maximum 38,400 baud
3.5 km (approx. 14,000 feet) using 18 AWG wire & maximum 19,200 baud
RS485 Limitations and Solutions
Note that RS485 "star" configurations are not recommended. If you have a situation where you need to run RS485 cable in more than two directions from the master device, then consider using an RS232 port on the master device and connecting a SIXNET RM-232-485-4U converter to it. This converter has four buffered RS485 ports, allowing RS485 cables to be run in a total of eight directions.
More than 32 RS485 devices
If you have a situation where you need to connect more than 32 RS485 slave devices to your master device, then consider using an RS232 port on the master device and connecting a SIXNET RM-232-485-4U converter to it. This converter has four buffered RS485 ports, allowing RS485 cables to be run in a total of eight directions. You can also connect up to 32 RS485 devices on each of the four ports, for a total of up to 128 RS485 devices.
Note: The RM-232-485-4U passes information bidirectionally between its RS232 port and the four RS485 ports as a group. It is not possible to connect an RS485 master device to one of the four ports and have the commands sent out the other three ports.
It is the customer's responsibility to review the advice provided herein and its applicability to the system. Red Lion makes no representation about specific knowledge of the customer's system or the specific performance of the system. Red Lion is not responsible for any damage to equipment or connected systems. The use of this document is at your own risk. Red Lion standard product warranty applies.
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