Lantronix Device Server MSS 485 - Manual (Page 11)

Overview
1. NET485 Overview
1.1 Overview
The NET485 product allows you to connect to an RS422 / RS485 transceiver to enable remote serial
communications over long distances (up to 4,000 ft.). Using RS485 two-wire mode, you can also connect
the NET485’s serial interface to multiple devices in a multi-drop network. The NET485 is designed around
the XPort-03 Serial to Ethernet device server.
1.2 Overview of RS422 / RS485
RS232 is an EIA standard transmission system and has been around since 1962. RS232 provides single-
ended data communications between a transmitter and a receiver. It allows for data transmission from one
transmitter to one receiver at relatively slow data rates (115k bits / second) and short distances (up to 50 ft.
at the maximum data rate).
While RS232 is well-known for connecting PC's to external devices, RS422 and RS485 are not as well
known. When communicating at high data rates, or over long distances in real world environments, single-
ended methods are often inadequate. RS422 and RS485 were designed to provide data communications over
longer distances, higher Baud rates and provide better immunity to external electro-magnetic noise.
RS422 and RS485 use differential data transmission (balanced differential signal). This offers superior
performance by canceling the effects of ground shifts and induced noise signals that can appear as common
mode voltages on a network. This also allows for data transmission at much higher data rates (up to 460K
bits / second) and longer distances (up to 4000 ft).
What is the difference between RS422 and RS485? Like RS232, RS422 is intended for point-to-point
communications. In a typical application, RS422 uses four wires (two separate Twisted Pairs of wires) to
transfer data in both directions simultaneously (Full Duplex) or independently (Half Duplex). EIA/TIA-422
specifies the use of one, unidirectional driver (transmitter) with a maximum of 10 receivers. RS422 is often
used in noisy industrial environments or to extend a RS232 line.
RS485 is used in applications where multiple devices want to share data communications on a single serial
network. RS485 can support up to 32 drivers and 32 receivers on a single two wire (one twisted pair) bus.
Most RS485 systems use a Master/Slave architecture, where each slave unit has its unique address and
responds only to packets addressed to it. However, peer to peer networks are also possible.
Specification RS-422 RS-485
Transmission Type Differential Differential
Maximum Data Rate 10 MB/s 10 MB/s
Maximum Cable Length 4000 ft. 4000 ft.
Minimum driver Output Voltage +/- 2V +/- 1.5V
Driver Load Impedance 100 Ohm 54 Ohm
Receiver Input Resistance 4 KOhm min 12 KOhm min
Receiver Input Sensitivity +/- 200mV +/- 200mV
Receiver Input Voltage Range -7V to +7V -7V to +12V
No of Drivers Per Line 1 32
No of Receivers Per Line 10 32
NET485 User Guide 1-1