Specifications

Chapter 1 - Overview RS-422 And RS-485 Applications Ebook
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RS-485 is used as the basis for many commercial and industrial data
communications systems. Industry systems such as PROFIBUS,
INTERBUS, and others are built on RS-485 technology. RS-422 was
historically used in older Apple computers and printers, and in video
editing equipment.
Often, end-users build systems themselves using RS-232 to RS-
422/485 converters, RS-485 smart switches, RS-485 repeaters, and
other off-the-shelf products. These devices are commonly used to link
programmable logic controllers (PLC), supervisory control and data
acquisition (SCADA) systems, remote terminal units (RTU) and other
equipment in custom networked systems.
The EIA/TIA-422 and EIA/TIA-485 standards do not define protocols.
They are simply physical layer standards (and even then, do not
specify connectors or pin-outs). This means that RS-422 and RS-485
can be implemented in many systems and applications. Often the
standards are used to extend serial links, or to network equipment that
was not previously interconnected. Systems using the ubiquitous
MODBUS protocol are often interconnected using RS-485 multidrop
networks.
Data Transmission Signals
There are several inescapable facts about data communications signals
that must be considered in any data communications system. Data, in
the form of electromagnetic signals, must be created, sent along some
kind of medium (which travels through a variety of environments) and
detected at its destination. For the purposes of this document the
signal is digital pulses and the medium is wire pairs.
Digital pulses are voltages that represent binary states - ones and
zeros. Voltages change from state to state, which creates rising and
falling edges. The number of pulses per second (baud rate) determines
how much data can be sent in a given period. The transmitters, media
and receivers must be able to create, transfer and detect the pulses
reliably at the baud rate at which the system is operating. The noise
picked up in transit can affect the receiver's ability to reliably detect