DCC Welcome Page‎ > ‎CMRI Notes‎ > ‎

RS422 vs RS485

RS422 and RS485 are hardware only standards.  These Standards do not define how data is to be sent, AKA the protocol, nor do they define any speed, AKA Bits Per Second (bps).  They exist ONLY to supplement other complete communication protocols, such as RS232, and allow these communication standards extend their communication in terms of physical distance and high speed while maintaining a robust reliability in doing so.

What both RS422 and RS485 buses standards have in common that they both use hardware differential signalling techniques to allow a combination of high speed communication over long distances who ground potential differences can be non zero.   However from a communication direction point of view, RS422 is not compatible with RS485, but RS485 can be made compatible with RS422.

FUNCTIONAL KEY BUS DIFFERENCE
The biggest difference between RS422 and RS485 is how they communicate with devices on a single pair of differential wires.

RS422: Each Bus only offers One-way communication.  A single transmitting (master) device to one or more receiving (slave) devices on a single pair of wires.  Two-way communication requires TWO RS422 buses in parallel.  One for each direction.  Hence a two-way RS422 bus requires 4 wires.

RS485: Each Bus offers Two-way communication.   Multiple devices can share a single pair of wires.  Each device has a transceiver allowing both transmitting and receiving capabilities.  This allows point to point two way communication between any given device anywhere on the bus to any OTHER device anywhere on the RS485 bus.  Hence two-way communication requires, at a minimum, ONE RS485 bus. 


HARDWARE: RS422 vs RS485 TRANSCEIVER CHIP SIMILARITIES

These chips allow a given device to access the RS422 or RS485 bus respectively.   Both chips contains a transceiver.  A transceiver contains both a transmitter and receiver in one part.  One side of the chip interfaces with standard logic signals from the local device it is connect to while the other side connect to the bus.  The transmitter take a digital signal from the device and translates it so it can drive the bus with the digital data.  The receivers listens to the bus digital data for the signal on the bus and translates back into a digital signal for the device.

HARDWARE: RS422 vs RS485 TRANSCEIVER CHIP DIFFERENCES

RS422 CHIP. 

1) The transceivers are fully isolated from each other driving TWO independent one way RS422 bus's in OPPOSITE directions.   The transmitter drives one bus and the receiver receives from a 2nd bus. Hence this is a 4 wire bus.
2) At a minimum, two RS422 chips are required to make a complete two way RS422 bus.  The transmitter of the first chip drives the receiver of the 2nd chip and the tranmitter of the 2nd chip drives the receiver of the first chip.
3) RS422 transmitter and receivers are always on and always connected to the RS422 bus.  They cannot be disconnected.
4) The transmitter is ALWAYS driving the bus to a known data state (mark or space)
5) Each RS422 bus can only be driven by one transmitter.
6) Multiple RS422 receivers can monitor the a given RS422 bus.

RS485 CHIP: 

1)
The transceivers are fully connected to each other internally working with ONE RS485 bus.
2) RS485 transceivers have Enable and Disabled functions.  The RS485 transmitter and receivers can be DISCONNECTED from the RS485 bus.
3) The ability to disable e tranmitter allows RS85 transceiver can drive the bus.

Unlike RS422 drivers, RS485 drivers have a enable pins on the transceivers which allows the chip to isolate itself from the bus.  This is what make bi-directional communication possible. 


Transmitting devices are NOT ALWAYS driving the bus to a know state.  Since any given device can transmit on the bus, all the other transmitters must disconnect themselves from the bus in order for that to happen.  When the transmitters are DISABLED, they electrically let go of the bus letting its state be determined by the transmitting transmitter.  The coordination between devices of who can transmit and when is done with the software protocol.    RS485  an also support RS422's  one way communication between a given device anywhere on the bus to all the other devices on the bus all at once.   It can switch between two way and one way on the fly between messages.


THE DIFFERENTIAL SIGNALLING ADVANTAGE

RS422 and RS485 require three wires to send data.  Two in the form of a twisted pair for data signalling and one for DC ground.

The digital signal data is converted from a standard logic signal reference to ground to become represented by the VOLTAGE POLARITY DIFFERENCE BETWEEN THE TWO SIGNAL WIRES.   Twisted Pair wire offers the properties of low mutual inductance between the two wires.

Differential Signalling Performance Benefits

1) takes the ground wire out of the high speed communication path by making no longer Representative of the signal ground.  Why?

1a) Eliminate Ground Wire Inductance.  Inductance is an electrical property that in high speed communication restricts the speed of communication.

1b) Differences in Earth Ground potentials.   The ground wire's only purpose now is to make the two earth ground potential difference between the devices establish a "common low ground voltage" within the range the RS422 and RS485 chips can operate with without being damaged.   Why?  Earth Ground or true 0V is a relative value.  It is relative to where your standing on Earth.  One can take a volt meter and stick the ground probe into the earth your standing one and hold the positive probe in you hanc and you will measure something close to zero volts.   Take that same meter connect to your same earth ground and extend the positive lead a long distance to the location of the other device and you would read a much LARGER NON ZERO voltage.   should be noted when there are two devices separated by long distance, they will NOT have the same earth ground reference point.  It made even worse by high power devices that leak current to Earth Ground.

2) takes the ground wire out of the high speed communication path.
2) reject noise from other adjacent wires since the ground is not involved.  All noise is common mode which simply means that it appear on both differential wires at the same time.