RS422 uses balanced transmissions on twisted pairs, but is point to point. It was supposed to clean up and modernise the RS232 protocol which was devised in the 60's for modems and had a lot of defunct control lines, as well as the ludicrous voltage levels. Unfortunatly the 'committee' came up with a full spec that requires a 36 way connector. Some clean up!
Note that it is the use of balenced differential lines that allows the speed/distance gains, but RS485/422 signal are ground referenced at the transmitter and must stay within a ground referenced window at the receiver, so althougth there is a high immunity to common mode noise (which on twisted pairs means all pick-up and crostalk), there are limits to the magnitude of common mode disturbances. Examples of true non ground referenced differential signalling may be found in ethernet and ISDN, which are transformer coupled. The upshot of this is when using 485/422 between areas that may be at slightly different earth potentials an isolating buffer should be used.
The REASON balanced differential lines can be used at higher speeds over longer distances is that the use of co-phased signals on a twisted pair cable allows noise to be self cancelling and hence the receiver sensitivity may be musch higher (the RS485 spec puts receiver hysteresis at 50mV), this means that the receiver is able to pick up a very highly attenuated signal. But if you do not respect the requirements, (twisted pair/terminators/no long stubs) then noise and/or reflection can quickly make things worse than they would be with 232!
Over the years, RS422 has generally meant pretty much like 232 but with balanced pairs instead of unbalanced NRZ signalling.
RS485 is the multi-drop version, a 485 spec transmitter can drive 32 485 spec receivers, and the transmitters can withstand working in conflict with other transmitters.
Because RS485 spec transmitters must have tri-stating and can withstand transmitter conflicts, they are suitable for half-duplex operation (which is what is normally used in industrial auomation) but full duplex can be done, it generally takes the form of a master slave arrangement where the master talks to all slave receivers on one line, and the slaves talk back to the masters receiver on the other. Because this removes the (time critical) requirement to commutate the 485 buffers on the master, this arrangement is very suitable for interfacing a PC master to a bunch of slaves as there is no need to control the UART on the PC, you could do it with no more than print/input commands in basic.
BTW, there was also a curious 'RS423' spec, which used two co-phased wires with 232 levels on them, the idea being that it could 'bridge the gap' between 232 and 422. Standards like this remind me of the definition of a camel; a horse designed by a committee;-) Fortunately it was not widely adopted!
>I was under the impression that RS-485 was basically, two wire RS-422 with >transmits and recieves on the same line.
There's 4 combinations of RS422/485 with 2 or 4 wires:
1. RS422, 2 wires. Gives you a system with one device as transmitter and 10 devices as listeners. This allows you unidirectional communication. Less useful for a network. Applications I know of are: printers, TV camera's, airport arrival/departure monitors.
2. RS422, 4 wires Gives you a system with two devices only (point-to-point). Bidirectional communication is possible in full duplex, however no network is possible.
3. RS485, 2 wires. Gives you a system with up to 32 devices. Software must determine access to the bus. Bidirectional communication is possible in half duplex; multi-master is possible. This is the most common wiring for a fieldbus.
4. RS485, 4 wires. Gives you a system with up to 32 devices. Bidirectional communication is possible. It enforces a master/slave relation between devices, which limits its usefulness.
Now, it is RS485 with 4 wires which many suppliers call "RS422" which is an error. Thus the idea exists that anything with 4 wires is RS422 and anything with 2 wires is 422. The summary above proves that that are 2 things not related to each other.