From Wikipedia:
RS-422 only specifies the electrical signaling characteristics of a single balanced signal. Protocols and pin assignments are defined in other specifications. The mechanical connections for this interface are specified by EIA-530 (DB-25 connector) or EIA-449 (DC-37 connector), however devices exist which have 4 screw-posts to implement the transmit and receive pair only. The maximum cable length is 1500 m. Maximum data rates are 10 Mbit/s at 12 m or 100 kbit/s at 1200 m. RS-422 cannot implement a truly multi-point communications network such as with EIA-485, however one driver can be connected to up to ten receivers.
So, according to the Wiki, there indeed is no standard for the pin assignments of RS-422 on a DB-9 connector.
In addition to that, different websites say different things:
So there probably indeed is no standard.
In normal serial communications links, you actually need two wires - one is the signal and one is the ground. The ground stays at zero volts and the signal wire swings between 2 voltages that represent data bits 0 and 1 respectively (in RS232, the signal swings between +25 volt and -25 volts).
Using voltage to signal is not great for long wires going though noisy environments because the wires act like antennas and the voltage difference can get lost in the noise.
RS422 uses current direction rather than voltage to transmit bits. Current going one way is a 1, reversed the other way is a 0. This is much more resistant to noise.
The RX+ and RX- are the two wires that that current for received data. You can imaging sending data down this wire using a battery as a transmitter. Connect the battery one way to send a 1, or flip it the other way to send a 0. Two wires, but they carry one bit of data. You can connect this one bit of data to a single serial input (using the the correct drivers to turn the current direction on thew wires in a voltage to show the pin).
Same goes for the TX+ and TX- wires. They carry a single bit. The driver will take a single bit of data represented as a voltage on your sereial send pin, and convert it to a current flowing one way or the other on the two TX wires.
ADVICE: Unless you are making lots of these, don't mess around with driver chips (unless you want to!). You can cheaply get prepackaged current loop dongles that have screw terminals and are very easy to use. They use the same chips you are looking at, but do all save you the hassles of figuring it all out. If you get one that uses TTL level signals, you can connect it directly to your Ardunio and treat it as just a regular serial connection. Here is an example of one (I've never used this one but looks as good as any other)...
http://www.amazon.com/INBOARD-RS422-DRIVER-ET-MINI-SHIPPING/dp/B00EXUGRUK/ref=sr_1_1?ie=UTF8&qid=1392255264&sr=8-1&keywords=ttl+rs422
Best Answer
RS-422 & RS-485 are similar in that they both use balanced, differential signalling. All of the RS-422 & RS-485 systems that I have seen or worked with use 5V levels.
A RS-422 driver is an output-only device. It will happily feed a RS-485 device so long as that RS-485 device is only ever receiving data. You will have a data collision if the RS-485 device ever tries to transmit on that pair.
The best way to think of this is that RS-422 is intended for systems that use separate Tx & Rx signal paths. That is: there is a single sender on each of the two data pairs (one sender at each end of the link).
Note that RS-422 can have multiple listeners.
RS-485 is designed for multi-drop systems where multiple devices can talk and listen to the same single conductor pair.