Because RS standards are merely recommended and technically general, lots of manufacturers develop products that are at best sub standard . They cut corners and cheat in order to manufacture cheaper products. We have tried not to make crappy products. That is why you don't see any port powered products put out by Integrity Instruments. A great Article on RS485 was written by Bob Perrin for Circuit Cellar Online, July 1999.
Data only flows in one direction.
That is to say, a device can be a receiver or a transmitter exclusively. A simplex device is not a transceiver . A good example of simplex communications is an FM radio station and your car radio. Information flows only in one direction where the radio station is the transmitter and the receiver is your car radio. Simplex is not often used in computer communications because there is no way to verify when or if data is received.