The chart on this slide details the major similarities and differences between RS-422 and RS-485. For the most part RS-485 devices are RS-422 compatible, but RS-422 devices are likely not RS-485 compatible. Both standards provide for very sensitive receivers that require only a 200 mV differential input, which coupled with the relatively large differential driver voltage allows for the attenuation that occurs over 4,000’ of twisted pair cable, or provides good noise margins in shorter networks. Similarly, both standards require receivers that work over wide input voltage ranges to account for noise pick-up, or to accommodate ground potential differences that occur between nodes that are far apart. The standards list 10 Mbps as the maximum data rate, but in reality there are compliant transceivers with data rates up to 100 Mbps. Note that the data rate is 2x the switching frequency, because there are two bits per period. For slow data rate applications, users should choose slew rate limited drivers to minimize potential edge effect problems. The shaded entries highlight the four major differences between RS-422 and RS-485. The biggest difference is that an RS-422 network allows only one transmitter on a cable, while the RS-485 standard allows multiple drivers. This enables bidirectional communication over a single twisted pair, while RS-422 requires at least two pairs for bidirectional communication. To accommodate multiple transmitters, each driver must be tri-state capable, and the standard requires that drivers tolerate bus contention without damage. Thus, the RS-485 driver short circuit current is specified for the whole common mode range of -7 V to +12 V, and most ICs include a thermal shutdown feature to protect against serious faults. The RS-485 transmitter also has more drive, in order to deliver its required 1.5 V differential voltage with two termination resistors. The RS-422 network is limited to only one termination resistor, so the standard requires a larger differential voltage into a lighter load. Since a common twisted pair characteristic impedance is 120 Ω, the RS-485 load spec of 54 Ω is two 120 Ω term resistors in parallel with the input resistance of 32 standard receivers. The 100 Ω load spec for RS-422 is one 120 Ω term resistor in parallel with ten RS-422 receivers. The RS-422 network may contain up to ten receivers and only one driver, while an RS-485 network may have 32 transmitter/receiver pairs. The 32 devices allowed on a 485 bus are “unit load” devices with an Rin of approximately 12 kΩ; if the input impedance is higher, then more devices are allowed on the bus. Note that the total number of devices allowed on the RS-485 bus is 32 unit loads, therefore the user may not always be able to have 32 transmitters and 32 receivers on a network. The disabled load of most transmitters is insignificant compared to the receiver input load, it is typically the receiver input resistance that sets the allowed maximum number of devices on a bus.