Comparison of RS-232, RS-422, and RS-485 Serial Communication Interfaces

Taken from: National Instruments

Hardware: Serial>>RS-485>>PCI-485/2

Problem: 
What are the primary differences between RS-232, RS-422, and RS-485 serial interfaces?

Solution: 
RS-232 is the most common serial interface and ships as a standard component on most Windows-compatible desktop computers. RS-232 only allows for one transmitter and one receiver on each line. RS-232 also uses a Full-Duplex transmission method. Some RS-232 boards sold by National Instruments support baud rates up to 1 Mbit/s, but most devices are limited to 115.2 kbits/s. Note that RS-422/RS-485 interface is not available on most IBM PCs. 

RS-422 (EIA RS-422-A Standard) is the serial connection used on Apple computers. It provides a mechanism for transmitting data up to 10 Mbits/s. RS-422 sends each signal using two wires to increase the maximum baud rate and cable length. RS-422 is also specified for multi-drop applications where only one transmitter is connected to, and transmits on, a bus of up to 10 receivers. 

RS-485 is a superset of RS-422 and expands on the capabilities. RS-485 was made to address the multi-drop limitation of RS-422, allowing up to 32 devices to communicate through the same data line. Any of the slave devices on an RS-485 bus can communicate with any other 32 slave devices without going through a master device. Since RS-422 is a subset of RS-485, all RS-422 devices may be controlled by RS-485. 

Both protocols have multidrop capability, but RS-485 allows up to 32 devices and RS-422 has a limit of 10. For both communication protocols, you should provide your own termination. All National Instruments RS-485 boards will work with RS-422 standards. 

The following table compares mode of operation, total number of drivers and receivers, maximum cable length, and maximum data rate. 

Specifications	RS-232	RS-422	RS-485
Mode of Operation	Single-Ended	Differential	Differential
Total Number of Drivers and Receivers on One Line. One driver active at a time for RS-485 networks	1 Driver 1 Receiver	1 Driver 10 Receiver	32 Drivers 32 Receivers
Maximum Cable Length	50 ft (2500 pF)	4000 ft	4000 ft
Maximum Data Rate (40 ft - 4000 ft for RS-422/RS-485)	160 kbits/s (can be up to 1Mbit/s)	10 Mbit/s	10 Mbit/s

For all serial communication protocols, you should provide your own termination. For information on termination, please refer to KnowledgeBase 0XE8M4L7: Do RS-485/422 Interfaces Have Built in Termination and Bias Resistors?. 

Note: NI 8430/8431/8432/8433 serial devices meet this specification without electromagnetic compatibility (EMC) or surge limitation. This means that under large surges, i.e. lightning strikes, the maximum length will not be 4000 ft. (1200 m) for 422/485 but 98 ft. (30 m) according to the specifications. Refer to the following linked manuals for more information.