A Quick Comparison of RS-232, RS-422, and RS-485 Serial Communication Interfaces 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/second. 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/second. RS-422 sends each signal using 2 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.
Note: NI 8430/8431/8432/8433 serial devices meet this specification without 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 manuals attached below for more information. Related Links: Products and Services: Serial Interfaces KnowledgeBase 1M9D8H6Q: Serial Communication Starting Point KnowledgeBase 1M9E1L6Q: Serial Communication General Concepts KnowledgeBase 0XE8M4L7: Do NI Serial boards come with Termination and Biasing Resistors Built-in?. Product Manuals: 8430/8431 8-Channel Hardware Specifications Product Manuals: 8432/8433 H2- and 4-Channel Hardware Specifications NI-Serial 3.4 Read-me NI-VISA 4.4.1 Read-me Attachments:
Report Date: 09/28/2004 Last Updated: 10/02/2008 Document ID: 3DRAGUOY |
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