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RS-232, RS-423, RS-422 and RS-485 Serial Interface Standard Summary

All four of these standards were written by the EIA (Electronics Industry Association) to allow peripherals and computers to communicate with each other regardless of manufacturer. There are identical government (MIL spec) and international (CCITT) standards.

These standards are for asynchronous serial communication. This means that data is transferred one bit at a time using a start bit, 7 or 8 data bits, stop bit(s) and an optional parity bit. The data is "self clocking" so that once the sender and receiver agree on the baud rate, the timing of the individual bits is based only on the start and stop bits.

Alternatives to these standards are synchronous communication and parallel communication. Synchronous communication requires an additional clock line between the sender and receiver. Parallel communication requires a clock line, usually some control signals and several data lines. Both parallel and synchronous communication systems can transfer data faster than asynchronous serial systems but require more wires in the interconnect cables. PC parallel ports are examples of parallel communication. IBM's SDLC and HDLC are synchronous serial communication protocols and have been typically used in mainframe communications.

RS-232, 423, 422 and 485 specify the communication system characteristics of the hardware such as voltage levels, terminating resistances, cable lengths, etc. The standards say nothing about the software protocol or how data is framed, addressed, checked for errors or interpreted.

RS-232

RS-232C is the oldest and most popular serial communication standard. It is the standard used on PC COM port hardware. It is designed to connect two systems only and is "single ended", meaning that it uses one wire for data and one for ground. It is a robust interface with speeds to 115,200 baud and will withstand a short circuit between any 2 pins. Maximum signal voltages are ±15 volts. Cable length depends on baud rate but is typically 50 feet maximum.

RS-423

RS-423 is similar to RS-232C except that it allows for higher baud rates and longer cable lengths because it tolerates ground voltage differences between sender and receiver. The maximum signal voltage levels are ±6 volts. Ground voltage differences can occur in electrically noisy environments where heavy electrical machinery is operating.

RS-422

RS-422, like RS-232, is used to connect only two systems. It uses differential, or "double ended" data transmission, which means that data is transmitted simultaneously on two wires between two stations independent of the ground wire. Each signal requires 2 wires with a ground present in the system. The advantage of this method over RS-232 is higher speeds and longer cable lengths - 4000 feet at a 100K baud rate, for example.

RS-485

RS-485 is an improved RS-422 with the capability to connect up to 16 devices (transceivers) on one serial bus to form a network. Such a network can have a "daisy chain" topology where each device is connected to two other devices except for the devices on the ends. Only one device may drive data onto the bus at a time. The standard does not specify the rules for deciding who transmits and when on such a network. That's up to the system designer to define.

Glossary

Asynchronous Communication - A protocol that allows bits of information to be transmitted between two devices at an arbitrary point in time, requiring a start bit and stop bit to identify the string of information being sent. Both devices need to communicate at an agreed upon data rate (baud rate) such as 19,200 KB or 115,200 KB. This protocol has been in use for 15 years and is used to connect PC peripherals such as modems.

Synchronous Communication - A protocol that allows bits of information to be sent at a fixed, synchronized rate between the transmitter and receiver. Sending synchronized information eliminates the need for a start and stop bit with each string of information.

Differential data - Data transmitted on two wires simultaneously at opposite polarities. Noise which is picked up on both wires is rejected since a differential receiver responds only to the difference in voltage between the wires.

Transceiver - A circuit which can connect to a shared bus for the purpose of receiving from or transmitting to the bus. A cooperative protocol for the bus ensures that only one transceiver on a bus transmits at any time.

If you need the full detailed specifications on the above-mentioned serial interfaces, please visit the EIA website at http://www.eia.org/eng/default.htm.

You can also visit National Semiconductor's website at http://www.national.com/apnotes/ to obtain useful application notes.

Another very useful resource is the book, C Programmer's Guide to Serial Communications, Second Edition, ISBN #0-672-30286-1, published by Sams Publishing.

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