EIA-485, also known as TIA/EIA-485 or RS-485, is a standard defining the electrical characteristics of drivers and receivers for use in balanced digital multipoint systems. The standard is published by the ANSI is replacing RS 232 or not ????

RS-232 is a standard means for two devices to communicate serially while RS-485 is a multi-drop serial network communication standard. While RS-485 can certainly do the same job as RS-485, the additional multi-drop nature of RS-485 makes its setup a bit more difficult and complicated for a simple point to point serial connection. The two standards each have their own purpose and I don't think either will be replacing the other.

Thanks Mark B and Mark Lochhaas i agree with you and i use both in feild ....

RS-422 is a full duplex implementation of serial communications, while RS-232 and RS-485 are half duplex. RS-485 has a much broader installed base and as such has seen the greater emphasis in commercial development. Interbus-S [Legacy and leading edge at its inception] is nowhere to be seen in the "automation market place" installed base. The improvements in RS-485 technology as a function of speed have surmounted the need to transmit and receive simultaneously to realize desired throughput. While more typical in a master slave environment, e.g. Allen-Bradley's RIO. you can find multi-master environments using the RS-485 wiring scheme, DeviceNet being one example, and surely Ethernet IP being another. Programming protocol would certainly dismiss more than one Master controlling any single device, but for multiple masters to be aware of sensor states across the entire spectrum of an implementation has numerous benefits.

PLC manufacturers typically provide the ability for Masters in a multi-master environment to pass messages back and forth between themselves. This allows one to cue another that it thinks a certain device may need to be changed to an appropriate state. Messaging also allows these Masters to cue the other that a sequence or process has been completed and that something is waiting the action of the other Master. Of course PLC programming models and instruction sets still rely on the programmer to take advantage of these capabilities.

RS-232's signal is referenced to ground, thus subject to noise and as mentioned above can be unreliable in distances greater than 15 meters, especially in noisy environments, factory floors being very noisy. USB [half duplex], like RS-422 and RS-485 use a twisted pair[s] and evaluates the differential voltage between say D+ and D-. Other portions of the specification involving the hardware limit its effective distance..

RS-422 is a full duplex implementation of serial communications, while RS-232 and RS-485 are half duplex. RS-485 has a much broader installed base and as such has seen the greater emphasis in commercial development. Interbus-S [Legacy and leading edge at its inception] is nowhere to be seen in the "automation market place" installed base. The improvements in RS-485 technology as a function of speed have surmounted the need to transmit and receive simultaneously to realize desired throughput. While more typical in a master slave environment, e.g. Allen-Bradley's RIO. you can find multi-master environments using the RS-485 wiring scheme, DeviceNet being one example, and surely Ethernet IP being another. Programming protocol would certainly dismiss more than one Master controlling any single device, but for multiple masters to be aware of sensor states across the entire spectrum of an implementation has numerous benefits.

PLC manufacturers typically provide the ability for Masters in a multi-master environment to pass messages back and forth between themselves. This allows one to cue another that it thinks a certain device may need to be changed to an appropriate state. Messaging also allows these Masters to cue the other that a sequence or process has been completed and that something is waiting the action of the other Master. Of course PLC programming models and instruction sets still rely on the programmer to take advantage of these capabilities.

RS-232's signal is referenced to ground, thus subject to noise and as mentioned above can be unreliable in distances greater than 15 meters, especially in noisy environments, factory floors being very noisy. USB [half duplex], like RS-422 and RS-485 use a twisted pair[s] and evaluates the differential voltage between say D+ and D-. Other portions of the specification involving the hardware limit its effective distance.

As a differential referenced signal, noise is seen simultaneously on both channels, D+ and D-, so the differential changes with the perceived noise, thus effectively canceling the noise, a huge factor in being able to transmit much greater distances without data corruption.

For Scott Clark - RS232 is a full duplex!

RS 485 replacing RS 232 or not - depended of your application, in some applications it's possible to use converters RS 485 - RS 232, in others no such possibility. If you use MODBUS RTU, for example, you can use RS 485 and RS 232 in any combination. Usually PC have RS232 COM port on board, but field instruments and controllers - RS485, so you can connect all RS485 equipment in network and polling all with PC trough RS232 - RS485 converter.

Eventhough RS485 has the multidrop connectivity, it can not be replaced RS232.Both will suit depend upon the application need.

You can use up to 31 device with RS232, each connected trough RS232-RS485 converter to RS485 network. so one of these 31 device can be MASTER and others will be SLAVE in case of MODBUS, or other master-slave protocol. You can, of course, connect only 2 device trough converters to RS485 network and use any PTP connection protocol. The restrictions in such a case are not all hardware signals transmission trough converters and RS485 network - RX and TX only. That means software handshake only and no hardware RI and other signals, described in RS232.