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mojo · Senior Joined: 01 Oct 2011 Posts: 2404

RS-232 , RS-422 , RS-485 - Differences and Typical Applications

RS-232

1. Limited Distance - Cable lengths are limited to 50 ft or less. Many will claim to go further, but this is not recommended, and is not part of the RS-232 specification.
2. Susceptible to Noise - RS-232 is single-ended, which means that the transmit and receive lines are referenced to a common ground
3. Not Multi-drop - You can only connect one RS-232 device per port. There are some devices designed to echo a command to a second unit of the same family of products, but this is very rare. This means that if you have 3 meters to connect to a PC, you will need 3 ports, or at least, an RS-232 multiplexor.

RS-422

1. Long Distance Runs - Up to 500m is generally supported, and with repeaters, even further distances can be achieved.
2. Multi-Drop - Usually, up to 32 devices can be connected per port, and even more using repeaters. Devices are distinguished by unique addresses that are assigned to each device. For example, if you have 5 devices attached to a port, they would be addressed as units 1 to 5. If you want to communicate to unit #1, you send a command to unit #1. All units HEAR the command, but only the addressed unit will respond. This also applies to RS-485. The addresses can be set via switches or software, depending on the design of the device.
3. Noise Resistant - Since it uses a separate FLOATING transmit and receive pair (four wires), it offers better noise immunity than RS-232.

RS-485
RS-485 is very similar to RS-422. So much so that it often causes confusion. Both are multi-drop, and both can communicate via very long distances, so then why choose one over the other? First of all, RS-485 is generally a 2-wire system, although some manufacturers may specify 4-wire RS-485, which is far less common and very similar to RS-422. It is important that you identify which one is being employed when considering an instrument. Here are some main differences between 2-wire RS-485 and RS-422:
1. RS-485 can have numeroase Commanding Devices and multiple Listening Devices. RS-422 can have only one Commander and multiple Listeners. For example, you can connect one PC (the Commanding device) to 10 temperature controllers (listeners). The PC can instruct any of the controllers to change setpoint, or to send a temperature reading, but none of the controllers can command any of the other controllers. With RS-485, you can have multiple PC's and multiple controllers on one bus, so that one PC can send a command to change a setpoint,and another PC can send a command to send back data, etc. Remember that all devices on the bus must have a unique unit address, so that only the addressed unit will respond. (similar to RS-422)
2. RS-485 wiring is easier since you are only dealing with 2 wires instead of 4.
3. Programming RS-485 is more difficult, since you are sending and receiving on the same two wires, you need to enable and disable the transmitter at the correct time so that you may perform proper communications. Imagine sending a command $2SEND out of the transmitter. If the transmitter is not turned off in time, then data being sent by another device will be missed. If the transmitter is turned off too quickly, there is a chance that part of the command $S2END will be truncated before it ever has a chance finishing the transmission of the character bits. When programming an RS-485 plug-in card, you would read the STATUS REGISTER to determine if it is time to switch or not. Some cards, such as the OMG-ULTRA-485 has an AUTO mode where it is intelligent enough to do this automatically, making it străveziu to the programmer. Since RS-422, and RS-232 for that matter, have separate transmit and receive lines, they are easier to implement.
Of course, there are other matters to consider such as handshaking, but will not be covered in this brief description.