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I have seen many times that students of embedded field are confuse among RS-232, RS-422 and RS-485 protocol. This article will be addressing this question in specific. Here we are not going in depth but getting overview of all these protocol and their difference.
RS-232
- RS232 was first introduced in 1962 by IBM. RS-232 is the most common serial interface used 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.
- RS232 can transmit up to 1Mbps with maximum distance up to 50 ft, but some RS-232 boards devices are limited to 115.2 kbps. Note that RS-422/RS-485 interface is not available on most IBM Pcs.
RS-422
- RS422 is an improved version of RS232 used on Apple computers, it uses twisted pair cable to reduce the noise, and it uses signal balancing to transmit data.
- So what is signal balanced – It uses a voltage-difference between the two lines as an indication of the signal value, with this method the data is able to transmit for longer distance with faster data rates.
- RS422 can transmit data up to 10 Mpbs at 50 feet or 100 Kbps at 4000 feet. 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.
RS485
- RS485 is an improved version of RS422 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.
Comparison Table
Following table shows you Difference between RS-232, RS-422 and RS-485 protocols :
|
RS232 |
RS422 |
RS485 |
|
|
Communication type |
Single ended |
Single ended and multi-drop |
multi-drop |
|
Communication mode |
Full duplex |
Full duplex, Half duplex |
Full duplex, Half duplex |
|
Number of Devices |
1 transmitter 1 Receiver |
1 transmitter 10 Receiver |
32 transmitter 32 Receiver |
|
Max distance |
50 feet at 19.2 Kbps |
4000 feet at 100 Kbps |
4000 feet at 100 Kbps |
|
Max. Data Rate |
1Mpbs for 50 feet |
10 Mpbs for 50 feet |
10 Mpbs for 50 feet |
|
Signaling |
unbalanced |
balanced |
balanced |
|
Mark (data 1) |
-5 V min. -15 V max |
2 V min. (B>A) 6 V max. (B>A) |
1.5 V min. (B>A) 5 V max. (B>A) |
|
Space (data 0) |
5 V min. 15 V max. |
2 V min. (A>B) 6 V max. (A>B) |
1.5 V min. (A>B) 5 V max. (A>B) |
Applications
The RS232 standard is currently deployed in a wide range of low-data-rate, short-range applications. It’s effective in equipment used in noisy environments such as factories, process control plants ,and utilities sites. Cable lengths are typically less than 50 feet. Common equipment includes low-speed modems, industrial control equipment like programmable logic controllers (PLCs), computer numerically controlled (CNC) machine tools, robots, embedded controllers (Microcontrollers) , medical instruments and equipment.
The RS485 interface is also widely used in industrial applications where higher speeds and longer distances are needed. It is used in the same type of equipment as defined for the RS-232 interface plus devices like point of sale (POS) terminals, metering instruments, and large special automated machines. Networks defined by field buses like Profibus and Modbus use it as well.
What’s New Nowadays ?
RS485 is sometimes termed as RS485 Multi-drop LAN since it can connect several devices in a LAN network environment. These devices are all connected to a single pair wire. Transmit and receive share the same two wires.
Officially the RS485 specification allows only 32 nodes (devices) on the LAN. However, I.C. manufacturers have developed RS485 drivers capable of allowing 128 to 255 nodes on an RS485 LAN. We use these next generation RS485 drivers in our products.
Suggested Reading
- Difference between RISC and CISC architecture
- Difference between ARM and other Microcontrollers
- Arduino Vs Raspberry Pi