Introduction
RS485 and RS232 are widely used serial interface standards for communications in embedded systems, industrial automation and other applications requiring simple point-to-point or multi-drop connectivity.
While both RS485 and RS232 transmit data serially over twisted pair cabling, there are important distinctions between the two interfaces in regards to capabilities, performance and use cases.
This article provides a technical overview and comparison of RS485 vs RS232 in terms of electrical characteristics, timing, cabling, maximum distance, data rates, number of nodes supported, interference immunity, routing topologies and more. Real world examples illustrate optimal application scenarios for each standard.
RS485 Overview
RS485, also known as TIA-485 or EIA-485, is a serial communications standard defined by the TIA/EIA in 1983. It specifies the electrical characteristics and timing requirements for robust transmission of digital data over twisted-pair cabling.
Some key attributes of RS485:
- Balanced differential signaling helps reject noise and interference
- Supports multipoint networks via daisy chaining up to 32 or more nodes
- Operates at data rates up to 10 Mbps over distances up to 4000 feet
- Widely used for industrial sensor networks, building automation, transportation and other applications requiring moderate data speeds and noise tolerance over twisted pair cable
RS485 uses differential balanced transmission and receipt of voltages to enable noise immunity and reliable communication in noisy environments. It can be implemented using simple transceiver chips that convert the single-ended logic levels found on most digital devices to differential signals on the transmission medium.
Now let’s look at some of the technical details behind RS485 and how it supports multi-drop networking.
RS485 Electrical Interface
RS485 signals are carried over a twisted pair of wires to achieve data transmission and receipt. One wire carries the differential bus signal labeled A (+), while the other wire carries the inverted bus signal labeled B (-):
By transmitting signals differentially over the two wires, noise and other common-mode interference gets cancelled out at the receiver. This enhances noise immunity significantly compared to single-ended interfaces.
The driver sends logic 0 by driving A low and B high. Logic 1 is indicated by driving A high and B low. Receivers determine the logic value based on comparing the differential voltage A – V
RS485 transceiver chips handle the conversion between single-ended logic levels and the differential signaling. They also manage the bus transmissions.
Multidrop Support
A major advantage of RS485 is the ability to connect multiple nodes (up to 32) on the same twisted pair through multi-drop bus connections:
The transmitters use tri-state outputs to enable multiple driver connections to the shared bus pair. Only one driver is active during a transmission, while all others are in a high impedance (Hi-Z) state.
This allows all nodes to share access to the same transmission medium. Slave nodes filter out messages not addressed to them based on higher protocol layers.
Differential Signal Immunity
RS485 provides robust interference rejection by leveraging differential signaling where the receiver determines logic value based on the voltage difference between the two wires rather than their absolute voltages.
This renders the interface much less susceptible to common mode noise events like ESD strikes, ground shifts and electromagnetic interference. Both wires are affected approximately equally, so the differential voltage remains steady.
Overall RS485 enables reliable serial communication in challenging electrical environments. It is far more robust than single-ended interfaces.
RS232 Overview
RS232, also referred to as EIA232, is an older serial interface standard introduced in 1960. It specifies connections between DTE (Data Terminal Equipment) and DCE (Data Communication Equipment) using a single-ended transmission scheme.
Some key characteristics of RS232:
- Single-ended signaling rather than differential
- Point-to-point communication between one transmitter and one receiver
- Typical applications include computer serial ports, test equipment and industrial controllers
- Data rates less than 20 Kbps up to distances of 15-50 feet
- Voltage levels ranging from -15V to +15V
While RS232 has been mostly superseded by USB for computer peripherals, it remains popular for simple low speed serial connections over shorter distances. However, noise immunity and maximum distance are both significantly less than differential signaling standards like RS485.
Now let’s take a closer look at the electrical interface used in RS232.
RS232 Electrical Interface
Unlike RS485 which uses differential pairs, RS232 is a single-ended interface with different voltage levels used to signify logic high versus logic low bits.
The diagram below illustrates RS232 voltage levels specified at the driver:
A logic 0 is represented by a voltage between +3V to +15V, while a logic 1 corresponds to a voltage between -3V to -15V. Receivers determine logic value based on these voltage thresholds.
This single-ended signaling provides much less noise immunity compared to differential schemes. Any noise or ground shifts can corrupt the absolute voltage levels leading to communication errors.
Point-to-Point Topology
Unlike RS485 which enables multi-drop networks, RS232 is designed for simple point-to-point communication between two devices:<img src=”https://drive.google.com/uc?export=view&id=1R7vwcSy38l6GzJv-6xdoZnp_LUOW2jpQ” alt=”RS232 point-to-point” width=”350″>
There is one dedicated transmitter and receiver on each end. No concept of bus sharing or collision detection is required. This makes RS232 unsuitable for multi-node topologies.
Limited Distance and Speed
The single-ended design and lack of advanced termination and equalization techniques also severely limit the maximum distance and speed achievable with RS232 compared to differential alternatives:
- Maximum distance limited to 50 feet at lower speeds
- Maximum speed limited to 20 Kbps at short distances
- Typical reliable rates only 2 Kbps up to 15 feet
Thus RS232 is only appropriate for relatively low throughput applications over shorter cable runs. More demanding applications require RS422, RS485 or other interfaces.
RS485 vs RS232 Comparison
Now that we’ve covered the key characteristics of both RS485 and RS232, let’s summarize some of the key differences between them:
As the comparison shows, RS485 provides substantial advantages for high noise environments and applications needing to span longer distances or support multiple node connections.
The only advantage of RS232 is minimal component count for very simple point-to-point links. For all other applications, RS485 is generally preferable.
Real World Application Examples
To better understand the optimal use cases for each standard, let’s look at some real-world examples of RS485 vs RS232 interfaces in practice:
RS485 Example Applications
Industrial Sensors: Connecting multiple temperature, pressure or flow sensors over twisted pair in a factory through daisy-chained RS485 links. Allows spanning large facilities over thousands of feet.
Building Automation: Interconnecting multiple HVAC controllers and devices using RS485 in a large commercial building for sending control and telemetry data across the facility.
Robotics: Connecting controller area network (CAN) or other serial buses between multiple servos, actuators and sensors in a robotics system using RS485’s multi-drop capability.
SCADA Systems: Implementing connections between field devices like RTUs and master controllers in utilities and other SCADA applications usingRS485 for long range communication.
POS Systems: Networking multiple point-of-sale terminals together using RS485’s noise rejection in a busy retail environment with magnetic stripe readers generating interference.
RS232 Example Applications
Computer Peripherals: Simple serial mouse, modem or printer connections to a PC using legacy RS232 ports. Low speed and distances involved match RS232 capability.
Lab Equipment: Instrumentation controllers and devices interconnected over short RS232 links rather than more complex fieldbuses. Suits low density of nodes.
** attenuated RS232 sending radio telemetry
Either RS485 or and
CNC Machines: Some CNC systems use RS232 for basic serial communication between drives, controllers and tactile feedback devices over short distances in electromagnetically noisy environments.
As these examples illustrate, optimal applications of RS485 vs RS232 align closely with their technical characteristics and trade-offs. Understanding these use cases helps apply the right standard during system design.
Conclusion
In summary, RS485 and RS232 represent two serial interface standards optimized for different applications:
- RS485 provides robust noise immunity and multi-drop connectivity ideal for industrial data links spanning long distances
- RS232 offers simple point-to-point communication over shorter ranges in less noisy environments
Engineers should consider parameters like data throughput needs, cable runs, number of nodes, and interference sources when selecting between RS485 vs RS232 during the design process.
For the majority of industrial, building and automation use cases where multiple node connections are needed, RS485 is the superior choice. RS232 remains suitable for basic legacy systems with minimal length and data rate requirements.
By matching interface capabilities closely with application requirements, optimal system reliability and cost can be achieved. Both standards continue to fill important niches for serial data transfer in embedded systems and automation.
RS485 vs RS232 FAQ
Here are some frequently asked questions regarding comparing RS485 and RS232 serial interfaces:
Q: Can RS485 devices communicate with RS232 devices?
A: Yes, with a protocol and electrical level converter that can translate between the different voltages and data formats. Native interoperation is not possible.
Q: What kind of cabling is used with RS485 and RS232?
A: Both typically use shielded twisted pair cabling, though RS485 is also robust over unshielded twisted pair. Coaxial cable can be used for RS232.
Q: Is RS485 full duplex or half duplex?
A: RS485 is half duplex, so data can only flow in one direction at a given time. RS232 supports full duplex communication.
Q: What is slew rate and does it impact RS485 vs RS232?
A: Slew rate limits how fast voltage levels can change. RS485 handles faster slew rates which helps enable higher data rates.
Q: How many RS485 nodes can connect to each other?
A: Up to 32 unit loads can be supported depending on power budget and data rate needs. More devices require repeaters.
