Did you notice yesterday that in the article titled “Understanding PLC Communication Protocols: Types, Meanings, and Their Advantages and Disadvantages” there was no mention of “the commonly referred to 485 and CAN protocols”? Many people think of the “485 communication protocol” as an independent protocol, but strictly speaking, RS-485 (or EIA-485) is not a communication protocol; it is a physical layer electrical standard. It only specifies electrical characteristics such as voltage, current, driving capability, and anti-interference ability, but does not define protocol content such as data frame format, addressing methods, or error checking rules.Because of this, I did not list it separately as a “PLC communication protocol”, but treated it as a physical layer carrier shared by various protocols, similar to Ethernet (IEEE 802.3), which is also not a protocol but the physical foundation for Modbus TCP, Profinet, EtherNet/IP, EtherCAT, and others.
In other words, the “485, CAN protocols” are the electrical standards (physical layer standards) for PLC communication protocols, determining how signals are transmitted over wires, anti-interference capabilities, transmission distances, node counts, and maximum speeds. Below, I will list the commonly used electrical standards for PLC communication protocols for everyone to review.
1. RS-485 / EIA-485
Maximum Rate:115.2 kbps ～ 50 Mbps (higher with special chips)
Maximum Distance: At low speeds, 1200 meters
Maximum Node Count:32～256 (more with repeaters)
Topology: Bus (daisy chain)
Typical Protocols:Modbus RTU、Profibus DP、USS、SiemensPPI/MPI、free port mode, various domestic PLC proprietary protocols
Main Scenarios: Almost all brands of PLC, over 90% of small and medium projects in China default to this standard
2. Industrial Ethernet (100M/1G/10G)
Maximum Rate:100 Mbps ～ 10 Gbps
Maximum Distance: Copper cable 100 meters, fiber can reach several kilometers
Maximum Node Count: Theoretically unlimited
Topology: Star, ring, linear combinations
Typical Protocols:Profinet、EtherNet/IP、EtherCAT、Modbus TCP、BACnet/IP、OPC UA
Main Scenarios: High-end systems from Siemens, Rockwell, Beckhoff, Omron,B&R, etc., mainstream choice for new factories
3. RS-232C
Maximum Rate:115.2 kbps
Maximum Distance:15 meters
Maximum Node Count:1 to 1
Topology: Point-to-point
Typical Protocols:Modbus RTU、PPI、DF1、free port
Main Scenarios: Communication between a computer/touch screen/barcode scanner and a single PLC, most common for debugging
4. CAN (Controller Area Network)
Maximum Rate:1 Mbps
Maximum Distance:40 meters @1Mbps, 1 kilometer @50kbps
Maximum Node Count:110 nodes
Topology: Bus
Typical Protocols:CANopen、DeviceNet、J1939
Main Scenarios: Rockwell systems, automotive electronics, engineering machinery, robotics
5. RS-422 / EIA-422
Maximum Rate:10 Mbps
Maximum Distance:1200 meters
Maximum Node Count:1 master, 10 slaves
Topology: Point-to-multipoint
Typical Protocol: Long-distanceModbus RTU
Main Scenarios: Early Mitsubishi A series, some weighing instruments, broadcast communication
6. Coaxial Cable (ControlNet dedicated)
Maximum Rate:5 Mbps
Maximum Distance: With repeaters, can reach1000 meters
Maximum Node Count:99 nodes
Topology: Dual coaxial redundant bus
Typical Protocol:ControlNet
Main Scenarios: Rockwell legacy systems (now gradually phased out)
7. Coaxial Cable (EarlyProfibus)
Maximum Rate:12 Mbps
Maximum Distance:100～1000 meters (varies with rate)
Maximum Node Count:126 nodes
Typical Protocol:Profibus DP (used purple coaxial cable before 2005)
8. Fiber Optics (Multimode/Singlemode)
Maximum Rate:100 Mbps ～ 10 Gbps
Maximum Distance:2～40 kilometers
Maximum Node Count: Unlimited
Topology: Star, ring
Typical Protocols:Profinet、EtherCAT、ControlNet redundant ring
Main Scenarios: Areas with strong electromagnetic interference, backbone networks across workshops or factories
9. 4～20 mA Current Loop + FSK Digital Overlay
Maximum Rate:1200 bps
Maximum Distance:2～3 kilometers
Maximum Node Count: Point-to-point or multipoint
Typical Protocol:HART
Main Scenarios: Pressure, temperature, flow transmitters, valve positioners, petrochemical and pharmaceutical industries
10. TTL Level (5V/3.3V)
Rate depends on the chip
Distance less than5 meters
Mainly used for internal module communication between PLCs or short-distance board-level communication
11. M-LVDS (High-Speed Differential Bus)
Rate100 Mbps and above
Mainly used forSERCOS III and other high-end motion control systems
12. M-Bus (Metering Bus, Telephone Line)
Rate2400 bps
Distance1000 meters
Maximum Node Count:250 nodes
Mainly used for centralized reading of water meters, electric meters, and heat meters
Proportion of actual use in China in 2025 (engineering experience value)
RS-485:65%～70%
Industrial Ethernet:25%～30%
RS-232:About5%
CAN Series: About3%
Others (HART, fiber optics, etc.): About2%
If only connecting one PLC and a computer/screen → RS-232 or RS-485 can both work
Thus, all commonly used electrical standards for PLC communication have been listed, which can be directly saved for documentation purposes.