PLC Communication Protocols


#1

Many different communication protocols exist, this is just a short list of the more common ones we have for our PLCs. The table below quickly gives you some of the key differences between them followed by a short description of each.

Specification CompoBus DeviceNet Ethernet/IP EtherCAT RS-232C RS-422 RS-485
Max Number
Of Devices
32 63 512 512 1 10 32
Max Cable Length 500 m
(1640 ft)
500 m
(1640 ft)
100 m
(328 ft)
100 m
(328 ft)
15 m
(50 ft)
1219 m
(4000 ft)
1219 m
(4000 ft)
Data Rate/Speed 750 kbps 500 kbps Up to 1 Gbps 100 Mbps 160 kbps 10 Mbps 10 Mbps
Internet Capable No No Yes Yes No No No
Power with
Communication
- 24 VDC
8 A
- - - - -

CompoBus
There are two modes to this protocol, high-speed and long-distance. When using the high-speed communication mode you can achieve 750 kbps but only at a max cable distance of 100 m. While in long-distance communication mode you can go up to 500 m of cable length but only at speeds of 93.75 kbps. Omron is the only manufacturer of this protocol.

DeviceNet
This is a digital network used to connect to simpler devices. Since it also provides power along with communication lines it can make installing much quicker. It utilizes CAN (Controller Area Network) which is the same network technology used in vehicles. Both DeviceNet and Ethernet/IP use CIP (Common Industrial Protocol) which is managed by ODVA (https://www.odva.org/Technology-Standards/Common-Industrial-Protocol-CIP/Overview).

Ethernet/IP
Not to be confused with TCP/IP (Transmission Control Protocol/Internet Protocol) which is what devices such as a computer uses to connect to the internet. Ethernet/IP (Industrial Protocol) uses the Ethernet structure to connect with various devices. Depending on how much the programming software does for you, this might require a steeper learning curve compared to other protocols.

EtherCAT
Originally developed by Beckhoff Automation, EtherCAT is a real-time Industrial Ethernet technology focused on short cycle times (<= 100 us). Each device reads the data in its frame and inserts data to the frame as it travels downstream. Data streams can be as small as a couple bits up to kilobytes information. If the controller only askes for information from one device then the data stream will be shorter and faster compared to gathering information from all devices.

RS-232C
This protocol was commonly found on older computers and was used for connecting printers, scanners, keyboards, etc. because of the cable length and device to device restrictions. With the development of RS-422 and RS-485, RS-232C is slowly being phased out and is still around to accommodate pre-installed systems. The C refers to the revision of this protocol. The primary difference between revisions is the voltage, A being 25V, B is 12V, and C is 5V. In 1962, revision C became the standard and if the protocol is not specified or listed as RS-232 then it is assumed to be revision C.

RS-422
Using differential data transmission, RS-422 was able to accommodate higher data rates and longer distances compared to RS-232C. This advancement also allowed up to 10 devices to be connected to a single driver. It was short lived and replaced with RS-485.

RS-485
Following in the footsteps of RS-422, this protocol was able to achieve a multi-point network where it can have multiple devices and drivers all running at the same time. Since it is still using the same base technology, RS-485 can be used to control some RS-422 devices.


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