I never expected that they would post such a "science popularization" post.
Some people have raised the question of choosing between PROFIBUS and PROFINET, so I compared a car from 2000 to one from 2018, and the result was unconvincing! Okay, then let's talk about fieldbuses. Since this is a popular science post, there's no need to be too academic (I don't have the ability to be). I can't go into too much detail; I only care about using them, not developing them. Just like when someone asked me about the principles of PLCs, I was completely baffled... Let's just chat casually, going wherever the conversation takes us.
I. What is fieldbus?
Simply put, devices need to communicate, and communication requires both hardware devices and software protocols. Fieldbus is what standardizes these two aspects. Let's take Siemens' PROFINET bus as an example; it's actually a variant of the TCP/IP protocol! We know that TCP/IP has a seven-layer architecture (look it up if you don't know). PROFINET modifies two (or three, I'm not sure) layers of that architecture, defining a communication format as its communication protocol. Therefore, it inherits the high-speed characteristics of TCP/IP. Hardware-wise, it's similar to TCP/IP, so we can use any switch, even the cheap ones costing a few dozen yuan, and connect a network cable—the network will work just the same.
II. Why use fieldbus?
For example, a PLC system controls 5 workstations, each with an average of 20 cylinders (each cylinder has 2 position signals, and the corresponding solenoid valve has 2 control signals), 5 analog signals, and 5 sensors. Without fieldbus, I would need to run all these cables to the main control cabinet. Let's calculate the total number of cables: 80 cylinders (20 * (2 + 2) = 80 points) + 5 analog signals + 5 sensors, 90 cables per workstation, * 5 = 450 cables! Looking at the densely packed terminal blocks, doesn't it feel incredibly complex? If the distance is far, running this bunch of cables would be incredibly cumbersome! However, if we use fieldbus modules, each workstation can be configured with a station, and each station can directly connect to the area's cables. The main control cabinet only needs one power supply and one bus to connect to each station. The wiring is significantly simpler, more efficient, and easier to maintain. There are other devices, such as industrial robots, which have dozens or even hundreds of communication signals with the PLC. Running cables for all of them? Are you kidding me? With fieldbus, one cable is all you need!
III. What are the fieldbus types?
The mainstream fieldbuses on the market include: PROFIBUS, PROFINET, DEVICENET, CC-LINK, EtherNet/IP, etc.
In the early 1990s, a series of fieldbuses were introduced. Siemens rallied a group of partners to develop PROFIBUS, AB (Alibaba and New Balance) gathered a team to develop DeviceNet, and the Japanese, realizing they had already mastered the technology, teamed up to develop CC-Link… and many more. Due to technological limitations at the time, they were all based on RS-485, so they all shared a common characteristic: terminating resistors were required. The Siemens PROFIBUS-DP (often abbreviated as DP) interface, if used with a PLC or a low-IP-level interface within a cabinet, looks exactly like a 9-pin serial port! It has a red DIP switch; switching it to ON activates the terminating resistor. As for Mitsubishi, I recently bought some Mitsubishi CC-LINK modules, and when I saw the terminating resistor, I was like, "What is that?!" It looked like just a single 0.5W ceramic resistor (no, there were two; different modes required different resistance values, and you had to look at the color bands to distinguish them)! Subsequently, with the development of Internet technology and the widespread adoption of network communication, major manufacturers, seeing the trend was not right, launched fieldbuses based on the TCP/IP protocol, such as PROFINET—PROFIBUS, EtherNet/IP—DeviceNet, and CC-LINK—CCLINK-IE (I have not used CCLINK-IE, but this is the general idea; please correct me if I am wrong), all of which were developed under this wave.
Fourth, since the basic architecture is similar, why bother with so many variations? Wouldn't it be better to unify it?
This is a good question, much like unifying the world's languages. To put it bluntly, it all boils down to two words: profit! Each bus protocol has a leader with several followers, forming their own alliance of interests. For example, if you choose a Siemens PLC, you either have the outdated PROFIBUS or the newer, faster PROFINET; there's no third option. Now, let's say I need a servo drive. Company A is aligned with Siemens and can provide PROFINET products. Company B is closer to AB and only offers EtherNet/IP. Choosing Company A allows you to fully utilize the convenience of the bus, while Company B only offers terminal control. Which would you choose? In reality, third-party vendors play both sides. For example, Turck can provide both PROFINET and EtherNet/IP products, but they don't offer them for free. Let's say Turck sells a PROFINET module for 2000 yuan and needs to pay Siemens a 200 yuan royalty fee; the same applies to EtherNet/IP. Of course, these prices are just my guesses; that's the gist of the matter. I remember posting something like this many years ago, when I was chatting with an engineer at LENZ. I said, "It feels like all these fieldbuses are just scrambling to claim territory, each with its own domain." He replied, "Yeah, we have to pay royalties to sell PROFINET bus communication modules, and they're not cheap."
5. Which fieldbus should I choose?
Haha, classmate, you're overthinking it. You basically have no say in this matter. When it comes to the brand of PLC used in project planning, the vendor usually has no say in the price. Once the PLC is decided, choosing a fieldbus is already a single-choice question.
6. Why doesn't our country have its own bus protocol?
This question is simpler. First, is our technical capability sufficient? I think even if it's lacking, it won't be by much. Second, who will play with us? The leading companies in the mainstream fieldbus market are all well-known companies in the industry, and they all have numerous subsidiaries that can command a large following. We are far behind in this area.
That's all I wanted to say. If you're interested, feel free to read on. If not, there's a cross in the upper right corner of your browser. See it? Move your mouse over it, and yes, the cross will turn red. Click it, and it won't waste your time.
So, what do you all think after reading this? Isn't it clear, concise, and easy to understand? This is the work of a master! Feel free to leave a comment and share this so more people can benefit!
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