Can the DO cards and relay amplification boards of Zhejiang University Zhongkong’s 300 system be extended using cables?
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Extend it by 50 meters and place the relay amplification board in an explosion-proof box in the power distribution room or the workshop; in this way, a 25-core cable can transmit 16 DO signals. There are various advantages, including cable savings: previously, a 2-core 1.5-square-millimeter signal cable was used to transmit one DO signal. Now, a 25-core 0.5 square millimeter cable can be used to transmit the 16 channels of signals from two DO cards. Previously, the start/stop signals for motors were sent to the control cabinet; these signals were of alternating current type and caused interference in the control cabinet. Now, the start/stop signal wiring is done on-site in the power distribution room, which facilitates wiring for multiple projects: the motor that needs remote control is wired accordingly, and those that don’t require it have their wiring removed, allowing for more rational use of resources. The DO signal drives the coil of the on-site shut-off valve in the workshop; the current is high, at 500 milliamps, and the voltage drop is significant. If the wire diameter is too small, it will not be possible to activate the solenoid valve. Now, output is done on-site; the relay amplification board is powered by a 24V power supply inside an explosion-proof box, with very short wiring. If this project is completed successfully and another project is launched, the layout can be done quickly, which facilitates future maintenance. The output from the DO output card should be a voltage signal with a certain current, used to drive the light-emitting diode on the side of the optocoupler. Therefore, when the wiring is extended, it is necessary to ensure that there is sufficient capacity to drive this optocoupler; moreover, any interference voltage is high in magnitude but low in power, that is, the current is small, so it is unlikely to interfere with the remotely transmitted DO signal.
It is possible. A 25-core cable is used to connect the DO card to the relay amplification board and extend it to 50 meters, thereby placing the relay amplification board inside an explosion-proof box in the power distribution room or workshop. This allows for centralized and simplified signal transmission, helps reduce the use of cables, minimizes interference, facilitates maintenance, and improves the efficient use of resources. .
The relay board serves to isolate signals (to protect the cards) and provide power; if it is too far away from the cards, how can the cards be protected?
This post was last edited by Xin Chang Xiao Xu on 2024-6-25 at 14:41. The function of the relay board is signal isolation and amplification; this is its most important role. The output capacity of the DO output card is very low, allowing it to drive only a current of a few dozen milliamps. There is also isolation within the card; it’s similar to optocoupler isolation. An optocoupler can be regarded as a transistor with isolation capabilities, as its base, collector, and emitter are electrically isolated from each other. When the control card receives a command from a person or from a program segment that tells it to turn a certain DO output on, this command is sent to the DO card through the data forwarding card. The CPU in the DO card then sets the corresponding DO output on, which causes the LED on the base to light up, and the collector-emitter pair on the other side of the optocoupler becomes conductive. The power supply for the DO amplification board comes in two types: one on the coil side and the other on the contact side. The coil side is connected to the power supply, and the signal flows to the optocoupler output side of the DO card; I think there is another stage of amplification here, with the amplified signal being used to drive the relay coil. The contact side corresponds to the relay’s output circuit; one terminal provides three outputs: one passive normally closed contact, one passive normally open contact, and another active normally open contact. The power supply on the contact side is used to supply power to the active normally open contact. If the wire gets longer, the current output will decrease at the same voltage; it is only when the current is too high that the components can be damaged. With a longer wire, the main concern is insufficient driving capacity. The 25-pin serial cable provided by Zhejiang University is about as thick as a thumb; the diameter of each wire inside it is around 0.1 or 0.2 square millimeters. Now that the cable has been made thicker, the increase in resistance shouldn’t be that significant, and the current flowing through it will be only a few milliamps. I’ll conduct an experiment to measure this. Either the output or input terminal of the optocoupler is equipped with a current-limiting resistor; a self-resetting fuse may also be present. The DO output module is not prone to damage.
Personally, I think it is not advisable from a safety perspective as well; having the relay board on site increases safety risks. Secondly, amid the current extensive safety inspections, the components of the DCS have been moved to the site, where many explosion-proof boxes can be seen. When asked what they were for, we explained that they contained the DCS relay boards. The experts then said that this did not meet safety requirements and required correction, which posed a serious problem.
Now the cabinets can be placed on site; do you follow the experts’ advice in everything?
Both domestic and foreign manufacturers already have solutions for placing cabinets on-site; you should also learn more new knowledge
Cabinets are generally required to be placed in blast-resistant cabinet rooms; currently, PLCs are an exception. It’s not a matter of whether experts agree or not – in my discussions with many designers, I disagree with placing them directly on site. If one insists on doing so, what then? Will you argue with the experts when they ask you to remove them?
There are too few experts with the necessary skills these days; those who understand something about electronics, microcontrollers, and safety-related topics are scarce. Often, they simply rely on their academic qualifications and years of experience in this field, and they consider anything they haven’t seen as unreliable and therefore avoid it.
The word “expert” is now used in a derogatory sense