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In this case, I had a very easy way to do this. In my case, I just had a simple question: Are you running? This includes speed control using Variable Frequency Drives (VFD) or inverters, as well as controlling simple PLC and HMI network systems. PLC systems are used to monitor and control operations, while HMI network systems allow interaction with machinery and control them. With Modbus RTU, sending and receiving are never simultaneous, so half duplex is as fast as full duplex. Half duplex has the advantage of only using three wires: ARX/ATX, BRX/BTX and GND. Since the difference between my low water level and my normal water level is about three inches, this worked perfectly for me. If on the other hand each terminal can accept only a single cable, a proper branch must be created using three auxiliary terminals for each instrument to be connected. In order to avoid signal reflections, a 120 Ohm termination resistance must be fitted on each end of the main cable.

The RS485 driver, with 22 Ohm isolation resistors and built in electro-static protection, allows 32 standard loads at 4000 ft. The foreword to the standard references The Telecommunications Systems Bulletin TSB-89 which contains application guidelines, including data signaling rate vs. RS-485 supports inexpensive local networks and multidrop communications links, using the same differential signaling over twisted pair as RS-422. To set up communications for your meter or counter, see the Serial Communications section of its user manual. An RS-485 cable is a type of communication cable used in serial communication systems. Henceforth, we shall consider Slave devices to be measuring instruments with serial communication, even if the cabling is similar for all Modbus devices. The RS485 system used for Modbus communication provides a main cable (Bus or backbone), to which all the devices have to be connected with branches (also known as stubs) that are as short as possible. On my Raspberry Pi, I utilize a Low Power Labs MightyHat which provides UPS power backup to the Pi, an LCD status screen and intelligent power control for the Pi. Provide a clean web interface to see the status of the system and allow me to start and stop filling the pool via the web.

Eventually the sprinklers will stop running and the pool will start to fill automatically. Initially I used a "blackout" time zone - i.e., if I knew my sprinklers were running from 3 am to 6 am, don't fill the pool during that time. One thing I also wanted to do was to monitor the filter pressure so I knew when to back flush our filter. The one-way valve prevents pressure from the pump from pushing water against the sprinkler valve when the pump is in operation. I also have a momentary switch (second on the left) that allows me to manually fill my pool when I want, and finally on the left side I have a DPDT switch that physically interrupts power from my system to the sprinkler valve and triggers a GPIO event to tell the system we have manually disabled filling the pool. Regardless of what triggers the pool filling routing (manually or automatically), I had to have a way to get the water into the pool.

It started with a problem with the water lines and valves that we inherited when we got the house, filter issues and a basically inept pool company that was a holdover from when we got the house. We were hit with a few setbacks, one of which was when they installed the fence they poured concrete around the lines going from the filter system to the pool. Now that the physical plant was installed and operational, I thought it would be nice to be able to fill the pool without having to get the hose out all of the time. Since my pool electrical panel is 240V and I have no neutral in the panel, I had to get a 240VAC to 24VAC transformer. Since we are required to have a vacuum breaker between our irrigation and our city water supply already, I choose to tie it into my existing sprinkler system to get water for the pool. I also added a smart meter which I can read in real time to show me that water is flowing and how much water over time I have been using. Over time I added in another piece to my pool control system.

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