connecting_external_devices
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connecting_external_devices [2018/07/10 10:35] – created akuzmuk | connecting_external_devices [2022/01/14 21:37] (current) – ↷ Links adapted because of a move operation 127.0.0.1 | ||
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====== External devices connecting ====== | ====== External devices connecting ====== | ||
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Wiring diagram of WEBHMI to devices with RS-485 interface. | Wiring diagram of WEBHMI to devices with RS-485 interface. | ||
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Probably (if you already have experience with this interface), after reading the phrase RS-485, you immediately decide to use a 2-wire circuit, connecting to the standard terminals A and B, since only 2 wires are really enough to transmit the signal (a lot of specialists adhere to this opinion). However, this is a generally accepted error. The EIA/RS-485 standard says that 3rd wire is needed: | Probably (if you already have experience with this interface), after reading the phrase RS-485, you immediately decide to use a 2-wire circuit, connecting to the standard terminals A and B, since only 2 wires are really enough to transmit the signal (a lot of specialists adhere to this opinion). However, this is a generally accepted error. The EIA/RS-485 standard says that 3rd wire is needed: | ||
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Sources for the formation this fallacy can be: | Sources for the formation this fallacy can be: | ||
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Thus, for a reliable connection it is necessary to use a 3-wire circuit. The operation of the three-wire circuit is shown in the following figure: | Thus, for a reliable connection it is necessary to use a 3-wire circuit. The operation of the three-wire circuit is shown in the following figure: | ||
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Here Vos is the transmitter bias voltage (measured relative to the common and midpoint of the voltage divider with the same shoulders connected to the A and B outputs), Vgpd is the voltage between the " | Here Vos is the transmitter bias voltage (measured relative to the common and midpoint of the voltage divider with the same shoulders connected to the A and B outputs), Vgpd is the voltage between the " | ||
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As for the 2-wire circuit of the isolated RS-485, it is also unstable for the following reason. The system can be under parasitic induction in this case, but the current noise " | As for the 2-wire circuit of the isolated RS-485, it is also unstable for the following reason. The system can be under parasitic induction in this case, but the current noise " | ||
- | ===== Connection | + | ===== Connection |
First of all, after connecting the device and creating a connection, you can create a register in a known state, for example, the bit " | First of all, after connecting the device and creating a connection, you can create a register in a known state, for example, the bit " | ||
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- | A dash line instead of value means that there is no connection. | + | A dash line instead of value means that there is no connection |
- | To deal with communication issues on the built-in RS-485 port TX/RX LEDs on the front panel will help with diagnostic - during | + | To deal with communication issues on the built-in RS-485 port TX/RX LEDs on the front panel could be used. During |
- | In the absence of RX - it is clear that the device does not respond at all, i.e. the format or speed of the exchange may be incorrectly configured. | + | In the absence of RX blinks |
- | If the RX LED flashes in an irregular manner, as if " | + | If the RX LED flashes in an irregular manner, as if " |
If the value is read but incorrect - the wrong data type is specified in the register description, | If the value is read but incorrect - the wrong data type is specified in the register description, | ||
connecting_external_devices.1531218937.txt.gz · Last modified: 2018/07/10 10:35 by akuzmuk