R15 is the series resistor for controlling the optocoupler U2 ( CT817C Receiving side). The 1k resistor has to be soldered parallel to R15. It is also important to note how U5 was soldered in, as the cables are soldered according to the pin numbers according to Table 1.įig: Position of U5 (unmodified board) Solder in 1k resistor This must be avoided at all costs, as we still need the pads to solder the cables. But you have to make sure that the pin is not heated for too long, otherwise the conductor track underneath will become detached from the circuit board. If you don't have this, you can heat up each pin of the IC individually with the soldering iron and carefully bend it up with a needle. The easiest way to unsolder it is with a hot air desoldering station. The IC U5 is not required because we do not use the Modbus and connect the serial TTL data signals (3.3V) directly to the Wemos D1 mini.
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