I want to send 7 sensor data from Arduino uno to nodeMCU. But using serial I can send only one. How to send all 7 at the same time with time sync between the both devices.
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You send the values one by one. A serial (UART) has a buffer. So you can send multiple bytes to a device; there the bytes are stored in a buffer, until you retrieve them.
Everything regarding timing will be handled, as long as you don't send more bytes than the buffer size, before reading them from the other device.
Arduino Uno nodeMCU Your UART UART Your Program Buffer Buffer Program ------- -------- ------ -------- Uno: Write byte 1 Uno: UART library 1 Uno: Write byte 2 Uno: UART library 1,2 UART devices (asynchronous) 2 1 nodeMCU: Read byte(s) 2 1 Uno: Write byte 3 Uno: UART library 2, 3 UART devices (asynchronous) 2,3 nodeMCU: Read byte(s) 2, 3
Above is just an example, you only read/write bytes, the communication between the buffers and over the UART line is asynchronous.