Difference between Arduino serial communication and Computer serial communication

Projects Programming
1

I'm trying to make a Wemos D1 Mini talk to a motorized serial device with a specialized motherboard. I have been able to get the device to respond to commands over serial-usb on termite on 8n1 9600, but it won't respond to the Arduino when issued the same commands. I bought a Max3232 thinking that it originally had to do with way the Wemos was communicated through UART, But I am still encountering the same issue. Is there a difference how the Wemos communicates through serial vs how termite or a regular computer does?

Here's a picture of my code configured for the Max3232.

sketch capture
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2

The MAX323 is a level shifter that converts a signal from TTL (0V to 5V) to RS232 (12V to -12V). The Wemos D1 probably uses 3.3V logic (0V to 3.3V) which will generally work with TTL (0V to 5V).

3

might be helpful to have a diagram with devices

the max232 not only shifts the level but the levels are inverted ~5V = ~-12V

are you sure this is typically true? are the switching thresholds that similar?

4

Works for me. 5V CMOS switches at about 2.5V so 3.3 is high enough to count as a 1.

5

of course a HIGH 5V output should be above the 3.3V switching threshold but is HIGH 3.3V output high above the 5V switching threshold?

6

Check the datasheet. Section 30.2 DC Charcteristics.

Max LOW input = 0.3V
Min HIGH input = 0.6V

Min HIGH output (20 mA, Vcc=3.0V) = 2.3V

Since 2.3V is well above 0.6V then, YES, an output on a 3.3V Arduino will be read properly by a 5V Arduino input pin.

7

Good chart:

Logic Levels - learn.sparkfun.com

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8

while i've seen a mix of technologies, CMOS, LS, don't know if i've ever seen a mix of voltages.

what happens when you drive a 3.3V with the output from a 5V device?

9

Some 3.3V inputs are 5 Volt tolerant; no worry.

For 3.3V inputs, non-protected, the input voltage will likely damage the input gate unless current limited by a high resistance.

Personally, I'm a fan of level-shifters on even 5V tolerant pins.

10

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