[JamesC]

Just to close the loop on this.

The issue here was that the MAX232 board that I mentioned above has an onboard 1k pull down resistor on the TX line. The Controllino MAXI and MEGA must have an internal pullup resistor installed on PD3 and that internal pullup resistor and the 1k pull down resistor were acting as a voltage divider.

When I removed the 1k pulldown resistor the board worked fine with Serial1.

[JamesC]

Disregard the above. Below is the correct description.

Just to close the loop on this.

The issue here was that the MAX232 board that I mentioned above has an onboard 1k pullup resistor on the TX line. The Controllino MAXI and MEGA must also have an internal 1k pullup resistor installed on PD3 and that internal pullup resistor and the RS232 board’s 1k pullup resistor were acting as a voltage divider. This is why I was seeing an oscillation between 5V and 2.5V

When I removed the 1k pullup resistor from the RS232 board it worked fine with Serial1.

[JamesC]

I’ve also tested this now with a CONTROLLINO MAXI with the same result as above.

I’ve also tried 24V and 12V power supplies.

[JamesC]

I should also say that I connected both MAX232 boards to a regular Arduino Mega on Serial1 and Serial2 and they both worked fine.

• This reply was modified 1 year, 7 months ago by  JamesC.

[JamesC]

I’ve found a work around by using the AltSoftSerial library.

I had tried using the SoftwareSerial library, but it’s incompatible with my code.

Having said that, it would be extremely helpful to have a fix for the Serial1 issue as it would mean the difference between having to purchase a Controllino MEGA over a MAXI (the pins used in the AltSoftSerial are only accessible on the MEGA) and saving €100+ per controller.

[JamesC]

Hi Jarda,

Thanks for the reply. I thought that “Serial” aka “Serial0” on pins 0 and 1 were used to communicate with a PC.

My issue is with Serial1 which is connected on pins 18 and 19.

James