UART RX problem

Hi guys,
I am working on a Projekt where I hook up an Arduino as Vending machine controller to a coin validator and a bill acceptor.
So when I only connect the coin changer everything is working fine, but if I also connect the bill validator I get no more signals from the devices not even the coin changer.
So my guess was that the pull up resistor value is wrong, so I testet 220 ohms, 1k 10k and 100k but nothing worked. Or is my guess completely wrong, what could cause this?

I hook up the devices via a y-cable so TX and rx lines just got split. I use the coinco guardian xl and the coinco vantage.

Greetings Tarcontar

Image from Original Post so we don’t have to download it. See this Image Guide



It's a nice picture - but what is it trying to show?

You can't simply connect two devices to an Arduino Rx pin because Serial idles HIGH and they will interfere with each other. You need diodes to keep them separate.

Also, how can you ensure that only one of the devices talks at any one moment?

The simplest option (assuming you are using an Uno) is probably to use SoftwareSerial to create an extra serial port.

If none of that makes sense please provide more info about what you are trying to do.


thanks for inserting the pic, I am on mobile phone so i attached the pic this way.

The MDB Protocol works as a bus, so the arduino is the master and talks to the slave devices via adresses, so there are no diodes etc needed.

Coin changer:

Bill validator:

MDB protocol:

I am guessing you are using the current loop interface, since you have an opto-isolator in the diagram. If one of the two wires from either machine is reversed, you will get the description you described.


What exactly do you mean by that? RX and TX line flipped?