Serial communications issue on pins 0 and 1 but not on USB?

Using Arduino Programming Questions
1

Hello,

I can post the code if needed however I have a simple sketch to control a power distribution unit via RS232.

I have serial communications set up at a baud rate of 9600 on the sketch and all terminal emulators.

When I open a terminal emulator on the USB connection COM port I see the data being sent from the Arduino without an issue. Simple output shown below,

0x00
Cycle
recycle P61=2

The issue is when I open a terminam emulator to a COM port that is connected to pins 0 and 1 (and ground) on the arduino I am getting ascii garbage similar to what you would see if you had mismatched baud rates as shown below.

ëöëY=åë5¿_
åë5¿_
9'5¿_9

If I am not mistaken the serial output should be the same over USB and from pin 1 and 0 yet the data from pin 1 is always garbage.

Any insights?

Thanks!

2

terminam emulator to a COM port that is connected to pins 0 and 1

How is your com port connected to pins 0 and 1 ?

...R

3

I am using a USB to serial adapter connected to a DB9 cable I made up that has TX, RX, and GND wired to pins 1, 0, and GND respectively.

Hopefully that answers your question!

4

RS232 and TTL serial levels are different voltages and inverted. Are you taking this into account?

5

I did not know this so I am not taking it into account. I assumed I could just read off of the TX/RX pins just like from USB and it appears I assumed wrong!

Could you enlighten me how to take this voltage difference into account?

I think you have given me enough information to google the solution! It appears as though I will need a hardware device plugged inline to do the conversion? Something such as this: SparkFun RS232 Shifter - SMD - PRT-00449 - SparkFun Electronics

Thank you for your information!

6

I don't understand where the DB9 connector comes into the picture. The usual USB-TTL converter plugs into a PC's USB port and provide a 5v TTL signal Rx Tx and Gnd at the other end - this for example

If you already have a serial port on your PC with a DB9 connector you will, of course, need an RS232 - TTL converter. But then you would not need any USB - serial connector.

...R

7

Robin2:
I don't understand where the DB9 connector comes into the picture.

I will eventually be using this to control a power distribution unit that has a RS232 interface with a DB9 connector.

Currently for testing though I wanted to check the output with my computer. I was using a USB to RS232 adapter into my computer due to lack of a serial port on my computer. So yes for talking to the computer a USB to TTL would work fine but for the end product I will need TTL to RS232.

Hope that makes more sense!

8

The MAX232 chip comes to mind.

9

Idrake:
Hope that makes more sense!

Yes, but it would help if you provided a schematic showing how things are connected.

...R

10