RS232 TTL converter

Projects Networking, Protocols, and Devices
1

Hi guys,

I bought this RS232-TTL converter
But I can't get it to work.
RS232 signal seems to be OK. I can see data on oscilloscope.
Connections:
RS232 device GND -> Converter GND (DB9 pin 5)
RS232 device TX -> Converter RX (DB9 pin 2)

Converter VCC -> Arduino 5V
Converter GND -> Arduino GND
Converter TX -> Arduino RX (pin 0)

Light on converter lights up, when it is powered.
Also when I hoop up my oscilloscope to converter, I can't see no voltage only some noise.
Is something wrong with my connections?
Thanks!

2

What are you trying to do with it?

I only use those converters to communicate with the Arduino Pro Mini clone which don't have the on-board USB port. I buy the USB-TTL converters because I don't have DB9 connectors on any of my current PCs.
something like FT232RL FTDI USB 3.3V 5.5V to TTL Serial Adapter Module for Arduino Mini Port | eBay

3

Ok, I'll bite! Where was the scope probe connected when you see noise?

Try connecting the RS-232 TX directly to the RS-232 RX and see if the TTL output of the adapter follows your TX data.

Paul

4

ieee488:
What are you trying to do with it?

I only use those converters to communicate with the Arduino Pro Mini clone which don't have the on-board USB port. I buy the USB-TTL converters because I don't have DB9 connectors on any of my current PCs.
something like FT232RL FTDI USB 3.3V 5.5V to TTL Serial Adapter Module for Arduino Mini Port | eBay

Hi. I am trying to read serial data. When i connect probe to RS232 device TX pin. I see -8 volts on oscilloacope. Which states it needs to be lowered and shifted. Which ise why I use RS232 - TTL converter.

5

Paul_KD7HB:
Ok, I'll bite! Where was the scope probe connected when you see noise?

Hi Paul

Probe was connected to Rs232 - TTL converter TX pin. And GND to GND. And VCC was connected to arduino 5V and GND also to Arduino GND.

Try connecting the RS-232 TX directly to the RS-232 RX and see if the TTL output of the adapter follows your TX data.
Paul.

I think I cannot see or log it out. There are no RX/TX led pins that would indicate me when there ise some data moving

6

rat97:
I think I cannot see or log it out. There are no RX/TX led pins that would indicate me when there ise some data moving

Something like this?

Serial.begin(9600);
pinMode(LED_BUILTIN, OUTPUT);
Serial.write(0xAC);
while(Serial.available() == 0);
if(Serial.read() == 0xAC)
  digitalWrite(LED_BUILTIN, HIGH);
while(1);

Pieter

7

PieterP:
Something like this?

Serial.begin(9600);

pinMode(LED_BUILTIN, OUTPUT);
Serial.write(0xAC);
while(Serial.available() == 0);
if(Serial.read() == 0xAC)
  digitalWrite(LED_BUILTIN, HIGH);
while(1);



Pieter

Yeah well Arduino serial works, but I can get no output from converter TTL pins :confused: It may be broken? Even if there is no RS232 signal, the converter RX/TX pin should give me 5 volts, right? If I power it up by 5 volts?

8

rat97:
Hi. I am trying to read serial data. When i connect probe to RS232 device TX pin. I see -8 volts on oscilloacope. Which states it needs to be lowered and shifted. Which ise why I use RS232 - TTL converter.

Then you have other problems! The RS-232 TX under "mark" condition, no data being transmitted, should be a + voltage, not a - voltage. The start bit on a data transmission begins with a + to - transition.

Paul

9

rat97:
Yeah well Arduino serial works, but I can get no output from converter TTL pins :confused: It may be broken? Even if there is no RS232 signal, the converter RX/TX pin should give me 5 volts, right? If I power it up by 5 volts?

No, the converter uses a charge pump circuit to give you more than 8 volts, + and -, for the RS-232 circuit. The RS-232 standard is the voltage can be anywhere from +- 3 volts to +- 25 volts. Anything between +3 and -3 is noise and ignored.

Paul

10

Paul_KD7HB:
No, the converter uses a charge pump circuit to give you more than 8 volts, + and -, for the RS-232 circuit. The RS-232 standard is the voltage can be anywhere from +- 3 volts to +- 25 volts. Anything between +3 and -3 is noise and ignored.

Paul

I am sorry you lost me :frowning:
This is the RS232 Serial signal that I receive from my device.

800×480 7.69 KB

I need to give it to Arduino, but yeah as we can all see it is a negative voltage and -8 volts.
So what I need is that RS232-TTL converter. But I don't know how to connect it.

11

rat97:
I am sorry you lost me :frowning:
This is the RS232 Serial signal that I receive from my device.

800×480 7.69 KB

I need to give it to Arduino, but yeah as we can all see it is a negative voltage and -8 volts.
So what I need is that RS232-TTL converter. But I don't know how to connect it.

I see that and it shows 2 volts peak-to-peak, with the correct + and - for the data. The -8 volt average is bogus. When did you last calibrate that device?

Somewhere in the manual there must be something that tells you the "voltage" is actually the cursor position on the display. That value tells me the cursor is somewhere way off the screen at the bottom.

Paul

12

The forum will not show your last post, which I got a copy of via email. I went back to your very first post on this thread to try to understand, again, what the problem is.

I see you have connected your adapter to pin 0 of the Arduino.

The following is directly from the Arduino documentation page:

Digital Pins 0-1/Serial In/Out - TX/RX (dark green) - These pins cannot be used for digital i/o (digitalRead and digitalWrite) if you are also using serial communication (e.g. Serial.begin).

So, you are connecting to the pins reserved for the serial communications between the arduino and your PC, using the USB connection.

Probably nothing wrong with your adapter.

Paul

13

Paul_KD7HB:
The forum will not show your last post, which I got a copy of via email. I went back to your very first post on this thread to try to understand, again, what the problem is.

I see you have connected your adapter to pin 0 of the Arduino.

The following is directly from the Arduino documentation page:

Digital Pins 0-1/Serial In/Out - TX/RX (dark green) - These pins cannot be used for digital i/o (digitalRead and digitalWrite) if you are also using serial communication (e.g. Serial.begin).

So, you are connecting to the pins reserved for the serial communications between the arduino and your PC, using the USB connection.

Probably nothing wrong with your adapter.

Paul

Yes I deleted my last post. Because I wanted to attach a better picture. I am using pin 0 and 1 only for serial read. There must be something going on with my connections then. :confused: But I can not figure out which pin I should connect to where...

14

rat97:
Yes I deleted my last post. Because I wanted to attach a better picture. I am using pin 0 and 1 only for serial read. There must be something going on with my connections then. :confused: But I can not figure out which pin I should connect to where...

That means you are not using the PC to power the Arduino?

Use your scope as a signal tracer! Power the adapter.

  1. get the trace from the TX wire from your mystery device, just like in the picture.
  2. Apply that wire to one of the possible connections on the socket of the adapter. Can't hurt anything.
  3. look at each of the output pins with the scope. A trace with 0-5 volts may be found on either connection.
  4. IF a trace is found, then you now have the correct input and output connections for the adapter.
  5. If no trace on either connection, move the data wire to the other socket connection and repeat the search for a trace on the board connections.
  6. If no output is ever found and there is still a good data trace from the mystery device, then the adapter is not working.

Paul

15

Paul_KD7HB:
That means you are not using the PC to power the Arduino?

Use your scope as a signal tracer! Power the adapter.

  1. get the trace from the TX wire from your mystery device, just like in the picture.
  2. Apply that wire to one of the possible connections on the socket of the adapter. Can't hurt anything.
  3. look at each of the output pins with the scope. A trace with 0-5 volts may be found on either connection.
  4. If a trace is found, then you now have the correct input and output connections for the adapter.
  5. If no trace on either connection, move the data wire to the other socket connection and repeat the search for a trace on the board connections.
  6. If no output is ever found and there is still a good data trace from the mystery device, then the adapter is not working.

Paul

Hi Paul.

My success so far.
I see some data moving from a converter.
This is what I see if I connect my oscilloscope to TX pin of the converter and oscilloscope GND to converter GND, leaving VCC pin unconnected.

800×480 7.79 KB

This is what I see if I connect Arduino 5V to converter VCC, Arduino GND to converter GND, and oscilloscope probe to TX pin.
800×480 8.09 KB

That seems quite right, but not really sharp corners, which is why I think my Arduino monitor shows me some boxes on console. The baud rate/bits etc should OK.

16

UPDATE:

Hell with that. I built my own RS232 shifter with a couple of transistors and resistors. Works like a charm. Thank you all for helping. Although if you still can help me, please do so, I just want to figure out, what is wrong

17

Hi,

Please note that the chinese RS232 to TTL converters have the pinout straight, i.e., TXD on the converter should be connected to TXD on board, etc.