Need help with RS232 wiring

[tsopkow]

Hi, I am trying to make an RS232 cable to connect a digital pressure module to a PLC. I purchased a cable that the maufacture supplies that uses a usb-->RS232 convertor (Note: The convertor supplies 5 VDC power to the module). I am able to communicate with the module using a terminal emulator with the command they supplied.

However, a PLC will not accomodate a USB connection so I need to make up my own cable. I have connected as follows

Module Rx pin to DB9 pin 3
Module Tx pin to DB9 pin 2
Module VCC pin to External Power supply V+
Module GND pin to External Power supply V-

This will only echo back the commands I send (even garbage commands).

I opened up the ends of the manufactures cable (See attached photos). It appears I have wired it the same, except for the GND, which I think could be the problem.

I think it is the GND that is the problem, the power and the communications could be two separate sources. Where do I connect the DB9 GND (pin 5) to?

Thanks in Advance.
Terry

 

[Diver300]

DB9 connectors always need pin 5 connected, in my experience. I think that all you have to do is to wire 0 V to pin 5 and to the -ve power input of the pressure sensor.

DB9 RS232 uses the original voltage levels, so logical 1 (idle) is between -5 V and -15 V, while logical 0 is between +5 and +15 V.

A lot of "RS232" converters use 0 - 3 V or 0 - 5 V with logical 1 (idle) at 3 (or 5) V and logical 0 at 0 V

A USB to RS232 converter can come in two forms, one that outputs at the original levels and the other that is just 0 - 3 V or 0 - 5 V. I don't know which yours is.

 

[For The Popcorn]

It looks like the converter only has a USB – UART chip, meaning it's using 0-5v (or 0-3.3v) signals. It needs a MAX232 chip to convert 0-5v signals to the bipolar levels required by RS232.

Look for a converter that explicitly states it's a USB – RS232 converter.

 

[Diver300]

It looks like the converter only has a USB – UART chip, meaning it's using 0-5v (or 0-3.3v) signals. It needs a MAX232 chip to convert 0-5v signals to the bipolar levels required by RS232.

Look for a converter that explicitly states it's a USB – RS232 converter.

I think that the OP could communicate to the pressure module using the USB converter and a PC. It was the PLC he was having trouble with.

 

[For The Popcorn]

Ok, I missed a detail....

If the computer can talk to the sensor using the pictured USB – UART module, then the sensor is using TTL levels, not RS-232 levels. I would expect this to be the case with the sensor, especially as it's powered by 5 volts.

Therefore, it's still an issue of RS232 levels vs. TTL levels. The DB-9 connector of the PLC will be using RS-232 levels, so a MAX232 chip is still needed.

Something like this. They are available with either male or female pins, start with whatever mates to the existing DB-9 connector. Pins 2 & 3 may still need to be swapped, with pin 5 (ground) connected between the PLC and converter.