[SOLVED] USB to RS232 exactly similar to COM1

[steve_rb]

Hi All,

I have purchased a few USB to RS232 converter but non of them couldn't give me exact COM port of a desktop PC. In my desktop PC running windows xp voltage of pin 3 of COM1 with respect to 5 (Ground) is -11 v when you turn the PC on and nothing connected to COM1 measuring ohmeter and pin 2 with respect to 5 is 0 v but non of the usb to RS232 converters I purchased does not have these voltages (only one model has both 2&3 5v with respect to 5 and the rest zero) and that is why when I connect my handled Niton XRF device to desktop PC it is working OK but when I connect to my laptop via USB to RS232 converters I purchased it is not working. Anyone knows what is the reason of this difference and which converter should I use to get my device to work? Manufacturing company selling C2G brand converter but it is not available locally. I want to find a replacement converter in local market.

thx

 

[KlausST]

Hi,

Read RS232 specifications.
There is no need for 11V. If Im not mistaken, then even 3V / -3V levels comply with RS232.

If your connected hardware does not work with levels below 11V, then this device does not comply with RS232.

Klaus

 

[steve_rb]

Device works with COM1 of my desktop computer and also with COM1 of my old heavy laptop. This means it comply with RS232. As far as I know COM1 is RS232. I checked voltages of pin 3 with respect to pin5 in both COM1 ports in idle condition. On my desktop it was -11 v and on my laptop it was -7 v. This means RS232 idle level is 1 (not 0) and between -5 and -15 volts as can be seen here (https://www.lammertbies.nl/comm/info/RS-232_specs.html) and this is the one I measured in both my old laptop and desktop computers COM1 port. Right?

 

[KlausST]

Hi,

This means it comply with RS232

No.
It just says it works with your computer, but it does not say anything about compliance.
*****
True: RS232 idle state = high = 1 = negative voltage (lower than -5V)
*****
...It seems I was mistaken with my statement about voltage levels.
They need to be at least +/-5V loaded at transmitter side.

*****
* There are several different pinnings at the connectors
* some DTE device do not wire all RS232 signals
* some DTE devices have different control signal (RTS, RTR, DTR) status at power up. They might change when opening the serial device at the PC software.
* some devices draw their operating current from the RS232 signal lines...maybe causing too much current

Klaus

 

[std_match]

...It seems I was mistaken with my statement about voltage levels.
They need to be at least +/-5V loaded at transmitter side.

The +/- 3V is correct for the RS232 receiver, so for short cabling it should work if the transmitter gives at least +/- 3V.

I don't think the voltage levels is the problem here.

Some older software can only handle COM1-4 correctly, bus USB serial ports can get COM5 or higher.
In Windows 7 (I don't know Windows 10) you can go to the device manager and force the USB serial port to be COM1-4.

 

[steve_rb]

Problem is almost solved but not quite. I have a USB-to-TTl converter and it gives me +5 volt at pin 2 and 3 with respect to pin 5. I have added a MAX3232 rs232-TTL converter and now I am having -9 v at pin 2 with respect to pin 5 and 0 at pin 3 but I need revers. I mean I need pin 3 to be at -9 and pin 2 to be at 0 with respect to pin 5. This will be recognized by my old device. I have connected TXD,RXD,GND and VCC on usb-to-TLL to the same signal pins on MAX3232 rs232-TLL converter. How can I make pin 3 to have -9 and pin 2 to have 0? This is exactly what I have on COM1 of my desktop PC. Have I done something wrong? People have claimed MAX3232 can give true RS232 but in my case it is almost true but voltages are reversed at TXD and RXD ?

 

[andre_luis]

What has not been clear here so far is the purpose of this port in your application, whether only communication or communication + power supply. Some older native COM port-based programmers intricately pull their power from the RS-232 bus itself, and this was no problem with native interfaces which as stated above have a reasonably higher voltage level than those commonly seen in USB converters.

 

[std_match]

Problem is almost solved but not quite. I have a USB-to-TTl converter and it gives me +5 volt at pin 2 and 3 with respect to pin 5. I have added a MAX3232 rs232-TTL converter and now I am having -9 v at pin 2 with respect to pin 5 and 0 at pin 3 but I need revers. I mean I need pin 3 to be at -9 and pin 2 to be at 0 with respect to pin 5. This will be recognized by my old device. I have connected TXD,RXD,GND and VCC on usb-to-TLL to the same signal pins on MAX3232 rs232-TLL converter. How can I make pin 3 to have -9 and pin 2 to have 0? This is exactly what I have on COM1 of my desktop PC. Have I done something wrong? People have claimed MAX3232 can give true RS232 but in my case it is almost true but voltages are reversed at TXD and RXD ?

You must have connected the MAX3232 wrong. The RS232 outputs from MAX3232 are on pins 7 & 14, and one of them should be connected to pin 3 (TXD) of a male DSUB-9 if you want to emulate a PC serial port (DTE).

 

[KlausST]

Hi,

draw a sketch.

Klaus

 

[steve_rb]

Yes I got the pin number wrong. Now it is OK and device working OK. Thank you all.