NEEDED HOWTO: USB to RS232 with External Power Supply 12VDC

[KnutOlafson]

Hi there,

Using standard USB-to-RS232 converters I have problem on some external devices when connected (communication do not work...). I suppose reason is that USB output voltage is low (5V) and cannot have more then 5V at RS232 side. This way, some devices that I connect to this cable using RS232 cannot communicate and needs RS232 connection with voltage closer to 12V.

Is there a way to have USB-to-RS232 converter that use external power supply which way I can have 12V on RS232 side of converter ??

Does anybody have solved this issue and how?

I am newbie at PCB production but can handle it somehow OR is there a way to find such product which I can buy under the reasonable price?

Thank you.
Knut

 

[etmabreu]

Usually those kind of converters have some kind of charge pump inside capable of providing around +/- 10V. This should't be a problem for any RS232 device. However, I found by experience that sometimes some converters do not perform as they should for some baud rates. Once I had a device at 19200baud and I was having trouble in the receiver, had to change the baud rate to 9600 to get a stable connection.

 

[KnutOlafson]

Usually those kind of converters have some kind of charge pump inside capable of providing around +/- 10V. This should't be a problem for any RS232 device. However, I found by experience that sometimes some converters do not perform as they should for some baud rates. Once I had a device at 19200baud and I was having trouble in the receiver, had to change the baud rate to 9600 to get a stable connection.

Well, it doesn't work with Cisco equipment or Satellite receivers with serial port when used converter. Using PC with only serial cable it works....

(edit) same story with car diagnostics equipment .... Many people is looking for older PCs and notebooks with serial port 'cause USB-to-RS232 do not work as it should ...

 

[tpetar]

Yes there is true USB RS232 converter with full RS232 signaling, also for laptops/notebooks you can buy extension cards with true RS232 ports.

 

[KnutOlafson]

Yes there is true USB RS232 converter with full RS232 signaling, also for laptops/notebooks you can buy extension cards with true RS232 ports.

Sorry but have to ask! Is there a commercial USB-to-RS232 converter with signal levels of traditional RS232 OR it has to be home-made ? If you have a link to commercial version I would appreciate if you can share it with us. Of course, in my previous message you can find applications for which I would like to have this device.

I am not interested for PCMCIA COM ports for laptops.

Thank you.

 

[tpetar]

Yes my friend there is commercial USB to True RS232 converters in markets, and you dont need external power supply of 12V to get 12V.

This is just a one example, but there is tons of real RS232 converters, google on Internet :

https://www.adafruit.com/blog/2011/01/24/back-in-stock-usbserial-converter-pl2303/

USB/Serial Converter – PL2303. This USB cable adds a DB9 serial port to your computer or laptop. Works with MacOS X, Linux and Windows. Note that this provides +-12V serial (RS232) not 5V serial (TTL), good for old devices like GPS, handhelds, programmers, etc with chunky DB9′s on the side.

 

[KnutOlafson]

Ok, but one I have is based on PL2303 chip and it doesn't work. I have to make second check.

 

[tpetar]

Ok, but one I have is based on PL2303 chip and it doesn't work. I have to make second check.

What do you mean ? Sorry I didnt catch concept of this post.

 

[KnutOlafson]

What do you mean ? Sorry I didnt catch concept of this post.

I had one converter that did't work with my satellite receiver and I hve sold it and have bought another one which is based on this particular chip. Problem remains Same as with old one. Both converters I have tested with Cisco equip I have at job and result was the same, too. Then I was reading on net that allll usb-to-rs232 sufering from low voltage problem since all they have is max voltage they have from usb port. Now I was wondering if it is possible to build some voltage booster at rs232 side of converter... That is an idea for gurus from this forum ... PCMCIA Com ports are expensive ...

Last few days i have figured out that those converters can have additional problems - some of them have only Rx and Tx for communication and some devices requires other Rs232 pins ... Or it is driver problem

Similar problems have people using diag tools in car services ....

 

[bigdogguru]

The issue maybe your laptop. Some laptop USB interfaces are notoriously under powered and have issues with many USB based devices.

Concerning adapters, I personally use:

Tripplite Keyspan USA-19HS


with a Toshiba Laptop and have successfully connected with and managed virtually every type of Cisco equipment manufactured, past and present, with the appropriate console port.

So, there definitely are USB Serial Adapters available which do perform as expected. However, I can also add from personal experience, not all USB to Serial Adapters are created equal, many of the less expensive models do not function properly in some circumstances.

The Keyspan USA-19HS utilizes the TI TUSB3410 chipset and is available on ebay, for sale in the $20 range.

BigDog

 

[KnutOlafson]

Thank you all on your help. I need to check my cable again against all odds I have mentioned and turn back here.

 

[tpetar]

Before buying I suggest to contact sellers and get more information about product if possible.

---------- Post added at 23:55 ---------- Previous post was at 23:40 ----------

You have this ?

UT-2303 USB to Serial Converter Cable
Permanent Link
**broken link removed**

 

[KnutOlafson]

This time I'm not buying new one untill get some digging on this one...Thanks.

 

[tpetar]

This cable working with some device, did you try ?

Change COM17 to some lower COM value, try in range of COM3..COM6

This is just small suggestion.;-)

 

[KnutOlafson]

When I connect pins 2 and 3 with screwdriver and type some text "hello world" in HyperTerminal I have reply "hello world" in HyperTerminal .... so, in basics it works... I don;t have any other particular device to test it now and I'll try in next few days when I find some free time to play with it. In futher, I have null-modem cable that I use for satellite receiver (it works when used on Desktop PC) which is few meters long and need to shorten it and try .... maybe it is too long for usb-to-serial adapter...

Never mind, I should do it for weekend or so..

Thank you.

 

[tpetar]

Ok, I just mean its ok and working, this "hello" test confirm that. ;-)

Yes my friend try shorter cable, that can be problem.

In 90'-00' I flashed many EchoStars receiver softwares even realtime decoding over RS232 port with RS232 cable without problems. Cable length was max 1,5m, I think maybe 1,2m.

 

[KnutOlafson]

Here are the "voting" results:

I have shortened pins 2 and 3 and have echo in HyperTerminal which means that cable is OK regarding communication logics.
Next, using voltmeter I have measured that I have voltage between pins (let's say between 3 and 5) MAX 6.38VDC. I have measured pins respective to the ground pin (pin no. 5). Between other pins to the ground it can be 5.05VDC ...

Next, I shall try to shorten my null cable which I use for flashing of my sat receiver and make reply here ...