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Author Topic: 5V usb to serial converter, reliable?  (Read 2261 times)
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gabriel
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« on: August 20, 2010, 12:05:03 12:05 »

Hello:
I bought recently a USB-to-serial converter, after doing some tests I noticed that outputs signals aren't +/- 8 V like old ones, but they are logic levels 0/5V.
is this normal? are they reliable? anyone has already experienced them? can we interface them directly to a microcontroller without a MAX232?

thanks
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romi007
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« Reply #1 on: August 20, 2010, 02:28:46 14:28 »

Check data sheet of converter, it should says that it is USB to RS232 with TTL output. You can use it directly to uC if it is powered by 5V.
Regards,
Roman.
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gabriel
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« Reply #2 on: August 20, 2010, 05:12:45 17:12 »

it's a converter without a datasheet nor a brand name, it comes just with a CD for the driver
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romi007
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« Reply #3 on: August 20, 2010, 06:04:16 18:04 »

Well, if you are shure it is 0/5V (TTL level) then you can connect it directly to uC.
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oldvan
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« Reply #4 on: August 20, 2010, 06:19:54 18:19 »

RS232 is inverted from TTL, so at the very least need to invert the signals
to interface to a uC UART.
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romi007
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« Reply #5 on: August 20, 2010, 06:30:54 18:30 »

I have MOXA converter (USB to RSR232) and never have to invert signals. Probably it is already done with USB/RS232 chip inside converter. But good to remember.
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DreamCat
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« Reply #6 on: September 15, 2010, 10:59:52 10:59 »

"Serial" just means the signal is serial transimission. maybe is RS232 standard, or TTL, or the other..
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Sideshow Bob
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« Reply #7 on: September 15, 2010, 12:41:51 12:41 »

Check data sheet of converter, it should says that it is USB to RS232 with TTL output. You can use it directly to uC if it is powered by 5V.
Regards,
Roman.
I have heard that cheap USB to RS232 converters do this trick. The problem is that it may not work well on all RS232 equipment. It is also true what the RS232 signals are inverted. But this always taken care of by the RS232 to TTL level converter IC. That is why the IO on say a MAX232 chip is drawn as inverters
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DarkClover
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« Reply #8 on: September 15, 2010, 02:24:07 14:24 »

Well, almost every USB to RS232 converter you can buy in store just has
TTL level on the RS232 interface.
To get the "real" levels you need some step-up electronic and op-amps.

A while ago i did that for a project. There was a step-up converter which created
~10V out of 5V USB voltage for driving the op-amps and they made the general
level conversion. It's quite complicated to do such upconversions because you don't
have enough current for every component so be sure to use low power design.

It worked well but if you just want to drive some microcontroller you only need
a RS232 level converter IC (e.g MAX232) and your cheap USB to RS232 adapter.
« Last Edit: September 15, 2010, 02:43:59 14:43 by DarkClover » Logged

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« Reply #9 on: September 15, 2010, 02:45:46 14:45 »

Also a lot of laptops that have a rs232 output port, the output is at TTL or 9V level never at original true - & + 12V Level.
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