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English => Interfaces, Pinouts, Cables => Topic started by: amciek on Oct 03, 2007, 22:03

Title: usb interface
Post by: amciek on Oct 03, 2007, 22:03
hej,

I would like to make usb interface to communicate my laptop with some equipment. This equipment requires "old" type of cable - RS232. I do not have this slot in my laptop so I started thinking about using phone cable. I found that it is some chip to convert usb into rs - it is prolific 2303. I found out that some of the phone cables has got this chip. I bought cable from samsung X600 - it has prolific 2303 inside. I found RX, TX using hyperterminal but the output voltage is to low. Should be 5V (this is my equipment requirement) but it is 3.3V and interface is not working. Of course drivers installed (emulating com).
My question is what kind of phones use 5V cable to communicate with PC.
I do not want to buy every phone cable and check ;).
I found that N-Gage Nokia N70 Nokia N71 Nokia N91, Nokia 1100 Nokia 1110 Nokia 1112 Nokia 1600, Nokia 2300 Nokia 2310 Nokia 2600 Nokia 2610, Nokia 3100 Nokia 3120 Nokia 3650 Nokia 3660, Nokia 6030 Nokia 6230 Nokia 6230i Nokia 6270, Nokia 6600 Nokia 6630 Nokia 6670 Nokia 6680, Nokia 6681 Nokia 6820 Nokia 6822 Nokia 7600, Nokia 7610 Nokia E50 Nokia E60 has got 5V battery.
But I am not sure if I can "think" based on that that communication "protocol" also based on 5V, not 3.3V.

Thanks for your help.
Title: Re: usb interface
Post by: amciek on Oct 06, 2007, 15:19
nobody knows? :-[
Title: Re: usb interface
Post by: amciek on Oct 15, 2007, 19:21
come on experts, do not be shy
Title: Re: usb interface
Post by: neilf on May 01, 2008, 20:39
NOT expert, & don't know what if any phones use 5 Volt
but,
standard RS-232 voltage levels are  + & -12 Volts

Look at    www DOT maxim-ic DOT com

Maxim make chips for (voltage) level conversion with only a small
number of other parts - very simple circuits !
 - bidirectional conversion up & down

MAX-232 = (computer) signals 5 Volt <-> +/- 12 Volt

MAX-3232 = (computer) signals 3 (or 3.3) Volt <-> +/- 12 Volt

& others

Maybe they have a 3V to 5V chip ?

if not
to do Your own level conversion You would have to use something
like
USB-Serial-cable_3V -> MAX-3232 -> 12V -> MAX-232 -> 5V    ????

Best to look at Maxim site & see what They have.
 also there look at Application Notes for relevant chips
-for circuits

hope this helps,
neilf [neilDOTfATexemailDOTcomDOTau]

PS I was waiting for a REAL expert to answer :-)
Title: Re: usb interface
Post by: Pantelis on May 28, 2008, 23:25
I wouldn't say I'm a real expert...but I was looking at this post and deal with this type of stuff everday...

Depends on what you want to do.  Are you creating a circuit board or do you just want a simple adapter that will 'Plug-n-Play'.

1.  Plug-N-Play
There are TONS of adapters out there.  A simple search on Google will get you what you need.  Let me point you in the right way...
ww.usbgear.com/USB-Serial.html

2.  Designing a circuit board...
This is more involved...and you have two problems...

a) correct voltage levels
b) correct protocol

a)  What neilf suggested is partly true.  The Maxim chip is by far one of the tried and true industry standard.  It is commonly used to convert signal coming from a micro-controller's UART port to an RS-232 port.  Generally...the micro's UART port operates on TTL (transistor-to-transistor logic) and is generally in the 3 volts category.  As mentioned before...RS-232 is in the 12 volt category...so yeah..a direct hook-up would mean blowing out transistors on your chip!  (Not good).

While I have seen the Maxim chip used to step up the voltages, I don't believe I've seen one to step them down.  Maxim probably makes a chip for that...just run some parametric searches on their site.

b)  So saying we take care of stepping the voltage down...RS-232 and USB operate in two different protocols.  RS-232's mode of transmission is dedicated pins for receiving and transmitting  (generally denoted Rx and Tx).  In contrast, USB has a differential pair (generally denoted D+ and D-).  A simple hook-up...well...won't do.

Luckily some smart people developed a chip that is really drop-in, load drivers on the host computer and go.  One that I have used many times is the SiLabs CP2102.
ww.silabs.com/public/documents/tpub_doc/dshort/Microcontrollers/Interface/en/CP2102_short.pdf



In my own personal experience...I have hooked up micro-controllers' UART port directly to the CP2102 chip and gotten USB on my projects.  It's obviously very nice since RS-232 has become legacy (hell...Dell CHARGES you an extra 2 bucks to include an RS-232 port) and some micro's don't have a USB port built into the die.  I have not had experience stepping down the voltages though from RS-232 and then going USB.  That will be something you have to try yourself.

Obviously...a simple adapter is the quick and easy solution.  Laying out a board is fun...but there are a lot that goes into it.  After the chips, filter resistors and capacitors, and connections for voltage supplies, you have to be pretty decent with a soldering iron to get the CP2102 chip on there (QFN package...).

I'm no expert...hehe...but yeah...hope it helps.

Pantelis

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