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Old 11-07-2011, 10:36 AM   #1
Frenko
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Question Kindle 3 serial cable

Hello folks,

As per object I'm trying to get a serial cable for my (bricked) kindle 3.

What do you think would happen if I took a standard 5V usb/ttl cable and lowered the voltage to 1.8? A friend of mine volunteered to do the trick, but I'm still not sure it's a good idea.

Would it work? Is there a chance it would damage my kindle?

Please advice.

f

PS: I know 1.8v adapter would be the optimum but 5V are a lot cheaper and waaay easier to find....
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Old 11-07-2011, 12:51 PM   #2
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If you want something cheaper, you can try 3.3V adapter, which might not damage your Kindle.
5V will surely fry your reader.
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Old 11-07-2011, 06:00 PM   #3
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Quote:
Originally Posted by seaniko7 View Post
If you want something cheaper, you can try 3.3V adapter, which might not damage your Kindle.
5V will surely fry your reader.
Thank you for your answer.

I'm not sure I made myself clear enough in my previous post.

This friend of mine suggested we used a resistor to lower the voltage to 1.8v... would it still fry my kindle? would it just not work?

At this point I think I'll just order a 1.8v adapter, but I'm curious...

Thank you for your time.

f
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Old 11-07-2011, 06:34 PM   #4
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Quote:
Originally Posted by Frenko View Post
Thank you for your answer.

I'm not sure I made myself clear enough in my previous post.

This friend of mine suggested we used a resistor to lower the voltage to 1.8v... would it still fry my kindle? would it just not work?

At this point I think I'll just order a 1.8v adapter, but I'm curious...

Thank you for your time.

f
It should not, as long as the dropping resistor is giving the correct output voltage. Ohm's law is your friend in finding the correct value for the resistor.

So if you need 1.8 volts and you can get that by using a resistor, you should be fine.

Imo!
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Old 11-08-2011, 06:38 AM   #5
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Quote:
Originally Posted by DustyDisks View Post
It should not, as long as the dropping resistor is giving the correct output voltage.
Thanks!

So if I measure the tension at 1.8v on the serial side of the cable I should be ok?

Quote:
Originally Posted by DustyDisks View Post
Imo!
I'm particularly interested in knowing if it's an opinion or a fact!

And, I don't know if it's a stupid question, but what about tx/rx? Can a transistor cause data losses or any other problem?

Thank you guys

f
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Old 11-08-2011, 07:04 AM   #6
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I think you'd need two resistors between GND and TX (of your serial adapter), where in the middle you could grab a lower-voltage TX. Choose high enough resistor values, as a RS232 interface isn't really made for feeding current through ohmic resistors. But my e-tech skills are quite rusty.

There will be more difficulties, however, if your serial adapter doesn't like 1V8 levels on its RX pin (in that direction - coming from Kindle - lowering voltage doesn't make sense).
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Old 11-08-2011, 07:54 AM   #7
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Quote:
Originally Posted by Frenko View Post
Thanks!

So if I measure the tension at 1.8v on the serial side of the cable I should be ok?


I'm particularly interested in knowing if it's an opinion or a fact!

And, I don't know if it's a stupid question, but what about tx/rx? Can a transistor cause data losses or any other problem?

Thank you guys

f
I would have to have your unit in front of me so I could read the resistances of your circuit. If I knew the circuit resistance then given the source voltage. 3.3 v and then make a good guess of the current needed

Then using ohms law the value of the dropping resistor could be calculated to be used to give your power supply a output voltage of 1.8v .

If you know anyone that is good with electronics, or amateur radio (ham) operator that knows their stuff. They could help you figure it out.

Just knowing your source 3.3v and desired output voltage 1.8v is not enough for me to tell you what value dropping resistor needed. I would have to have a good ideal of the current/resistance of the circuit before that can be calculated accuracy, but I figure something between 60 and 80 ohms , but that is only a best guess. You could try these values and see if the supply voltage drops across them to give you the 1.8v you want.

Imo is just that , my opinion or best guess.

Hope this helps!

Charles

Edit: On the data lines I do not think anything needs to be done, if the supply voltage is correct. The data pulses (Square wave, assuming digital) along the lines tx/rx should be controlled in the unit its self and their voltages are of no concern. The only concern is the supply voltage feeding the circuit.

Last edited by DustyDisks; 11-08-2011 at 07:58 AM.
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Old 11-08-2011, 08:23 AM   #8
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Quote:
Originally Posted by hawhill View Post
I think you'd need two resistors between GND and TX (of your serial adapter), where in the middle you could grab a lower-voltage TX. Choose high enough resistor values, as a RS232 interface isn't really made for feeding current through ohmic resistors. But my e-tech skills are quite rusty.

There will be more difficulties, however, if your serial adapter doesn't like 1V8 levels on its RX pin (in that direction - coming from Kindle - lowering voltage doesn't make sense).
Rereading the thread, you do have some very good points. The 5v or 3.3v adapters may not work at a 1.8v level.

Rethinking , it may be better for the op to buy a correct 1.8v usb ttl serial adapter and be done with it.

The op may only use it once, but having it would give peace of mind. Also more electronics are going in the 1.8v direction.

Frenko

I would recommend that you forget about any modifications and buy the correct cable/adapter. Then you can rest assured that no boggy men are hiding any where in your setup to fry or mess up anything in your hardware.

Here is a link in the proper direction. The TTL-232RG-VREG1V8-WE, sounds like exactly what you need.

http://www.ftdichip.com/Products/Cab...BTTLSerial.htm

Last edited by DustyDisks; 11-08-2011 at 08:25 AM.
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Old 11-08-2011, 09:07 AM   #9
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All right then, I'll play it safe and buy the FTDI 1.8v adapter.

Thank you so much for your advice guys, you've been very helpful


Cheers,
f
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