| Author | Message | ||
     Kurosawa |
Hi Court, I want to figure out the correct value for a resistor to protect the Palm E2 and TX, which can't be fed more than 3.3 volts on their TTL lines. The ECM puts out 5 volts on the data link (TTL lines). What I need to know to make the formula work is what's the max current that is drawn into the data receive line, and what's the max current that's pushed out the data transmit line. Yeah, I know this terminology is bad, and my electronics ability is even worse, but I hope you know what I'm asking. Thanks in advance! | ||
| Reuel |
You can make a voltage divider with resistors. Connect the 3.3 volt TTL to ground through a 650 ohm resistor, and to the 5 volt signal through a 330 ohm resistor. That should put you right at 3.3 volts for a 5 volt input. (Message edited by Reuel on March 18, 2008) | ||
| Kurosawa |
OK you're going to have to type reeeeal slow 'cause I'm hard of understanding. On the Palm end I have a 3.3v TX and a 3.3v RX line. On the Buell end I have a 5v TX and a 5v RX line. Now where exactly would these resistors go? Thanks in advance! (Message edited by kurosawa on March 18, 2008) | ||
| Reuel |
One's TX is another's RX. The resistors I'm suggesting have worked between 5 volt controllers and 3.3 volt devices, so should work with your application.  In this diagram, the upside down tree is ground, otherwise known as common, or negative from the battery. You'll do best if your third conductor ground is a shielding for a 2 conductor wire for the TX and RX wires. If you add up both resistors and divide by 5, then multiply by 3.3, you should get the value of the bottom resistor. If you're close, but not exact, you want the bottom resistor to be less than the result rather than more. TX from 3.3 to RX on 5 will work fine because typical 5 volt inputs see a logic high around 2 to 2.4 volts or so. | ||
| Kurosawa |
Thanks a huge bunch Reuel! | ||
| Reuel |
For posterity: We who posted here did more research and discovered that the signal of the 5 volt TX already had a 1000 ohm resistor before the output, which allows a direct connection to the 3.3 volt RX. The 3.3 volt TX can also be connected directly to the 5 volt RX because TTL levels require 2 to 2.4 volts for a logical high. |
