I am using a Bluetooth modem device that has 3.3v TTL output
Its instructions state that its TXD can be sent to a 5v TTL RXD input.
For Inputs from 5v to 3.3v inputs it gives a resistor divider circuit to use.
But straight in for 3.3v ouput to 5v input is shown.
The Pololu 8 servo board has 5v TTL input
i have been running the pololu board with the 3.3v TTL TXD connected to 5v Input Data pin of the pololu.
It all works well in testing.
I use it at 38400 baud
(doesnt seem to like 56700 even on hard wired cable to PC to the RS232 inputs)
I have not run 5v output of the pololu to 3.3v RXD of the modem as yet but will do using voltage level drop circuit given by manufacturer.
Now I am about to buy a batch of 20 of each to fit them to equipment I sell.
I read the threads about the Chumby TTL
Jan advises that there may be certain conditions were the threshold might reach 4v logic.
What conditions would that be.
(both pololu and modem device are on same master board plugged into, fed with 3 voltage levels to handle modem at 3.3, pololu V+ at 9v and Servo load input at 5v or thereabouts.)
Why would 4 volts be an issue
As I understand it the signal only arrives when the input pin is pulled low, as there is no flow of current at high between 3.3v TTL and 5v TTL.
How would the pololu even know the 3.3TTL line input is 3.3v if it is not drawing current from the 3.3v side wehn it is HIGH.
I assume both outputs and inputs are floating, current only flowing were the output on the 3.3TTL pulls to LOW
At which point both inputs are LOW
Is there a flaw I am making in these assumptions ?, if any.
In short, if it works now on the bench, why will it fail to work at some later time.
(If I read the Chumby thread right)
Would that be if the servo input voltage went below 5v by too much ?
VDD is 9v and 3.3v on the modem will not drop as they come from another source.