A common technique for a ‘RS232’ receiver was to use a zener diode, resistor and a ttl inverter. Not quite to spec, but worked in most cases. Similarly, you can use a ttl device to output ‘RS232’ for short distances.
I think what it's saying is that it will accept either TTL (unipolar) or RS232 (bipolar) levels.
RS232 transmitters are usually inverting - so, if you skip the transmitter, you still need to provide an inverter...
I've skipped through the thread, so maybe I'm repeating something already mentioned.
RS-232 is a physical interface standard.
In days gone by it was one of the two many I/O methods for PC's, (the other being the Centronics parallel interface).
RS-232 signal levels are:
0 = +3 to +15 V
1 = -3 to -15 V
These are measured with respect to a common Ground connection between the two devices.
The -3 to + 3 V range is a no man's land, which improves the noise immunity and data integrity for the signal transmission.
In practice, many old RS-232 devices routinely used +/-12V, some original IBM printers and other devices used +/-15V.
When Laptops first starting hitting the market, and size, weight, and power consumption were an issue, many laptops clamped the negative signal to ground, and didn't bother generating a true negative voltage.
Because most devices used a "standard" RS-232 line driver chip set, (1488 and 1489, IIRC), and the receiver worked fine with the ground level replacing the negative level, it worked.
RS-232 inverts the signal. But as one typically has an RS-232 chip driver on both ends of the connection, the signal polarity ends up "correct", (i.e. non-inverted).
TTL is typically 0 and 5 V levels.
So, if one "inverts" the signal, and uses 0/5 V signal levels, one could in fact call it (pseudo) RS-232.
To drive the LCD it appears that one could use the USART module if one inverted the output pin polarity.
On the Xmega's this is a simple set the bit in the register process to invert the I/O pin's polarity.
One could easily do this in hardware with a 2N7000 and a 10K resistor.
This assumes one is only sending data to the LCD, and doesn't need to read data back out from the LCD.
One can often define the output signal polarity when using a software USART, also; which would be another option.
Again, without having read the entire Thread, it looks like the OP might simply have to connect "logic level" (i.e. 5V) signals to the LCD, but INVERT the usual USART output.
Cross post with others who are less wordy than I am...
So any device that accepts a converted TTL signal should be described as a device that accepts TTL?
A device which accepts 0/5 V (logic level) signals can generally be considered TTL compatible.
This doesn't take into account whether or not the signal was inverted somewhere along the data processing chain.
There are lots of "details" about specific switching thresholds and current drive capabilities, but for a simple analysis on your connectivity issue they aren't important.
any device that accepts a converted (sic) TTL signal
What the device sees needs to be a TTL-level signal; that is all - it neither knows nor cares if that signal has been "converted" from something else.
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