Is it possible to use Tx, Rx pins (from Arduino Uno) with Rx,Tx that use CMOS technology ( 2.8V)? Or must I use any voltage convertor?
2.8 is too low.
A 5V Arduino needs a minimum input High signal of 0.6 x VCC = 3V.
Do you mean Tx, RX must have at least 3V otherwise Arduino will not understand those signals?
And can you please suggest the simplest way how to increase those Rx,Tx voltages coming from CMOS device( and having only about 2.8V) to Arduino Rx,Tx pins ?
Thank you
One way is with a part like this
http://www.digikey.com/product-detail/en/TXB0102DCUT/296-32601-2-ND/1910149
A side is powered from the lower CMOS level, B side is powered with the Arduino 5V level.
TXB0102 is far better, but MOQ is 250.
Here is the work around.
sonnyyu:
Plan D;-Use 8-channel Bi-directional Logic Level Converter IC
1.2 V to 3.6 V on A Port and 1.65 to 5.5 V on B Port
http://www.adafruit.com/datasheets/txb0108.pdf
$3 overkilled.
How about $1.20 for 1 then?
http://www.digikey.com/product-detail/en/TXB0102DCUR/296-22862-1-ND/1740009
$8 (from Adafruit) is for a chip soldered to a DIP-style breakout board.
The TXBxxxx chips are cool, but they're overkill for something like a serial port where the pins are not bidirectional. You can probably get by with a transistor or two and a handful of resistors.
Would a pullup resistor work?
Something like this is probably good. The 74LCX06 is an open-collector inverter that works down to about 2V but has I/O tolerant to 7V, so it's useful for a lot of things. (You can get a non-inverting version (74LCX07), but then it's somewhat less useful.) Less than $0.50, one at a time.
It does seem to only come in SMT packages 
