Among other things such as timing, connector polarity, etc this standard said that logic level '1' was to be represented by a negative voltage and logic level '0' was to be represented by a positive voltage. In the beginning, before there was USB and I2C and SPI, the term 'serial communication' generally referred to RS-232 (recommended standard #232).

'The ATmega328 provides UART TTL (5V) serial communication, which is available on digital pins 0 (RX) and 1 (TX). The key properly is that 0V < VOL < VIL < VIH < VOH < VddīTW these voltages apply to fast changing signals not just DC voltages, TTL outputs rise to a lot higher than 2.5V in a millisecond, its how high they rise in the specified signal rise-time (~ 15ns) that matters for true compatibility. TTL is an entirely separate (and obsolete) bipolar logic family - basically TTL and Arduino will inter-work, but you can’t strictly speaking guarantee that a TTL output can drive an Arduino input high since TTL outputs are only guaranteed to generate 2.5V when high (most will in fact give a lot higher, and the problem is easily cured with a pull-up resistor).Īny logic family is characterised by 4 voltages, VIL, VIH, VOL, VOH (being the guaranteed input voltages and guaranteed output voltages assuming fully loaded according to the family specifications.

Without a load on the pin you are measuring the +5V rail (unless your multimeter has a very low resistance). The output transistors on Arduino pins have about 40 ohms resistance if I remember right - however they have an absolute max rating of 40mA current too, so any safe use of the output pin will produce 1.6V or less for LOW and 3.4V or more for HIGH.

Is digi 003 compatible with logic