TTL Serial Details

william chan

I have heard that for Inverted Logic, idle is at low.
Does this mean that the Start Bit for Inverted Logic is high ( logic 0 ) ?

Does this also mean that for True Logic, idle is high? and Start Bit is low ( logic 1) ?


Is the Stop Bit always equal to the idle level?

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PJAllen

Rightwise, in RS232, idle is a negative voltage, the start bit is a positive voltage and the stop bit is a negative voltage.

"Ideally", TTL would interface with a MAX232 (or similar), which is an inverter.

So, TTL, in this situation, would have idle a positive, the start bit is 0V, and the stop bit is 0V.

Chris Savage

william chan said...(trimmed)
Does this mean that the Start Bit for Inverted Logic is high ( logic 0 ) ?
Does this also mean that for True Logic, idle is high? and Start Bit is low ( logic 1) ?

William, did you mean Logic high is logic 1 and logic low is logic 0?· They look backward to me.

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Chris Savage
Parallax Engineering

william chan

Sorry,

Somebody said...

Does this also mean that for True Logic, idle is high? and Start Bit is low ( logic 1) ?

this one should read (logic 0).

Does this mean that we should use Inverted logic for driving an IR transmission because idle is Low voltage leading to zero LED current? ( we don't want the IR LED to be continuously on when idle ).

To receive such an IR transmission, should the receiver should be True Logic
because most Sharp IR receivers pull the data pin low when receiving a 40Khz IR signal?

I tried this but the receiver kept receiving wrong byte values.....

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Mike Green

Yes, the receive end should be true logic because the usual IR receivers are low true. The transmit end depends on how you're driving the IR LED. You should transmit the start bit and other zero bits and leave the IR LED off for idle.