[moved] [TXD and RXD state (0 or 1) in RS485 and RS232 in initial condition

[dharithothi]

My query is what should be TxD and RxD signals initial state in communication. Means is there any defined standard which state that in initial state RxD should be pulled high or TxD should have low level??? Query is for both RS232 and RS485 stds.

 

[FvM]

There's definitely a standard, see e.g. https://en.wikipedia.org/wiki/Rs-232 and https://en.wikipedia.org/wiki/RS-485

You see that the initial or idle RS-232 line level is low (negative voltage). The internal UART (logic level) signal is inverted against external signal.

The RS-232 terminilogy still shows the connection to teleprinter (TTY) current loop interface, with loop current on ("mark") as idle state and and current off ("space") as active signal.

 

[KlausST]

Hi,

It is relatively good specified.
You are mixing RS485 and RS232, but they have different signalling and different voltage levels.

The data lines are all push pull, so they need no pullup or pull down ( as long as you don't need any (optional) fail Safe features).
The differential signal lines of RS485 must be properly terminated with about 120 Ohms.

The digital state in "idle" is high. A start bit is low, a stop bit is high.

s there any defined standard which state that in initial state RxD should be pulled high or TxD should have low level??? Query is

Both RS232 and RS485 are standards, you should read.
Wikipedia is a good place to start. There are links to related documents.

Klaus

- - - Updated - - -

Hi again,

...FvM was faster...

You see that the initial or idle RS-232 line level is low

This is the most confusing: "1", 0", high, low, mark, space.

Wikipedia says for RS232:

For data transmission lines (TxD, RxD and their secondary channel equivalents) logic one is defined as a negative voltage, the signal condition is called "mark". Logic zero is positive and the signal condition is termed "space".

Bold is the idle state.

Klaus