Active low transmitter question

[LEDave]

'you might want to send a heartbeat to indicate tx link is active and working'

Good idea.

'why the tx once only obsession ?'

Not an obsession, I'm just trying to understand the mechanism here.

Another Question:

If an input pin is internally held HIGH and you put an external HIGH onto that pin, does the input pin 'see' the external HIGH as a state change? Or does it ignore it because it's internally held in a HIGH state already.

[richard]

logic pins by definition have two states high or low.
a state change will be caused by any action that causes the pin voltage to reach
the level of the logically opposite threshold
so a low state is maintained until vih is reached
a high state is maintained until vil is reached
see data sheet for vih/vil threshold values

[LEDave]

'a high state is maintained until vil is reached'

Ok, cheers, so:

If an input pin on the transmitter is internally held HIGH and you put an external HIGH onto that pin (from the pic) no Tx would occur as no state change has taken place, is that correct? (vih would would always equal vih).

[richard]

no state change has taken place, is that correct? (vih would would always equal vih).

more like ( ̶v̶i̶h̶ Vin would would always ̶e̶q̶u̶a̶l̶ exceed vih)

the state will remain high till Vin is less than or equal vil where its guaranteed to go low

[LEDave]

'the state will remain high till Vin is less than or equal vil where its guaranteed to go low'

So switching Vin LOW (from pic) causes a Tx?

And

Transmitter will continue to Tx until Vin goes High?

[Ioannis]

No. Only when there is a state change. We said that. Transmission happens on change once and only.
But you shouldn’t care at all. You should care only for your logic state. And maybe your voltage levels not to fry your module.

Ioannis

[LEDave]

'No. Only when there is a state change. We said that.'

Yes you did Ioannis, I'm just trying to understand the mechanism here.

'You should care only for your logic state. And maybe your voltage levels not to fry your module.'

The data sheet does say [High impedance inputs, LVCMOS/LVTTL compatible, 5V tolerant] would you say I could leave 5v on the Input pin to transmitter and switch to 0v/ground to cause a Tx safely? There are two module versions below, should I go for number 2 to be safe? Or is there a low voltage 12f683 that you know of, or is 5v fine?

1/ Part Numbering 2.4 -3.6V Version

2/Part Numbering 3.6 -15V Version