Why 74LV066D can work?

Hi, ALL,

I found a circuit, which used 74LV066D to switch on or off of the RS232 signal.
The power supply is 3.3V.
But we know RS232 is >3V (+) or <-3V (-), so I doubt how 74LV066D can work?

Does 74LV066D can output more than 3.3V signal?
Or just output 3.3V and GND signal, so how about the negative signal of RS232?

Anyway, the circuit can work all the temperature. I really worry about it.
Who can comment?

Best,
Tony Liu

Hi,

first: it is 74LV4066.

Then:
* your schematic doesn´t show any voltages (except 3V3 supply)
* I assume it is just a UART application to transform 3V3 levels to 5V levels and back
* Because it can´t work with true RS232 levels I assume the signal named "RS232" is just the often misused word "RS232" instead of "UART".

But to be sure we need to see the rest of the schematic with all the signals and voltages.

Klaus

KlausST,
Yes, you are right.
One side it is linked to STM32 UART port. This port I am sure about.
The other side should be linked to RS232 port, but I am not sure about it, I guess so.
So STM32 UART port output 5V TTL, not real RS232, right?
Best,
Tony Liu

Hi, KlausST,
Yes, you are right.
RXD etc. nets are linked to STM32 UART port.
And the nets with "R" prefix are converted by a IC, MAX3237, then linked to one laptop USB port with a USB/DB9 cable.
So I think the process is as following:
1. STM32 UART port output 5V TTL signal
2. 74LV4066D is one switch to switch the direction of the UART signals, because there are two directions, only one can be selected.
3. MAX3237EIDBR as relay or level compatible function, change 5V TTL to 3.3V or 5V??? to a DB9 serial port
4. Then by a link cable, link to the USB port of laptop.
I am a little confusion about the level convertion here.
Who can comment?
Thanks.
Tony Liu

- - - Updated - - -

I understood a little:
MAX3237 converter TTL levels to a standard RS232.
Then I see.
Thanks.