I'm using a EM-406A GPS module for a project and the datasheet requires that the input pin RX (which takes TTL) be kept HIGH when not in use.

To do this, it suggests "from Vcc, connect a 470ohm resistor in series with a 3.2V Zener diode to Ground. Connect RX input to Zener's cathode to pull the input HIGH". Vcc in this case is 4.5V-6.5VDC."
(http://www.usglobalsat.com/store/download/46/em406a_ug.pdf)

My question is, why is this particular configuration used, as opposed to say, a resistive voltage divider? If possible, please also note the behavior of the RX pin voltage when either logic high or logic low are provided.

I think it has something to do with the output characteristics of the TTL transistors that are providing the input, but my understanding of transistor logic is still weak.

 

jump a bump to request help. thanks.

 

The zener just gives you a fixed high input level compatible with the logic supply level. Not specifically stated what the logic supply level is - 3.3V??

That's probably the only reason. It's usually good practice to follow the manufacturer's advice. And this isn't a huge issue in terms of cost.

 

Yeah the cost isn't the issue, I'm just trying to understand how the circuit works.

You asked what the logic supply level was - I think the RX pin referenced above is expecting between 1.5-2.0V, since it wants a TTL HIGH signal. That is, if i'm reading the follow chart correctly: http://www.interfacebus.com/voltage_threshold.html

Does that answer your question? Let me know your thoughts, thanks

 

There's 3.3V & 5V TTL.....

 

The Vcc voltage range is pretty big. A resistive divider would give a lower but still variable level. Using a Zener in this way gives you a more consistent level, higher than the minimum required, but not too high.