Series resistor + zener diode used to provide logic 'HIGH' ?

Thread Starter


Joined Dec 15, 2009
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."

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.


Joined Mar 6, 2009
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.

Thread Starter


Joined Dec 15, 2009
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:

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


Joined Dec 26, 2010
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.