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.
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.