Decoder enable issue

Hi,

For the jumper placed in J1,J4
To select decoder 1, i will make A3 =1;A4=0;A5=0;
To select decoder 2, i will make A3 =0;A4=1;A5=0;
To select decoder 3, i will make A3 =0;A4=0;A5=1;

On doing so, the second decoder is not enable, that means A4 will not become high. The first and third decoder enable pin becomes high when i want to enable it respectively. The problem is only with the middle decoder only.

But when kept open all the jumpers J1 - J4, the A4 pin becomes high for the second case.

What could be the issue here?

Thanks,
Murugesh

Remove the jumpers you don't need them. The pin RA4 is open-drain output so you have to use pull-up on that pin if you want to make it high.

A normal digital output can be driven HI and LOW by the PIC- it's what you would normally think of a digital pin.

Normal digital pins have a pull-up transistor and a pull-down transistor. One transistor pulls the pin HI (by connecting the pin to +V), and the other pulls the pin LO (by connecting the pin to GND). An open-drain output has no pull up transistor and only has the pull-down transistor. This means that instead of being able to make the pin go HI or LO like a normal digital pin, the pin can only go LO or high impedance/floating. This means you need a pull-up resistor to make the pin HI.

The "drain" on the LO transistor is "open" (not connected to a pull-up transistor). Open collector means the exact opposite (the drain on a MOSFET is the equivelant of the collector on a BJT)- it can go HI, but goes high impedance instead of going LO and a pull-down resistor is needed.

Thanks jayanth.

But when i measure the voltage at the A4 pin without connect it to decoder, the high and low values are correctly obtained. How?