Need help multiplexing EUSART output

[Kamikaze47]

Hi, The PIC i'm using has only 1 EUSART and I need to send data to 2 different devices without using software serial like SEROUT or SEROUT2.

This is what I want to do:

<img src="http://ii.net/~steven/files/pic1.jpg">

And thus PORTB.6 would control which of the 2 outputs receive the data, and the other output would just be +5v which is the idle voltage anyway.

However i'm having trouble getting it to work. I tried this as a first step:

<img src="http://ii.net/~steven/files/pic2.jpg">

But the data is not getting through. I'm using a Phillips HEF4071PB IC for the OR gates.

Does anyone have any idea why this would not work?

[Jumper]

Those look like AND gates to me and I guess that is what you need to have if you want to use PORTB.6 as enable.

Since you have GND to one side of the AND gate in your pricture.... GND AND anyvalue is still ZERO.

Try to connect one side to 5V or use the PIC connection and set the pin accordingly.

But if you use AND gates the output that is NOT enabled will be low. but if you want the opposite use a NAND.

/me

[Kamikaze47]

They are OR gates.

AND gates would be idle low so they wont work.
NAND gates would be idle high, but the 1s and 0s of the serial transmission would be inverted, so that wont help.

That is why I went with OR gates - idle high, non-inverted signal, logic 0 to enable, logic 1 to disable.

It seemed like a perfect plan, so i don't know whats going wrong.

[Acetronics2]

The obvious number is ...

4081 AND Gates ...
or
4052 Triple 2 Input multiplexer

or two series resistors, one to each addressed device ... and grounding ( or VCCing ) the signal ( past the resistor ! ) with another PIC pin , if NO transmission, and use the PIC pin as an input for transmission.

may be too simple ?

Alain

[Kamikaze47]

or two series resistors, one to each addressed device ... and grounding ( or VCCing ) the signal ( past the resistor ! ) with another PIC pin , if NO transmission, and use the PIC pin as an input for transmission.

may be too simple ?

I just did a few tests. This idea seems to be working. The signal would not get through the series resistor until i reduced the value to about 200 ohms. That means a 25mA draw whenever the signal is low. I guess that's not too bad.

[Kamikaze47]

The only thing that worries me is the possibility of reduced reliability due to the series resistors.