Serial Communication between 1 master PIC and 3 slave PIC

[jameschia]

I am using PIC16F877A to do some serial transmission.
Diagram below is the connection between master PIC and slave PIC.

The master PIC will trigger RA pins to ask one of the slave PIC to send some data to ensure there is only one slave PIC will send the data to master PIC every single time.

All of the transmit pin of slave PIC is connected parallel with receive pin of master pin.
This circuit work perfectly when only 1 or 2 slave PIC connected parallel to master PIC, but it fail to transmit the data when 3 PIC connected parallel.

Any idea?

 

[Pommie]

That is because you have bus contention. You can't just join outputs together, you need diodes and a pullup to form an OR gate.

Mike.

 

[jameschia]

Pommie, can I know what is the pullup i means?
And for the diode, is it need to be connected like the figure below?
Any requirement on the diode or any diode can be use?
View attachment 62044

 

[Pommie]

The diodes need to be the other way around and where they join needs a 10k resistor to the 5V line.

Look up diode AND gate.

Mike.

 

[jameschia]

Is it something like this?
By the way, I should form an And gate or Or gate because you telling different gate in different comment.

 

[jameschia]

I tried the method and it work perfectly. thanks a lot!!!!