Multiple PICs on usart network

[sayzer]

1. Two Schottky diodes on TX lines could be one solution. This way, one TX line does not effect the other.

or if not using diodes,

2. After each Transmit, you may disable USART and make TX pin input pin. When you need to send again, make TX pin an output pin and enable USART, then transmit.
If one TX line is an input pin, it will not effect the network during the transmission of other TX line.

These two are only suggestions and are not tested.

[Demon]

Can't comment on the diodes, but disabling USART also turns off receive. That's why I disabled only transmit.

[dhouston]

The URL I referenced in post #4 shows diodes in TX lines. There's no coding required. Schottky diodes would be best.
http://cool-emerald.com/?p=711

[JoseJimenez]

Mr Demon,


I definitely solve this.
You will need a couple of transistor per each Tx output and 3 resistor.


As you said, if you disable the usart, also Rx line will be disable.
If you use diodes, you will need a conversor as MAX 232...
Problem with diodos, is due to signal keep positive if Tx is idle. Plus this, Impedance for Tx and Rx line are so sensitive.


If want to communicate several PIC without using interfacing, you can do it using the schematic that I will post next.


/jose p.

[Demon]

The URL I referenced in post #4 shows diodes in TX lines. There's no coding required. Schottky diodes would be best.
http://cool-emerald.com/?p=711

How fast a baud rate do you think this can handle?

[dhouston]

How fast a baud rate do you think this can handle?

I don't know. I seldom need more than 9600bps with embedded devices.

[Demon]

I needed 115200 at 48mhz, not sure diodes could handle it. Switching coms in software doesn't add hardware limitations.

[timmers]

Attached is a pdf schematic similar to Jose P's posting, but halving the number of transistors per channel.
Also need to add a TX BUSY bus to prevent collision. (Hold to ground when transmitting, tristate when idle).
A similar circuit ran fine at 115200 baud.

Almost any signal diode in series to the TX pins (cathode to the TX pin) with a bus pull up resistor on the anode could also work fine.

Tim.