BUSY line technique

[Ron Marcus]

(cut and pasted from the guts of another thread)

Hi,

I'd appreciate a description of the BUSY line technique between 2 PICs please.

I am using a Master 16F628 with multiple Slave 16F628 using USART communication (HSERIN/OUT via TX/RX lines). I'd like to be able to use a BUSY line to indicate when the line is busy.

At first thought I would think that HIGH = NOT BUSY and LOW = BUSY, that way there is no power signal at the same time as communication, less risk of EMF disruption.

It is not known which Slave will be connected to the system. There might be none, or there might be several, that's why I'm looking for a flexible way of Slaves to send data.

Robert

First of all, the EMF is usually induced by switching signals. Having a line high or low should make little difference. If it was rapidly switching states, it would produce noise. It helps your design if you use an industry accepted format for handshaking. I2C seems perfect, because the accepted states are low, or high impedance. There is a pull up resistor on the line to keep it high unless a slave or master pull it low to initiate data transfer. Look at the I2C function in PBP. This may be all you need. You may connect as many nodes to the line as you want, as long as they are in a high impedance state (input). One of these nodes taking the line low is the signal for the master to begin polling the slaves.

Hope this is helpful,
Ron