Multiple PICs on usart network

[HenrikOlsson]

Hi,
This....

TXSTA.5 = 1                                 ' Enable USART TX function
HSEROUT (slave sends reply only when spoken to by master)
TXSTA.5 = 0                                 ' Disable USART TX function

...is also a bad idea.

The HSEROUT will load TXREG with the last byte to be sent, then the program continues. If the next line turns off the transmitter the last byte won't be sent properly.

Poll TXIF to determine when you can turn off the transmitter.

If this is for anything more than messing around, use RS485 or something designed for the purpose.

/Henrik.

[Demon]

...
The HSEROUT will load TXREG with the last byte to be sent, then the program continues. If the next line turns off the transmitter the last byte won't be sent properly.
...

Weird, nothing unusual came up when I was doing my tests back then; no bytes lost, no data loss. I'm going to have to test this with a higher volume of data transfer and add the polling of TXIF as you suggest.

I just read a RS485 app note and my head hurts.
http://www.analog.com/static/importe...tes/AN-960.pdf

Robert

[tumbleweed]

TXIF only means that you can load the TXREG.

You need to poll TRMT to know when the byte has been shifted out of the TSR (Transmit Shift Register) and you can disable the output. TRMT gets set halfway into the STOP bit.

[HenrikOlsson]

Robert,
I've done quite a bit of work with half duplex RS485 where you have a pin telling the tranceiver chip if it should operate in TX or RX mode.
If I do

TxRx = Tx   ' Switch tranceiver to TX mode
HSEROUT["Test",10,13]   ' Send data
TxRx = Rx  'Sitch tranceiver to RX mode

The last byte (13) will not go thru properly because the tranceiver chip is switched from Tx to Rx mode in the middle of it. I'm pretty sure you'll get the same thing when disabling the USART. It's possible though that a low oscillator frequency in combination with high baudrate "masks" the problem.

Tumbleweed,
TRMT is the correct bit, thanks for pointing that out!

/Henrik.

[richard]

they might get away with it because the last chr is 13 , so that means the last 4 bit and the stop bit all leave the tx line high anyway

[Demon]

It was 64MHz at 115,200 with a checksum that always balanced.

I can't explain it but it worked. But I'm going to do intensive testing soonday and report back here.

Robert