Time padding on RS232

[cylence]

I am trying to simulate a particular serial protocol (DMX512, used by stage lighting equipment) using the USART
of the PIC16F877. The bps speed is at 250KHz (1 bit = 4u sec. wide).
I need to manually craft the packets so they can be 100% compatible with the equipment.
Obviously the idle state for the serial communication is always a "high" state.
I need to put the level to a low state for at least 88 uSec (20 bits), this is
the "break code" of the packet according to the DMX protocol. Here is a simplified sample of my code to acheive this :

void main()
{
setup_adc_ports(NO_ANALOGS);
setup_adc(ADC_OFF);
setup_psp(PSP_DISABLED);
setup_spi(FALSE);
setup_timer_0(RTCC_INTERNAL|RTCC_DIV_4);
setup_timer_1(T1_DISABLED);
setup_timer_2(T2_DISABLED,0,1);
do
{
output_C(0x0); //low state
delay_us(128); //Break code delay.
putc(0xA); //sample data
}
while(1);
}

in my ".h" I have the following :

#include <16F877.h>
#device ICD=TRUE
#device adc=8
#fuses NOWDT,HS, NOPUT, NOPROTECT, BROWNOUT, LVP, NOCPD, NOWRT, DEBUG
#use delay(clock=20000000)
#use rs232(baud=250000,parity=N,xmit=PIN_C6,rcv=PIN_C7,bits=8)

Note that I need to programm a 128 usec delay in order to acheive an actual 88 usec delay at the output, wich is the
execution latency (around 40 usec).
My problem is that the USART does my "time padding" but not at the correct state. I need it to be at a low state for that
period of time but the USART always outputs it at 1.
Can someone let me know how to override this or even tell me if its feasable to override the state of the serial pin while
idling or a better way to acheive this ? Your help would be greatly appreciated.
Thank you.
_________________
Gravity Works.

[cylence]

Yes, I am actually using a 75176 transceiver at the output of my PIC USART.
Transceiver works well, my problem is with the logic state of my time padding.

[asmallri]

There are several ways to do this. Here is a couple:

1. You can take control of the TX pin away from the UART and do the break yourself.

2. You can change to baud rate of the UART to a lower rate and send a 0x00 character. Once the character has gone you can then change the rate back again.

[cylence]

I'm guessing I should use the "SET_UART_SPEED()" command to reduce the transmit speed, right ?
Also how can I take the controll away from the UART ? Do you have some example code ?

As for my latency explanation, I didn't program this to serialize a previous character.
I have probably misunderstood some concepts. I simply noticed on my logic analyser that when I tried to
output a level to a certain amount of time, the result displayed was always shorter than the time
programmed. So I streched it to 128 usec to be able to see a delay of 88 usec on my logic analyser. This is surely not
the best way to acheive it, but its all I've came up with so far.

Thanx in advance ;)

[Guest]

there r a dmx decoder:dm118

http://www.freeweb.com.tw/sd/upload/product/File/Product/DM118X%20_SD.pdf


http://www.skylinedynamics.com/