>
>im planning to use a continuos loop in the transmitter, so it would send
>the byte over and over again, and the transmitter would catch the byte and
>do whatever it has activated.
>
>but how do i make the receiver recognize the start bit and stop bit????
>

Basically, the receiver sits waiting for a (say) falling edge. That
indicates the start bit, and thereafter the receiver samples the input port
periodically for the length of time to grab one byte. If the last bit
(where the stop bit should be) is received as high, then you (may) have a
good byte - otherwise your receiver was triggered by a glitch, or in the
middle of a byte.

As long as the transmitter pauses occasionally, for longer than one byte's
transmission time, the receiver will finish its sequence and be in a state
of looking for the start bit's falling edge. So the next falling edge from
the transmitter (start bit) will be picked up as such by the receiver, and
the two will be in synch.

Actually, you probably ought to consider a bit-level coding scheme, such as
Manchester coding. To transmit a 1 bit you send 10, and to transmit a 0
bit, you send 01. So the byte 11011011 goes out as 1010011010011010. It
halves the data rate, but you can pick up errors much more easily.
Obviously, if the receiver gets more than two 1's or 0's in a row, there's
an error.

Then you can deliberately transmit an 'illegal' signal -such as 11110000 -
which causes an error in the receiver and allows you to reset.  You know
the data that follows is the start of a new byte.

And you can easily tell the difference between 'line-idle' and 'line-dead'.
The former gives you ...0101010101010.... and the latter probably all sorts
of random stuff.

There are other advantages to this kind of coding, too - some
communications channels don't like long strings of 000's or 111's, because
they drift off frequency (or whatever). So you may _need_ to use some kind
of coding system.

If you just want to start off with a simple serial link (no coding) you can
use the UART on the '84 as the receiver, and write a simple bit-bashing
routing on the '71 as a transmitter. It's simpler to write transmitter code
from scratch than receiver. The UART in the PIC will sort out all the start
and stop bits, as long as you transmit them right.

Don't forget to include lots of redundancy in the codes - transmit each
nibble at least twice, and compare them.


Good Luck

Alec

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