Hi Neil,

> I have an application where I want to send half duplex serial data between
> two PIC 16c74 microcontrollers through a one wire link (+ gnd of course))
> The thoroughput required is 32 bits of data (the value of 4 registers)
> needed to be sent 30 times per second

Since you want to send 4 octets (32 bits) 30 times per second, anything
at or above 1200 baud would suffice.  If you're going to put any sort of
checksum on each "packet", you might want to use 2400 baud.  Generally,
the slower the link, the less noise susceptibility.

Depending on the distance between the PICs, you might be able to get
away with TTL signal levels between the devices.  I'd use that for a
link up to several feet.  Beyond that distance, a differential signal
such as RS-422, RS-423, or RS-485 would work well.  Driver chips are
easier than RS-232 since they use lower, single ended voltage swings.
And you have control both the physical interface on both ends.

> I want to keep this simple. I don't want to start playing with
> UARTS and USARTS, even though a USART is built into the PIC (its
> seems quite complex for a beginner like me) and a one line link
> (asynchronous) is a must. Perhaps someone could suggest a simpler
> method.

Bite the bullet and learn about UARTs.  There's a reason that UART
stands for Universal Asynchronous Receiver Transmitter.  It does the
octet parallel to time dependant bit serial data conversion that you
say you want for this project.  If you find a chip by another name
that does the same function, you're deluding yourself if you think
you're not working with a UART/USART.

You can still get external UARTs that are programmed totally with
jumpers (i.e. no software setup).  The interface is 8 bit parallel
with a couple more lines for handshaking.  If you have enough pins
free on the PICs you're using, one end can write octets and the
other end can read octets.  Nothing simpler.

                                                Lee

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