🚀 go-pugleaf

In article <1992Aug28.135419.3725@hubcap.clemson.edu>
dawill@hubcap.clemson.edu (david williams) writes: 

>Since the RxD and TxD lines are tied
>together, a pontential point of error is going to be in the timing
>of transmitted characters.  Suppose you just received a block that needs
>to be responded to.  If the MiniBoard transmits at the same time the
>sending computer does, the two bytes will collide at the Miniboard
>interface.  This means we're going to have to use a half-duplex
>scheme in which the MiniBoard "knows" when the sending computer is
>guaranteed not to be transmitting.  A good example of this sort 
>of scheme is the kermit protocol.

You are absolutely right, the Mini Board serial circuit will only be
reliable in half-duplex protocols, where each side knows whether it's
its turn to transmit or receive.

This isn't so horrible, though.  For most applications, this works
just fine.  See for example the protocol used by the DLM downloader,
or the HEXMON program supplied with the Mini Board.

>   Even worse, what if we try to hook the MiniBoard up to another 
>serial port that works the same way? (sending side supplies the negative
>voltage) - No communications will be possible! 

Believe it or not, two Mini Boards can communicate to each other.
Think about it:  the Mini Board's own circuit doesn't need a negative
voltage for serial line true.  The "negative" voltage will simply be
zero volts (there is a 10K resistor pulling the RxD input to ground).
When either board needs to transmit +5v, it can do so, and the other
board will interpret it properly.

>   yep.  Oh well is right.  Get out the x-acto knives when you get 
>the miniboard.

I think this comment is a bit harsh.  The circuit works just fine for
most protocols and most computers, and has been tested on a variety of
clone and true-blue PCs, Macintosh models, and Unix boxes.  The main
situation in which the circuit will be problematic is if your host
computer can't deal with +/-5 volts instead of the standard +/-12
volts.  And as I mentioned in a previous message, computers with this
limitation are fairly unusual.

	- Fred