ComLynx collision detection

[+karri]

Reading comments in another thread I realized that when Lynx is sending only the send interrupt is active. After the message has been sent the receiver is allowed to interrupt again.

 

This may be a logical flaw that makes reliable communication impossible or at least very difficult.

 

Perhaps the sending should read the sent byte before it sends the next byte and if the byte was garbled the transmission would abort with a collision error.

 

Perhaps the sending code

@tx_irq:
        ldx     TxPtrOut    ; Has all bytes been sent?
        cpx     TxPtrIn
        beq     @allSent

        lda     TxBuffer,x  ; Send next byte
        sta     SERDAT
        inc     TxPtrOut

could be replaced by something like

        if (a byte was recently sent by me) {
              read the byte
              if (it was not equal) {
                   there was a collision
                   abort mission
              }
        }
        send a byte
        mark that a byte was sent by me

I also noticed that sending does not appear to start before Lynx receives a character. Obviously the Lynx PUT routine should generate a sending buffer empty interrupt to get the thing going.

 

Comments?

 

--

Karri

Edited August 21, 2014 by karri

[Songbird]

Wouldn't you only run into collisions when first detecting other players? Because once you've negotiated all the players, you could send in sequence and thus a given player would never broadcast until his turn arrived.

[+karri]

Makes sense. Besides, thanks for the sources. There is one very interesting and clever approach. When you stop sending a sequence of characters and switch the interrupts back to RX you also read the last byte. If the last byte is different from what you sent you had a collision while writing.

 

There can always be breaks because interference, bad connections...

[Songbird]

Agreed. The idea of checking the last byte is a good one, as a fairly reliable way of detecting an issue. But basically, I would not expect ComLynx to be robust enough to survive such situations. Unless they happen frequently in actual games, I suppose...