FullDuplexSerial Stop Bits

TomS

I'm using the Parallax Full Duplex Serial object and have noticed that when transmitting, it uses two stop bits. I would like to modify the code to send only one stop bit but of course I don't know assembly programming. Anybody have an answer for me (other than learning assembly)?

All help appreciated
Tom

Peter Jakacki

Hi Tom,

Usually it's not a problem if 2 stop bits are transmitted as most applications are not trying to push the limits of a already limited communications medium. The FullDuplex object does not actually send 2 stop bits but it effectively does so because it transmits an idle bit before the start bit. The code snippet around line 252 (comments added) shows that a stop bit is appended with #$100 and then room is made for a start bit and a padding bit by shifting left 2 bits. The data is transmitted lsb first so the first bit is actually an idle bit.

                        or      txdata,#$100          'Add a stop bit after the 8 data bits
                        shl     txdata,#2               'make room for a start bit plus a padding bit
                        or      txdata,#1               'force the padding bit to an idle/stop bit 
                        mov     txbits,#11            'count 8 data + 1 stop + 1 start + 1 idle/stop
                        mov     txcnt,cnt

I haven't tried it but you could try this.

                        or      txdata,#$100          'Add a stop bit after the 8 data bits
                        shl     txdata,#1               'make room for a start bit plus a padding bit
                        mov     txbits,#10            'count 8 data + 1 stop + 1 start
                        mov     txcnt,cnt

*Peter*

TomS

Thanks Peter,

I'll give it a try.

Tom

TomS

It Works!

Thanks Again Peter. I'm not making the change for speed but for compatability.

Tom

Mike Green

From a compatibility standpoint, it shouldn't matter. To any receiver attached to the Propeller, a serial stream with 2 stop bits is indistinguishable from a serial stream with 1 stop bit and a little bit of internal delay between characters transmitted due to computation or other I/O. The side of FullDuplexSerial dealing with receiving an external serial stream only needs one stop bit, doesn't even check for that, and will accept essentially any delay between characters.

TomS

Thanks for the good info Mike. I'm replacing a commercial device that uses one stop bit with my propeller driven device and I want the change to my device to be totally transparent. You're right, it probably wouldn't make any difference though.

Tom