At 16:29 19/06/98 PDT, you wrote: >>Date: Tue, 16 Jun 1998 05:52:20 GMT >>Reply-To: pic microcontroller discussion list > >>From: Nowhere Man >>Subject: Re: [OT] X-modem protocol >>To: PICLIST@MITVMA.MIT.EDU >> >>Remember also that the 'window size' is 1. An ACK must be sent to the >>sender before the next packet will be sent. If the checksum doesnt >>match, a NAK must be sent. After 10 NAKs, the transfer should abort. In the old days (before error correcting modems) it was most advantageous to use xmodem or similar protocols for trans-atlantic BBS calls, especially when connected via satellite. Due to the delays (with other protocols) each end could NAK the other and both timeout! At least with xmodem each packet was acknowledged. With an error prone link a windowed protocol could (say) send 10 packets then slide the window back 9 and send 9 then slide it back and.. you get the picture. The transfer time was actually longer than using Xmodem as result. Regards Adrian WWW WWW Adrian Gothard WWW WW WWW White Horse Design WWWWWWWWWW +44-385-970009 (Mobile/SMS), +44-118-962-8913/4 (voice/fax) WWWW WWWW whd@zetnet.co.uk, http://www.users.zetnet.co.uk/whd --- Developers of GPS satellite-based tracking systems for vehicles/helicopters