Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!ucbvax!WSMR-SIMTEL20.ARMY.MIL!WANCHO From: WANCHO@WSMR-SIMTEL20.ARMY.MIL ("Frank J. Wancho") Newsgroups: comp.dcom.modems Subject: xyzmodem problems Message-ID: Date: 24 May 89 20:44:00 GMT Sender: daemon@ucbvax.BERKELEY.EDU Organization: The Internet Lines: 21 Xmodem was written by Ward Christenson(sp) back in the late 70's. Ward Christensen wrote MODEM, then MODEM2, as described. Keith Petersen modified the program to be used on an unattended Remote CP/M system, and called it XMODEM. Although people dialed into an RCP/M system using the MODEM program and transferred files using the XMODEM program on the remote end, the protocols were basically the same, except the name XMODEM, instead of MODEM, stuck. The correct term is the Christensen Protocol, which applies to the underlying protocol, originally using 8-bit checksums, with the 16-bit CRC option added a short time later. The 1K blocks were added to XMODEM later. The "batch" mode was NOT a part of XMODEM because it was desired to limit connect time, forcing the caller to transfer only those files one by one which would complete in the allotted time. Because the protocol is half-duplex, a clear 8-bit path is required only from sender to receiver. It would have been trivial to have the sending end honor flow control. It simply wasn't needed at the time. --Frank