Path: utzoo!attcan!uunet!husc6!uwvax!oddjob!mimsy!eneevax!umd5!umbc3!cbw1!brian From: brian@cbw1.UUCP (Brian Cuthie) Newsgroups: comp.dcom.modems Subject: Re: US Robotics / Micom incompatibility Keywords: US Robotics, Micom Message-ID: <180@cbw1.UUCP> Date: 12 May 88 02:41:48 GMT References: <267@sagpd1.UUCP> <21560@oliveb.olivetti.com> Reply-To: brian@cbw1.UMD.EDU (Brian Cuthie) Distribution: na Organization: CBW, Columbia, MD 21046 Lines: 41 In article <21560@oliveb.olivetti.com> jerry@oliveb.UUCP (Jerry Aguirre) writes: > >Eventualy I found an option in the modem labled 'allow overspeed data' >that basically allowed 1221 BPS instead of limiting it to 1205 BPS. >This fixed the problem without requiring two stop bits to be set. I >only tested this for output (from Micom to modem) as I had no need for >continuous input in the other direction. Most high speed modems are actually Synchronous modems with Asynchronous converters. This conversion from Async to Sync, and back, is done (usually) at the bit level. The reason that there is an overspeed option is that Synchronous modems are within .01% of spec'd speed. Async tolerances, however, are much more liberal (-2.5% to as high as 3%). This obviates a need for more bandwidth on the Sync side of the channel. To put 3% overspeed Async data on a 0.01% tolerance line, stop bits are deleted in the Synchronous data stream. However, in order to have time to re-insert the stop bits on the receiver side, the clock for the Asynchronous transmitter of the Sync to Async converter (part of the receiver in the modem) must run 3% overspeed. This doesn't always make the DTE equipment happy. Note that the amount of overspeed tolerance need not be a full 3%. Most modems allow several selections since, in effect, you are choosing the speed at which you will throw data at the DTE on the receiving end and not all DTEs can survive too much overspeed. Also note that modems from different manufacturers may not behave as expected when selecting options such as overspeed. It is possible that, although the modem on the transmitting end is deleting stop bits to allow for the overspeed data, that the receiving modem does not know that it must run it's Async converter faster. Thus it is possible to overrun the receiving modem at the *synchronous* level. Hope this hasn't been too confusing... -brian -- Brian D. Cuthie uunet!umbc3!cbw1!brian Columbia, MD brian@umbc3.umd.edu