> -----Original Message----- > Have a look with a 'scope probe around the ribbon & note the > difference in RXD & TXD-induced spikes coming from the ribbon. > Anybody have any idea why RS232-in-converted-to-TTL should > be worse than TTL-converted-into-RS232-out? > > 73, VR2BrettGraham >
Brett, Are you asking about spikes on the TTL level side or the RS-232 level side? If it is on the TTL side, and the RS-232 side at the same time period has a voltage beyond the 'region of uncertainty' between +3 volts and -3 volts, one can only blame the receiving device, but I do believe the problem goes further than that. In the proper RS-232 world, the minimum levels (+/- 3 volts) are the minimum that can/should be expected at the receiver end, while the driver voltage can go anywhere greater than those values (up to +/-25 volts) and still meet the spec. The designer of the driving circuits have to make a decision about how much cable they want to drive and what the safety margin they want to use above the minimum. Some laptops are limited on the power supply end of things and may chose to drive the positive signal with only 5 volts but will usually drive the negative with something like -12 or -15 volts. They lose some noise immunity on the positive side by doing that, but since in RS-232 signalling the negative voltage is the logical 1 (or ON signal), they can usually get away with it unless the noise source puts extra negative going spikes on the cable. I would tend to believe that something has failed in Tom's laptop rather than thinking that it is a design flaw - yes, most laptops cannot drive the 200 ft. cables that used to be common in the RS-232 world, but that world was usually not driven by laptop serial ports anyway. In a typical home or small office setup, the 5 volt positive level still provides more tha a volt of noise imunity on the positive signalling level, and since the negative signalling level is driven to a -12 volt level, there is plenty of margin for noise on the level that will register a logical 1 at the receiving end. 73, Don W3FPR -- No virus found in this outgoing message. Checked by AVG Anti-Virus. Version: 7.0.323 / Virus Database: 267.6.9 - Release Date: 6/11/2005 _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
