Bob, thanks for the heads up confirming my suspicions... as I see it right now, even with 1uF tantalum + .1uF right on pin 7, with NO xmit activity on its input, the '233s RS232 output driver "breaks into song" all by itself a few seconds after power-up. :^( NOW.... for the interesting discovery! I said earlier that I'd swapped out the '233, and saw same symptoms. Just now, I swapped it again, but this time I yanked a '233 that I knew to be working out of another project (fortunate use of sockets!) ... and damn if it WORKS! The two that I'd been using initially were Maxim "engineering samples" (wonder if that's a synonym for "rejects"?!). Methinks it's time for a few words to Maxim. Anyway, Caveat Emptor, E Pluribus Unum, Trust No One, and Keep on Truckin'! Jim ----- Original Message ----- From: "Bob Axtell" To: Sent: Friday, August 01, 2003 8:52 PM Subject: Re: [PIC] RS232 w/ MAX233 question... > I hate to add to your woes, but I have to tell you that that's the one that > also sings out RF real bad, tried filtering & caps (worked OK then) but > finally had to scrap their use by going overbudget on 4 sets of filters (I > couldn't get it to pass radiation tests). 5 more caps was cheaper than 4 > filters. Yep, that's the very one. As Yosemite Sam used to say, "that's mah > story and ah'ma stickin' to it." You might have to drop back to less > exotic (and cheaper) part... > > In all fairness to Maxim, the problem seems to be that most folks don't > bypass the Max233 with a tantalum cap large enough (i.e. very low delivery > impedance) to get the current it needs instantaneously and a 0.1uF RF > killer to quash the RF it generates, both across the line right at the > MAX233. We did all of that stuff, didn't help. > > Sometimes, when something is too good to be true, it isn't. I know 10 > engineers personally that all went thru that expensive learning session. > > --Bob > > At 08:25 PM 8/1/2003 -0700, you wrote: > >The '233 doesn't require external caps. There are no bad/broken > >connections: like I said, it works -- when it starts up. Tried another > >chip, same behavior. It does "seem" like an RC time-constant effect, in > >that the time required for the symptom to surface after power-up decreases > >as I increase the frequency of powering-up the circuit. I see no > >misbehavior on the PIC side of things - i.e. the inputs to the MAX233 are > >solid. Mystifry me, man! > >Jim > >----- Original Message ----- > >From: "Patrick J" > >To: > >Sent: Friday, August 01, 2003 6:40 PM > >Subject: Re: [PIC] RS232 w/ MAX233 question... > > > > > > > ok, sounds to me like your caps is either wrong value/bad solder/or > >broken? > > > tried another chip? > > > > > > > TXSTA/TRMT = HIGH before sending), and the output of the '233 turns to > > > s*** > > > > after a few seconds! (the P-P amplitude of the waveform drops to about > > > 6v, > > > > and it looks like it's broken into oscillation). The input to the '233 > >is > > > > much difference. Hmm? > > > > Jim > > > > > > -- > > > http://www.piclist.com hint: The list server can filter out subtopics > > > (like ads or off topics) for you. See http://www.piclist.com/#topics > > > >-- > >http://www.piclist.com hint: The list server can filter out subtopics > >(like ads or off topics) for you. See http://www.piclist.com/#topics > > --------------- > NOTICE > > 1. This account can accept email & attachments up to 10M in size. > 2. Federal Monitors: At request of client, some attachments are encrypted. > Please DO NOT delay traffic; please reply with credentials for password. > -------------- > > -- > http://www.piclist.com hint: The list server can filter out subtopics > (like ads or off topics) for you. See http://www.piclist.com/#topics -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.