The bus should be connected as a "daisy-chain" (ie. no "star" connections), with a terminating resistor at each end (ie. two resistors in total).
A 2V swing is *more* than enough! Your problem is either a common-mode problem (if you read the standard, it says you need a common potential/connection between all the nodes, if you want this stuff to actually work properly), as suggested by some of the others. The other possibility is noise pick-up, which you might be missing if your CRO timebase is too long. Joe. > -----Original Message----- > From: Peter Vogel [SMTP:[EMAIL PROTECTED]] > Sent: Friday, September 20, 2002 9:48 AM > To: Multiple recipients of list CHIPDIR-L > Subject: Re: Strange LTC485 behavior > > Are you terminating the lines at one end or at both ends? My > recollection is that they should be terminated at one end only. > > On Thu, 19 Sep 2002 03:53:29 -0800 > Elbert Smit <[EMAIL PROTECTED]> wrote: > > > Hello Chipdir members, > > > > We are using LTC485 RS-485 transceivers and found some strange behavior. > > > > > We have a test setup with two devices, one using a Sipex RS232/RS485 > > (selectable) transceiver in a industrial PC, and one of our own devices, > > using the LTC485. > > > > These two devices will only communicate if we remove the termination > > resistor of 120 ohm at one side, leaving the bus unterminated. > > > > When the resistor is placed, the voltage swing of the A and B channel is > > still about 2 Volt, which should be sufficient, according to the RS-485 > > documentation I have. Only differences below 200mV are undefined. > However, > > the 2 Volt voltage is not enough to operate for some of the devices. We > > tried several LTC485's and found out that some of them worked fine with > > termination, but some didn't. Testcable length is an awsome 50 cm ;-) > > > > Any ideas or clues? > > > > Kind regards, > > > > Elbert Smit > > -- > > Author: Elbert Smit > > INET: [EMAIL PROTECTED] > > > > Fat City Network Services -- 858-538-5051 http://www.fatcity.com > > San Diego, California -- Mailing list and web hosting services > > --------------------------------------------------------------------- > > To REMOVE yourself from this mailing list, send an E-Mail message > > to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in > > the message BODY, include a line containing: UNSUB CHIPDIR-L > > (or the name of mailing list you want to be removed from). You may > > also send the HELP command for other information (like subscribing). > > > -- > Author: Peter Vogel > INET: [EMAIL PROTECTED] > > Fat City Network Services -- 858-538-5051 http://www.fatcity.com > San Diego, California -- Mailing list and web hosting services > --------------------------------------------------------------------- > To REMOVE yourself from this mailing list, send an E-Mail message > to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in > the message BODY, include a line containing: UNSUB CHIPDIR-L > (or the name of mailing list you want to be removed from). You may > also send the HELP command for other information (like subscribing). -- Author: da Silva, Joe INET: [EMAIL PROTECTED] Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
