Post by thread:[OT}balanced line

> From: Frans Gunawan > I would like to ask, why with using balanced line(rs485) data can be > transmitted longer than using unbalanced line(rs232)? > I can't find good explanation about this. if possible, detail please... Balanced line (RS-422 & RS-485) operates by comparing two lines relative to ground. If line one is at a higher potential than line two you have a 1. If line one is at a lower potential than line two, you have a 0. RS-232 takes the absolute potential on a single line relative to ground. Without checking I think the formal RS-232C specification calls for a transition region between -3 and +3 volts, with the region above +3 volts (but below 25 volts) being 1. Remember the "RS" in RS-232 stands for "Recommended Standard." So if all you have is a +5V power supply you can work out a "practical" protocol that uses a narrow transition region around +3 volts and calls anything below +2 a 0 and everything above +4 a 1. Clearly this "practical" protocol is awfully vulnerable to anything that changes the absolute value of the signal. That's why it is cable length (resistance) sensitive. On the other hand, each of the RS-422/485 balanced lines are equally effected by line impedance so the differential can still be detected. That's why it is less sensitive to cable length. Cheers, Win Wiencke Image Logic Corporation spam_OUTImageLogicTakeThisOuTibm.net

The simple answer is that the noise cancels out. I believe the impedance of the line is a lot lower also - around 120 ohms for 485 and 2K for 232. Any external noise will be equally induced in the same polarity in both the 'true' and 'not-true' signals. The lower the impedance of the line, the harder it is to induce a noise signal to it. Same principal for low z balanced microphones. Another example is your telephone. Lots of unshielded wire running throughout your house and yet not much hum. Does this help? {Quote hidden}>---------- >From: Frans Gunawan[SMTP:.....fKILLspam@spam@POBOXES.COM] >Sent: Thursday, November 06, 1997 6:02 AM >To: PICLISTKILLspamMITVMA.MIT.EDU >Subject: [OT}balanced line > >I would like to ask, why with using balanced line(rs485) data can be >transmitted longer than using unbalanced line(rs232)? >I can't find good explanation about this. if possible, detail please... >thanks >http://www.poboxes.com/f >f >

Hi Frans: 1. Balanced lines (with RS422/485 differential drivers and receivers) rejects common mode noise. That's why TV, radio and recording studios use balanced lines for all routings for high quality sound (mostly reducing AC 60 Hz and florescent light hum). 2. RS422/485 line impedance is usually used with 50 or 120 ohms termination, lower than RS232's 1 K unterminated - so signal reflections are much less. This gives higher bandwidth to a given cable and allows longer signal lines (until the wire's series resistance gets large compared to the termination). 3. RS422/485 outputs are lower voltage swing, slew-rate controlled, which helps crosstalk and reflections. See this URL for more details: http://www.maxim-ic.com/AppNotes.htm -Wayne {Quote hidden}> ---------- > From: Frans Gunawan[SMTP:.....fKILLspam.....POBOXES.COM] > Sent: Thursday, November 06, 1997 3:02 AM > To: EraseMEPICLISTspam_OUTTakeThisOuTMITVMA.MIT.EDU > Subject: [OT}balanced line > > I would like to ask, why with using balanced line(rs485) data can be > transmitted longer than using unbalanced line(rs232)? > I can't find good explanation about this. if possible, detail > please... > thanks > http://www.poboxes.com/f > f >

The less than simple answer is that RS-485 has a much better common mode rejection ratio. If one observes the same signal in both transmission wires, the error can be corrected very easily. Since RS-485 carries both wires as signal wires with some impedance, the common-mode noise can be eliminated. This is true for RS-232, but to a much lesser extent since the ground side of the common mode signal will usually be substantially less than the signal side. {Original Message removed}