MAx232 unused inputs
in [Design]
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From: lighthouseman on 6 Jan 2010 07:54 Here's a nice simple one for you.... If my design has 5 CMOS serial ports, and I use something like 3 x MAX232, which have 2 drivers/receivers on each chip, then what should I do with the unused inputs/outputs on the 3rd chip? Should I leave the CMOS inputs floating or tie them up or down? What about the RS232 inputs? Should I leave them floating or tie them to VSS or VCC?
From: Jan Panteltje on 6 Jan 2010 08:10 On a sunny day (Wed, 6 Jan 2010 04:54:29 -0800 (PST)) it happened lighthouseman <kenny(a)kmillar.co.uk> wrote in <9885e2a0-1802-483c-b9e8-147d1a110a1c(a)j14g2000yqm.googlegroups.com>: >Here's a nice simple one for you.... >If my design has 5 CMOS serial ports, and I use something like 3 x >MAX232, which have 2 drivers/receivers on each chip, then what should >I do with the unused inputs/outputs on the 3rd chip? > >Should I leave the CMOS inputs floating or tie them up or down? >What about the RS232 inputs? Should I leave them floating or tie them >to VSS or VCC? Unused CMOS inputs should always be connected to either ground or Vdd. Unused outputs should not be connected. Unused MAX232 inputs can go to ground or Vdd.
From: Rich Webb on 6 Jan 2010 08:18 On Wed, 6 Jan 2010 04:54:29 -0800 (PST), lighthouseman <kenny(a)kmillar.co.uk> wrote: >Here's a nice simple one for you.... >If my design has 5 CMOS serial ports, and I use something like 3 x >MAX232, which have 2 drivers/receivers on each chip, then what should >I do with the unused inputs/outputs on the 3rd chip? > >Should I leave the CMOS inputs floating or tie them up or down? >What about the RS232 inputs? Should I leave them floating or tie them >to VSS or VCC? Tie the CMOS inputs high through a moderately large resistor. The MAX232 inputs may be left unconnected, since "[t]he inputs of unused drivers can be left unconnected since 400k? input pull-up resistors to VCC are built in (except for the MAX220)." -- Rich Webb Norfolk, VA
From: petrus bitbyter on 6 Jan 2010 09:29 "lighthouseman" <kenny(a)kmillar.co.uk> schreef in bericht news:9885e2a0-1802-483c-b9e8-147d1a110a1c(a)j14g2000yqm.googlegroups.com... > Here's a nice simple one for you.... > If my design has 5 CMOS serial ports, and I use something like 3 x > MAX232, which have 2 drivers/receivers on each chip, then what should > I do with the unused inputs/outputs on the 3rd chip? > > Should I leave the CMOS inputs floating or tie them up or down? > What about the RS232 inputs? Should I leave them floating or tie them > to VSS or VCC? You'd *never* leave unused logic inputs undifined. Usually CMOS are tied to GND, TTL should be pulled high. Ancient TTL needs a pullup resistor in the range 1-10k, for LSTTL this resistor can be omitted though I prefer to use it. Some components have internal pullups or (pulldowns) but if you are not sure you'd better use an external. petrus bitbyter
From: David Chapman on 6 Jan 2010 12:58
In article <hi224u$ffp$1(a)news.albasani.net>, Jan Panteltje <pNaonStpealmtje(a)yahoo.com> writes >On a sunny day (Wed, 6 Jan 2010 04:54:29 -0800 (PST)) it happened >lighthouseman <kenny(a)kmillar.co.uk> wrote in ><9885e2a0-1802-483c-b9e8-147d1a110a1c(a)j14g2000yqm.googlegroups.com>: > >>Here's a nice simple one for you.... >>If my design has 5 CMOS serial ports, and I use something like 3 x >>MAX232, which have 2 drivers/receivers on each chip, then what should >>I do with the unused inputs/outputs on the 3rd chip? >> >>Should I leave the CMOS inputs floating or tie them up or down? >>What about the RS232 inputs? Should I leave them floating or tie them >>to VSS or VCC? > >Unused CMOS inputs should always be connected to either ground or Vdd. >Unused outputs should not be connected. >Unused MAX232 inputs can go to ground or Vdd. FWIW, if you should encounter the entirely pin-compatible MAX3232 (which allows operation down to 3volts) BE WARNED that for some reason Maxim have omitted the 400K pull-up resistors that are on the CMOS inputs of the MAX232. If your driving circuit needs them you'll have to add them externally. - Dave -- David C.Chapman - (dcchapman(a)minda.co.uk) -------------------------------------------------------------------------- |