MAx232 unused inputs
in [Design]
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From: miso on 8 Jan 2010 16:43 On Jan 8, 6:23 am, Jan Panteltje <pNaonStpealm...(a)yahoo.com> wrote: > On a sunny day (Thu, 7 Jan 2010 19:05:08 -0800 (PST)) it happened > "m...(a)sushi.com" <m...(a)sushi.com> wrote in > <7eab3bb5-dd81-4a6c-bca0-b3f139585...(a)j5g2000yqm.googlegroups.com>: > > > > > > > > >On Jan 6, 3:01 pm, Jan Panteltje <pNaonStpealm...(a)yahoo.com> wrote: > >> On a sunny day (Wed, 6 Jan 2010 14:32:54 -0800 (PST)) it happened > >> "m...(a)sushi.com" <m...(a)sushi.com> wrote in > >> <36580fbc-79ed-4e00-9d44-9c5326aa8...(a)22g2000yqr.googlegroups.com>: > > >> >The reason I mention you don't really need a 232 chip is every 232 > >> >chip on the market adds jitter to the signal. > > >> You can try a MAX-panteltje: > >>http://panteltje.com/panteltje/wap54g/max_pantel.txt > >> but then you need control over RTS and DTR. > > >> Having the MAX232 is rather universal, and always works. > > >> It is hard to keep the > >-------- > >Did the Maxim police cut you off at mid-sentence? > > That is a remaining line from your own posting, did you not recognise the text? > > I use the MAX232 a lot too. > Just not the 3.3V version, and that only because it is not in the shops here. > I am not Joerg, who can only afford 2 cent transistors. > Poor guy ;-) > > >How do you insure the 1n4148 doesn't hog all the current, preventing > >the NPN from turning on. Does this just depend on the ratio of > >junction area between the diode and transistor? > > They are reverse connected, > the diode conducts when the input is negative, protecting the be junction from > too much revere voltage, > and the be junction conducts when the voltage is positive. > > >There are devices that only need to receive 232. In that case, a home > >brew receiver is fine. Rolling your own transmitter is another story. > >There are cases where the output is shorted. You really can't > >controller the external world. But a bit of resistance in series with > >the output would do the trick. > > In this case the current limit is set by the DTR and RTS lines... > So it is always in spec if i connects to a real RS232 port. > > >I made a data slicer that didn't use a 232, though I've seen designs > >swearing you need one. By skipping the 232 chip, I made the design > >much easier to phantom power from the transmitter. If you ever do > >this, shoot for single supply use since most of these 232 chips just > >charge pump invert the boosted signal, so the negative supply is not > >as beefy. > > >[Given your website and hobbies, I suspect you know what a data slicer > >is.] > > Actually electronics is not only a hobby, also made a living with it. > > To 'do the RS232 thing right' there are many ways. > In an other design I made negative with a simple sine wave oscillator, > rectifier and pot core. > That then also powers other stuff. > Creating a negative supply almost never is a problem, > here are some details of what is one directory level up: > The generator: > ftp://panteltje.com/pub/8052AH_BASIC_computer/dc_dc_converter_detail/.... > The EPROM programmer switch: > ftp://panteltje.com/pub/8052AH_BASIC_computer/dc_dc_converter_detail/.... > The RS232 interface: > ftp://panteltje.com/pub/8052AH_BASIC_computer/dc_dc_converter_detail/.... > Hope this helps:-) I didn't notice the diode was reversed. My point about trying not to use the negative supply is not that it can't be done, but it is higher impedance and lower efficiency IF it is derived from the boosted positive supply. I did a 232 with shared inductor rather than caps. In that case it didn't derive the negative supply from the positive supply, but still a boost DC/DC is beefier than an inverter since the boost just adds to the input supply. I don't have the 232 spec handy, but I don't recall a current limit on the transmitters when I was designing those 3V parts you don't buy. ;-) The lack of a current spec made driving a serial mouse a roll of the dice. One of the datasheets for my chips had some blurb about how it could drive 10 (or something like that) popular serial mice. It didn't mention the one mouse it couldn't drive. Prior to the design, I went to Fry's and bought what serial mice we didn't have handy, then measured the current the mice needed. That produced an internal spec, but as luck would have it a newer design was more power hungry.
From: Jan Panteltje on 8 Jan 2010 17:21 On a sunny day (Fri, 8 Jan 2010 13:43:44 -0800 (PST)) it happened "miso(a)sushi.com" <miso(a)sushi.com> wrote in <9e8aee52-fbcd-40a8-9ca5-244b36542b76(a)l30g2000yqb.googlegroups.com>: >My point about trying not to use the negative supply is not that it >can't be done, but it is higher impedance and lower efficiency IF it >is derived from the boosted positive supply. I did a 232 with shared >inductor rather than caps. In that case it didn't derive the negative >supply from the positive supply, but still a boost DC/DC is beefier >than an inverter since the boost just adds to the input supply. > >I don't have the 232 spec handy, but I don't recall a current limit on >the transmitters when I was designing those 3V parts you don't >buy. ;-) The lack of a current spec made driving a serial mouse a >roll of the dice. One of the datasheets for my chips had some blurb >about how it could drive 10 (or something like that) popular serial >mice. It didn't mention the one mouse it couldn't drive. Prior to the >design, I went to Fry's and bought what serial mice we didn't have >handy, then measured the current the mice needed. That produced an >internal spec, but as luck would have it a newer design was more power >hungry. Yes, indeed, we discussed that here a while back I think. This because of my temp_pic and freq_pic designs, http://panteltje.com/panteltje/pic/temp_pic/index.html http://panteltje.com/panteltje/pic/freq_pic/index.html those just limit the DTR voltage with a zener to about 4.7V. From what I have measured, the zener current stays below the max value on my USB to RS232 adaptors, and on the serial COM1 and COM2 ports of my PC. The currents are different though, resulting in a different voltage. It does make some difference on temp_pic in the calibration. Also depends on the type of zener used I guess. I can live with it, it all stays in one piece., and saves MAX232 chips or dedicated hardware, doing it 'the mouse way'.
From: miso on 8 Jan 2010 23:49
On Jan 8, 2:21 pm, Jan Panteltje <pNaonStpealm...(a)yahoo.com> wrote: > On a sunny day (Fri, 8 Jan 2010 13:43:44 -0800 (PST)) it happened > "m...(a)sushi.com" <m...(a)sushi.com> wrote in > <9e8aee52-fbcd-40a8-9ca5-244b36542...(a)l30g2000yqb.googlegroups.com>: > > > > >My point about trying not to use the negative supply is not that it > >can't be done, but it is higher impedance and lower efficiency IF it > >is derived from the boosted positive supply. I did a 232 with shared > >inductor rather than caps. In that case it didn't derive the negative > >supply from the positive supply, but still a boost DC/DC is beefier > >than an inverter since the boost just adds to the input supply. > > >I don't have the 232 spec handy, but I don't recall a current limit on > >the transmitters when I was designing those 3V parts you don't > >buy. ;-) The lack of a current spec made driving a serial mouse a > >roll of the dice. One of the datasheets for my chips had some blurb > >about how it could drive 10 (or something like that) popular serial > >mice. It didn't mention the one mouse it couldn't drive. Prior to the > >design, I went to Fry's and bought what serial mice we didn't have > >handy, then measured the current the mice needed. That produced an > >internal spec, but as luck would have it a newer design was more power > >hungry. > > Yes, indeed, we discussed that here a while back I think. > This because of my temp_pic and freq_pic designs, > http://panteltje.com/panteltje/pic/temp_pic/index.html > http://panteltje.com/panteltje/pic/freq_pic/index.html > those just limit the DTR voltage with a zener to about 4.7V. > From what I have measured, the zener current stays below the max value > on my USB to RS232 adaptors, and on the serial COM1 and COM2 ports of my PC. > The currents are different though, resulting in a different voltage. > It does make some difference on temp_pic in the calibration. > Also depends on the type of zener used I guess. > I can live with it, it all stays in one piece., and saves MAX232 chips or > dedicated hardware, doing it 'the mouse way'. It never occurred to me anyone would be hanging a zener directly on a transmitter. Most of these 232s want to do about 8V out the transmitter. I think some resistance would be nice before hitting the zener. I think the mice use regulators, but now that I think about it, I never curve traced the mouse. On one of my 232 chips, the apps guy found it wouldn't start up if 3 transmitters were shorted to a negative supply. Value escapes me, but it was a legal voltage for the pin, even though in the scenario used, it was very unlikely. My boss said "I bet it doesn't start up if you attach it to the AC line. This is nuts! I had to tweak ESD, so I made some changes to the transmitter, but really, you can't build chips for every conceivable customer abuse. Given an emission microscope, it was possible to make the transmitters work under these conditions, but such antics are really a waste. |