deanr@sco.COM (Dean Reece) (08/25/90)
Hi, I need some help with RS232 drivers/receivers I'm building a project that needs a standard RS232 port, but only has available supplies of +5, +12, and +24 volts. I thought that I might be able to use the 0(GND), +12, and +24 kinda like the +/- 12v supply usually called for. I couldn't think of any trivial way to do this, however. If I can't get it really simple, I'll probably just use a MAX232 and not bother, but I've got lots of 1488/9 chips to use up, and I prefer to save my MAX232 for 5V only projects. Note that all the logic uses +0 as GND and are TTL or HCT compatible devices. Schemes I've thought of (not simple enough to replace MAX232): Tie the 1488(xmitter) GND to +12, +/- go to +24/+0. Feed the TTL XMIT line to a 7407 high voltage (30v) driver. The 7407 output is pulled up to +24 via a 1k resistor. This would then supply the signal to the 1488. -or- Connect the 1488 supplies as above. Use an opto-isolator to shift levels. This would work for the receiver as well. Thanks for your thoughts and suggestions (TIA) -deanr ______________________________________________________________________ | Dean Reece Member Technical Staff |"The flames are all long gone | | The Santa Cruz Operation 408/458-1422 | but the pain lingers on" | |___________deanr@sco.com_______________|___________________Pink_Floyd_|
ftpam1@acad3.fai.alaska.edu (MUNTS PHILLIP A) (08/26/90)
In article <7574@scolex.sco.COM>, deanr@sco.COM (Dean Reece) writes... > >Hi, I need some help with RS232 drivers/receivers >I'm building a project that needs a standard RS232 port, but only has >available supplies of +5, +12, and +24 volts. I thought that I >might be able to use the 0(GND), +12, and +24 kinda like the +/- 12v supply >usually called for. I couldn't think of any trivial way to do this, You could use a charge-pump inverter to generate -10V or so from the +12V supply. This takes as little as an LM555 and some discretes. I just use the MAX233 (MAX232 with integrated caps) myself. If you're really lazy, I once saw a data sheet for a DB25 connector with the MAX232 integrated. Philip Munts N7AHL NRA Extremist, etc. University of Alaska, Fairbanks
touch@dsl.cis.upenn.edu (Joe Touch) (08/26/90)
In article <7574@scolex.sco.COM>, deanr@sco.COM (Dean Reece) writes... > >Hi, I need some help with RS232 drivers/receivers >I'm building a project that needs a standard RS232 port, but only has >available supplies of +5, +12, and +24 volts. I thought that I >might be able to use the 0(GND), +12, and +24 kinda like the +/- 12v supply >usually called for. I couldn't think of any trivial way to do this, How about the following... Remember that an RS232 needs only a 6V level shift, 3V on either side of ground (lower than -3V is a logical '1', greater than +3V is a logical '1'). First convert the signals to CMOS levels, i.e. 0V and 5V for 0 and 1. Then use variable voltage followers to set up a 4.5V and 9V references. Send the 0-5V signals into a CMOS driver powered by the 9V circuit. The output will be signals will a shift of 0-9V. Connect the output of the CMOS driver (high current) to the RS232 data line. Connect the signal ground of the RS232 to the 4.5 V ref. I think this is a quick way to solve the problem, and uses only 2 cmos high-current drivers and 2 voltage references. Joe
whit@milton.u.washington.edu (John Whitmore) (08/26/90)
In article <7574@scolex.sco.COM> deanr@sco.COM (Dean Reece) writes: > >Hi, I need some help with RS232 drivers/receivers >I'm building a project that needs a standard RS232 port, but only has >available supplies of +5, +12, and +24 volts. I thought that I >might be able to use the 0(GND), +12, and +24 kinda like the +/- 12v supply >usually called for. Uhh... no, the third-wire ground on most machines will cause your 12V supply to burn up if you actually connect +12 to the RS-232 (or any other) grounded point. Since you aren't starting from +5 only, the MAX232 is overkill; just make a 555 oscillator with a charge pump rectifier like so: +12 | ------- | | C1 | 555 +----| |----+-----|<|-----+---- -12V output | | | | -------- - - C2 | V - GND - | | GND GND With a frequency of 50 kHz on the '555 (I've omitted the timing components), and C1= C2= 1uF, this should easily supply several 1488's with their negative power. John Whitmore
george@wombat.UUCP (George Scolaro) (08/27/90)
In article <7574@scolex.sco.COM>, deanr@sco.COM (Dean Reece) writes... > >Hi, I need some help with RS232 drivers/receivers >I'm building a project that needs a standard RS232 port, but only has >available supplies of +5, +12, and +24 volts. I thought that I >might be able to use the 0(GND), +12, and +24 kinda like the +/- 12v supply >usually called for. I couldn't think of any trivial way to do this, One method, which I have used successfully is to use a chip like the LT1054 (made my Linear Technology, T.I. and probably others). It is an 8 pin minidip, add 2 caps and wire it for inversion. Feed in +12 and out comes -12 (less a small drop). It will supply up to 100ma. In reasonable volumes, the LT1054 + 14c88 + 14c89 comes in at about the same cost as a MAX232. The main point though is that the MAX232 does not have enough drive (+12/-12) to transmit RS232 on long cables at high baud rate. On a test using 1000ft of 6 wire flat telephone cable we were <9600 baud to get reliable data transmission with the max232, but with the 14c88/14c89 combo we were up at 38400 baud. Observing the output levels on the end of the cable using a max232 as the driver showed the signal looking very 'rounded' and not reaching +/-3V. The 14c88/14c89 though, still exceeded +/-3V at 38400. As an alternative to the max232 (if you need 3 tx and 3 rx) is Motorola's recent MC145407, which has +5V only operation (with external caps). It has sufficient drive to run itself and one MC145406 (3 tx and 3 rx). These are CMOS parts. regards, -- George Scolaro george@wombat.bungi.com [37 20 51 N / 122 03 07 W]
psfales@cbnewsc.att.com (Peter Fales) (08/27/90)
In article <7574@scolex.sco.COM>, deanr@sco.COM (Dean Reece) writes... > >Hi, I need some help with RS232 drivers/receivers >I'm building a project that needs a standard RS232 port, but only has >available supplies of +5, +12, and +24 volts. I thought that I >might be able to use the 0(GND), +12, and +24 kinda like the +/- 12v supply >usually called for. I couldn't think of any trivial way to do this, This may or may not be what you are looking for, but if you just want to talk to a single specific RS-232 device, I have had good luck using a TTL inverter as an RS-232 driver. I don't recommend this technnique for a commercial product (or anything that someone besides yourself might use), but many RS-232 receivers will accept 5 volts as logic 0 and 0 volts as a logic 1 (despite the fact that the spec says the that the logic 1 must be more negative than -3 volts). Again, don't sell it and dont't blame me if it doesn't work, but I have gone up to 9600 baud over short distances without problems. -- Peter Fales AT&T, Room 5B-420 N9IYJ 2000 N. Naperville Rd. UUCP: ...att!ihlpb!psfales Naperville, IL 60566 Domain: psfales@ihlpb.att.com work: (708) 979-8031
owens@acsu.buffalo.edu (bill owens) (08/27/90)
In article <1990Aug26.234711.23808@cbnewsc.att.com> psfales@cbnewsc.att.com (Peter Fales) writes: >In article <7574@scolex.sco.COM>, deanr@sco.COM (Dean Reece) writes... >> >>Hi, I need some help with RS232 drivers/receivers > >This may or may not be what you are looking for, but if you just want to >talk to a single specific RS-232 device, I have had good luck using a >TTL inverter as an RS-232 driver. > >I don't recommend this technnique for a commercial product (or anything >that someone besides yourself might use) I'm presently working on s/w for a device that uses inverters as RS-232 drivers. The technique has the additional advantage that you can output a 1 to the latch, and the inverter flips it to the 'correct' level of 0 volts. I didn't design the thing, but it has been a successful commercial device for 7 years... Bill. Bill Owens owens@acsu.buffalo.edu 108 Computing Center uunet!acsu.buffalo.edu!owens Buffalo, NY 12460 716/636-3511