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'MAX232 Problem'
1997\07\30@170502 by Mailing List Account

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       I'm using a MAX232CPE with a 82C50 to send/recieve serial data,
i've got reasons not to be using the PIC for this.  Sending works fine,
but receiving gives me a constant stream of characters (usually the same
one) that aren't being sent from the other side (a PC running Telix).

       Has anybody experienced anything like this?  I'm absolutely sure
that the software is right, so i'm assuming it's the MAX232 or the cable.

                                               Philip Lalone

1997\07\30@201330 by Mike Keitz

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On Wed, 30 Jul 1997 13:04:58 -0700 Mailing List Account
<spam_OUTmlistTakeThisOuTspamalphax.com> writes:
>        I'm using a MAX232CPE with a 82C50 to send/recieve serial
>data,
>i've got reasons not to be using the PIC for this.  Sending works
>fine,
>but receiving gives me a constant stream of characters (usually the
>same
>one) that aren't being sent from the other side (a PC running Telix).
>
>        Has anybody experienced anything like this?  I'm absolutely
>sure
>that the software is right, so i'm assuming it's the MAX232 or the
>cable.

Start with the obvious.  Use an ohmmeter to make sure your PIC system's
ground is connected to PC ground.  Measure the voltage at the MAX232's
receiver input with the PC connected and running, it should be negative 8
to 12 volts (more negative than -3V anyway to comply with the standard).
If it's around 0V something is wired wrong and you are just getting
whatever couples in capacitively.  If you have an oscilloscope, use it to
be sure no pulses are appearing on the MAX232 input.

Do the erroneous characters stop when you short the MAX232 output/8250
input to +5V? (this won't hurt anything since everything should be at
idle anyway)  If not, the problem is in the 8250 or the software.  This
sounds like it may be bad software (especially likely when the programmer
swears up and down that the software is good) responding to interrupt
requests from other than the receive character ready.  It's been a while
since I worked with an 8250 but I think only the character ready
interrupt clears itself when the receive register is read.  The other
ones (parity, etc. error or change in handshake status) need to be
cleared seperately.
>
>                                                Philip Lalone
>


'MAX232 Problem'
1997\08\01@023236 by Alex I. Torres
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  Hi All PICers !

b>         I'm using a MAX232CPE with a 82C50 to send/recieve
b> serial data, i've got reasons not to be using the PIC for
b> this.  Sending works fine, but receiving gives me a constant
b> stream of characters (usually the same one) that aren't
b> being sent from the other side (a PC running Telix).
b>
b>         Has anybody experienced anything like this?  I'm
b> absolutely sure that the software is right, so i'm assuming
b> it's the MAX232 or the cable.

Philip,  check your cable, and try to use another chip.
Many times I use this chip, not MAX232 but ADM232 and ADM202 by
Analog Devices - its more cheaper then MAXIM, and I have never
seen this chip not working! I often use it not only for TTL<>RS232
convertion, but also for providing bipolar power for OP!

  Best Wishes, Alex Torres.
  Kharkov, Ukraine, exUSSR.
  E-Mail To : .....altorKILLspamspam@spam@cook.kharkov.ua   via InterNet
              or 2:461/28             via FidoNet

P.S. And I also use Telix for testing :)
--- GoldED 2.50.A0531+

1997\08\01@032329 by John Payson

picon face
>  b>         I'm using a MAX232CPE with a 82C50 to send/recieve
>  b> serial data, i've got reasons not to be using the PIC for
>  b> this.  Sending works fine, but receiving gives me a constant
>  b> stream of characters (usually the same one) that aren't
>  b> being sent from the other side (a PC running Telix).
>  b>
>  b>         Has anybody experienced anything like this?  I'm
>  b> absolutely sure that the software is right, so i'm assuming
>  b> it's the MAX232 or the cable.
>
> Philip,  check your cable, and try to use another chip.
> Many times I use this chip, not MAX232 but ADM232 and ADM202 by
> Analog Devices - its more cheaper then MAXIM, and I have never
> seen this chip not working! I often use it not only for TTL<>RS232
> convertion, but also for providing bipolar power for OP!

I'm not sure if I fully understand what the original poster is doing,
but I had a somewhat similar problem awhile ago after swapping I/O
cards on a PC.  The problem I had was that there are two different
pinouts for the 10-pin "post" connectors found on many motherboards
and I/O cards.  Some cards require a cable that maps pin 1->1, 2->2,
3->3, etc. (numerical order) while others expect 1->1, 2->6, 3->2,
4->7, 5->3, etc. (positional order when using crimp-on ribbon cable
connectors).  Unfortunately, there's no particularly good way to tell
what type of cable any particular card requires and sometimes Murpy
will have the wrong cable "sort of work" just enough to distract your
attention from the real problem.

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