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'16C63 UART Problems?'
1997\02\23@143534 by Kevin Russell

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This is pretty long...

I have two boards with 16c63 parts connected to LTC485 485 drivers.  Pin C5
is used to enable the driver.  The receiver is enabled at all times.

To test the hardware I wrote two simple programs, one for receive, and one
for transmit.  The transmit code turns on the 485 driver and sends
alternating + and - with a 500 ms period.  The receive code disables the
driver, and listens for incoming characters.  When it receives a +, it
turns on an led, when it received a -, it turns off an led.

It didn't work, so I did the following:

1) I hooked up the RX data pin on the receive side to a PC via a MAX233.
It show that at the PIC, the + and - characters are being received.

2) I pulled the 485 driver and connected the PIC directly to a PC via the
MAX233 and sent + and - characters from the keyboard.  It worked fine.

3) I then put the 485 driver back in.  I did leave the MAX233 connected to
the transmit side so I could send out some info for debug purposes.  Based
on that info, when connected to the other PIC, the rx PIC receives two
characters, +- or -+ depending on when I hook it up, and then waits for
ever for another character.

Conclusion:

When connect to the PC the rx side works.
When connected to another PIC the rx side does not.

Any ideas?

Thanks,

Kevin Russell
spam_OUTkevin.russellTakeThisOuTspamnull.net

1997\02\23@164918 by Andy Kunz

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At 11:22 AM 2/23/97 -0800, you wrote:
>This is pretty long...
>
>I have two boards with 16c63 parts connected to LTC485 485 drivers.  Pin C5
>is used to enable the driver.  The receiver is enabled at all times.

Is your 485 connection terminated properly?

Andy
==================================================================
Andy Kunz - Montana Design - 409 S 6th St - Phillipsburg, NJ 08865
         Hardware & Software for Industry & R/C Hobbies
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'16C63 UART Problems?'
1997\03\02@072053 by Dorin Dogaroiu
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Long question, long answer !

Some time ago, I experiment the receiver problems when I connected
on RS422 ( which is the almost the same as RS 485 ) some control
systems using 80c552 ( an expanded version of 8051) and a PC. The
protocol was like this : the PC queries the stations one at the
time.  The PC transmitter was activated, it sends an address ( that
all the stations listen), then it was deactivated and the PC
listens for the response.

       It appeared the following problem : in the idle periods of
the bus ( when neither transmitter was enabled ) the bus floats
and the receiver TTL output ( which are all enabled) goes in an
impredictable state ( because of the input histeresis) or even
oscillates.
       The solution I found was like this : I prepolarised the
bus so it keeps the polarity when neither transmitter is enabled. I
used two solutions :
1. Two 1k resistors, one from the + line to VCC, other from - line
to GND ( VCC and GND of the RS422/485 driver ). This gives an
aprox. 120 mv between lines when the bus is idle
2. I replaced one 100 ohm  terminator resistor with 2x200 ohm
resistors, one from + line to VCC, other from - line to GND.
This gives an 1V voltage between lines in the idle periods.

               I hope it is useful for you,

                                       Dorin

---------------------------------
Dorin Dogaroiu
Research Institute for Computers
Bucarest, Romania
Email : .....dogaroiudKILLspamspam@spam@pcnet.pcnet.ro
       ddorinspamKILLspamkappa.ro
---------------------------------


 >This is pretty long...  >
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