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'16c74 serial port HELP Part2'
1998\02\17@135428 by Mike Barrett

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Thanks for the suggestions re my problems with serial port on 16C74 (and
the example code Mark).
But I still cant get my own program to work.
I now have a simple prog that reads the state of a pin on portB (switch to
ground pullup to 3k to +5V) and if low then outputs an ASCII character down
the TX port. It does this by passing the ASCII code to a reg and then
calling a subroutine. This works fine!
BUT if I read a character from the RX port (see code below)
and then pass that to the reg  and then call my subroutine to send it to
the RS232 port  it it does not work.
Having started to pull my hair out I put a scope on the TX (output) pin of
the 16C74. When my routine outputs a character (when I press the switch)
the output goes from +5V to 0V when I send it the same ASCII code from the
PC to the '74 and the code then reads it and echo it back, the signal on
the TX port goes from +5V to +1.5V instead of to 0V. The signal code looks
corrects (timing and bits) the program seems to be working but something is
happening to the output level of the '74.
Because it does not go to 0v the RS232 driver does not output anything....

BTW the Output from the 16c74 goes to a maxim MAX232 TTL to RS232 converter

Anyone any ideas???


Mike Barrett

; code to read data from RX reg and send to TX port
BTFSS   PIR1,RCIF               ; SEE IF DATA IN BUFFER, else goto NO_DATA
       GOTO    NO_DATA
       MOVF    RCREG,W ; MOVE DATA INOT W
       MOVWF   TX_CHAR
       CALL    SERIAL_SEND
NO_DATA
       rest of code...

=============================================
Mike Barrett                E-mail: spam_OUTbarrettTakeThisOuTspamcambridge.scr.slb.com
Schlumberger Cambridge Research      Phone: 01223 325200
High Cross,  Cambridge, CB3 OEL. U.K    Fax: 01223 327019
=============================================

1998\02\17@213312 by Mike Keitz

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On Tue, 17 Feb 1998 17:53:35 +0000 Mike Barrett
<.....barrettKILLspamspam@spam@CAMBRIDGE.SCR.SLB.COM> writes:
> the signal
>on
>the TX port goes from +5V to +1.5V instead of to 0V. The signal code
>looks
>corrects (timing and bits) the program seems to be working....
>Anyone any ideas???

Do you have both of the PIC Vss pins (pins 12 and 31 on the 40-pin DIP)
connected together and to your circuit ground?  Also, both Vdd pins have
to be connected together to the power supply.

Other than that, maybe you're doing (or not doing) something to the TRIS
registers?

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