On Tue, 1 Jul 1997 12:09:32 -0700 david@PAPDEV.COM writes:
>You may drive your average cheap sensitive gate SCR with a 0.1 uf
>capacitor between the PIC output port and the SCR gate, and a 1K or so
>resistor between the  SCR gate and SCR Cathode.  Make sure with this
>arrangement that the SCR Cathode is at PIC signal ground.

When driving heavy or reactive loads such as this, it is a very good idea
to structure your software so as not to require reading back the port.
When a port is read, the value returned is the state of the pins, not the
value in the output latch.  So if an external load is holding an output
pin down, it will read as 0 even though it is trying to output 1.  The
data book touches on this.  A common pitfall are the BSF and BCF
instructions, these internally read all 8 bits of the port, set or clear
one bit, then write all 8 bits back.  If you try

        bsf     PORTB,0         ;SCR # 1 on
        bsf     PORTB,1         ;SCR # 2 on

with a large capacitor connected to port B0, the second BSF will end up
writing a zero back to the bit 0 output latch, and only a very short (one
instruction) pulse will appear at B0.

A better way to do this is

        bsf     mirrorb,0       ;SCR 1 will be on
        bsf     mirrorb,1       ;SCR 2 will be on
        movfw   mirrorb
        movwf   PORTB           ;Write new SCR drive

Now the RAM location 'mirrorb' has a reliable copy of the last bits
written to port B.  In a lot of cases it won't be necessary to read back
the outputs anyway, so just use

        movlw   b'00000011'     ;SCRs 1 and 2 on (rest of B
outputs 0)
        movwf   PORTB

instead of the BSFs.
>
>I've made no mention of any hazards, or otherwise here.  So be
>careful!
>60+ volts can be deadly.

For software development, build a low voltage version of your circuit,
driving it with 12V or so AC from a transformer and using suitable
low-voltage bulbs.  You can touch, poke, and prod this circuit with
relative impunity.  Once the software is done, the PIC should plug right
into the live full voltage version and work flawlessly (yeah, right.  At
least most of the bugs will be out of it).