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Re: Output boards, SPI vs I2C

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On 3/26/2017 3:49 PM, David Lesher wrote:
> rickman <gnuarm@gmail.com> writes:
>
>>> Not sure I follow; there is SPI on the board, but thinking out
>>> loud, we would have to add selection to the existing SPI users,
>>> the display, touch screen, and flash.....
>
>> Flash???  The other stuff is up to you.   I"m just talking about the 24
>> relays.  You didn't mention the other items.  Is everything going to be
>> on the 8 digital outputs?
>
> I was talking about using the existing I2C or SPI system to also
> drive the outboard relay boards. It's mentioned only in passing in the
> writeup.

Yes, I saw that.  I don't have the various web pages open at the moment, 
but you may have trouble using the built in SPI port because each port 
will need an enable.  If you go this route you will need to use I2C.


>> The display, touch screen and other things they talk about connecting
>> through SPI or I2C connect directly to the CPU.  This will not be the
>> same as using the digital outputs.  If you want to use the dedicated
>> I/Os on the CPU for the relay drivers you might as well not use the PLC
>> but get a CPU card instead and save some bucks.  What does the PLC unit
>> do for you exactly?  Does it provide a way of programming it that you
>> understand well that an Arduino won't?
>
> No, because I'm not the programmer at all; (To paraphrase Bob
> Pease) <https://tinyurl.com/kby566u> I just solder wires.
> Two people working with it suggested this platform for this project.

I'm no expert on PLC, but if you aren't using any of the hardware I 
would suggest you get a microcontroller board instead.  There are TONs 
of good products that are smaller and more cost effective.  But I 
believe PLCs often have a special language for programming, ladder 
logic.  I didn't think it was used much even for PLCs though.

Arduinos mostly are programmed in Python.  Everything can be programmed 
in Forth.  I'm working with Mecrisp these days.  Commercial versions of 
Forth are SwiftForth from Forth, Inc https://www.forth.com/ and VFX from 
MPE (Microprocessor Engineering) http://www.mpeforth.com/.

Not likely you can change the programmer's point of view, but if they 
are suggesting using a PLC just to get a CPU they aren't very informed 
in my opinion.


>>> Do you echo krw's concerns on I2C cabling?
>
>> It's not an absolute on leaving the CPU board.  But both SPI and I2C are
>> sensitive to noise and poor impedance matches.  If your wire runs are
>> short you should be ok.  Use ribbon cable with every other wire a ground
>> or twisted pairs with a signal wire mated with a ground.  How far apart
>> are the PLC and the relay boards?
>
> A few inches; directly adjacent.

Make sure your power connections are solid and direct.  A few inches 
won't hurt.  I2C isn't magic.  It uses a pullup resistor, about 4.7 
kohms sticks in my mind.  All devices on the bus are open collector.  It 
should be pretty robust.  You can use a smaller value pull up resistor 
to get a bit more noise robustness.  Just check all the data sheets to 
make sure they can pull the lower value resistor to ground.

-- 

Rick C

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Output boards, SPI vs I2C David Lesher <wb8foz@panix.com> - 2017-03-25 02:46 +0000
  Re: Output boards, SPI vs I2C krw@notreal.com - 2017-03-24 23:25 -0400
    Re: Output boards, SPI vs I2C David Lesher <wb8foz@panix.com> - 2017-03-26 04:13 +0000
  Re: Output boards, SPI vs I2C rickman <gnuarm@gmail.com> - 2017-03-25 14:04 -0400
    Re: Output boards, SPI vs I2C David Lesher <wb8foz@panix.com> - 2017-03-26 04:11 +0000
      Re: Output boards, SPI vs I2C rickman <gnuarm@gmail.com> - 2017-03-26 02:37 -0400
        Re: Output boards, SPI vs I2C David Lesher <wb8foz@panix.com> - 2017-03-26 14:41 +0000
          Re: Output boards, SPI vs I2C rickman <gnuarm@gmail.com> - 2017-03-26 12:47 -0400
            Re: Output boards, SPI vs I2C David Lesher <wb8foz@panix.com> - 2017-03-26 19:49 +0000
              Re: Output boards, SPI vs I2C rickman <gnuarm@gmail.com> - 2017-03-27 10:49 -0400
                Re: Output boards, SPI vs I2C Jasen Betts <jasen@xnet.co.nz> - 2017-03-27 18:41 +0000
                Re: Output boards, SPI vs I2C Martin Riddle <martin_ridd@verizon.net> - 2017-03-27 19:07 -0400
                Re: Output boards, SPI vs I2C Jasen Betts <jasen@xnet.co.nz> - 2017-03-28 08:03 +0000
                Re: Output boards, SPI vs I2C Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-03-28 10:44 -0400
                Re: Output boards, SPI vs I2C Jasen Betts <jasen@xnet.co.nz> - 2017-03-28 18:59 +0000
                Re: Output boards, SPI vs I2C Martin Riddle <martin_rid@verizon.net> - 2017-03-28 15:37 -0400
                Re: Output boards, SPI vs I2C David Lesher <wb8foz@panix.com> - 2017-04-03 17:49 +0000
                Re: Output boards, SPI vs I2C rickman <gnuarm@gmail.com> - 2017-04-03 15:18 -0400
                Re: Output boards, SPI vs I2C David Lesher <wb8foz@panix.com> - 2017-04-04 02:59 +0000
                Re: Output boards, SPI vs I2C rickman <gnuarm@gmail.com> - 2017-04-04 00:54 -0400
                Re: Output boards, SPI vs I2C David Lesher <wb8foz@panix.com> - 2017-03-28 05:08 +0000
                Re: Output boards, SPI vs I2C rickman <gnuarm@gmail.com> - 2017-03-28 02:16 -0400
  Re: Output boards, SPI vs I2C Martin Riddle <martin_ridd@verizon.net> - 2017-03-27 19:03 -0400
    Re: Output boards, SPI vs I2C David Lesher <wb8foz@panix.com> - 2017-03-28 03:28 +0000
      Re: Output boards, SPI vs I2C rickman <gnuarm@gmail.com> - 2017-03-28 00:38 -0400
        Re: Output boards, SPI vs I2C David Lesher <wb8foz@panix.com> - 2017-03-28 14:47 +0000

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