Digital IO Extension Board - RS485

Discussion in 'The Projects Forum' started by tudstudent, Mar 30, 2015.

  1. tudstudent

    Thread Starter Member

    Jun 6, 2009
    With this topic I would like to request feedback for a project I am busy with for some time now.
    I am looking for feedback on the schematics as well ass the board layout/routing. I am no electronics engineer and this schematic is what I think should be working in "my theoretical world". However I would greatly appreciate if some of you experts would take a look.

    Primary functions:
    • Switching 7 12V push button led lamps (build in the push button) (X2 X3 X4 X5 X18 X19 X20);
    • Switching of 2 solenoid's (24VAC) - (X1 & X17);
    • Switching of VAC lamps/outlet (230VAC) - (X13)
    • Detection of 7 push buttons pressed (X6 X7 X8 X9 X14 X15 X16);
    • Detection of 230VAC lamp switch on/off - (X12);
    • (1 transformer -> 1 primary - 2 secondary )
    • X11 = 16 VAC input from a transformer
    • X10 = 24VAC input from a transformer
    Items which I am unsure about:
    • RS485 circuit in general (1 resistor or jumpers with additional 2 resistors to ground en 5V?);
    • Ground plane?
    • 60 ohm resistor on the 24VAC to power the solenoid?
      (solenoid characteristics: 370 mA inrush, 210mA holding, 50 HZ)
    The print needs to be manufactured by a local company. Furthermore the print will be assembled in a watertight ABS enclosure.

    Thank you in advance.
  2. JDT

    Well-Known Member

    Feb 12, 2009
    I have only had a quick look but I think your VO14642AT opto-mosfet devices are incorrectly wired - check the data sheet.
    Unless your symbol is incorrect. You seem to have N-channel devices with the source going to the positive supply.
  3. tudstudent

    Thread Starter Member

    Jun 6, 2009
    Symbol is correct, I did it wrong.
    Thanks, I will correct it.
  4. tudstudent

    Thread Starter Member

    Jun 6, 2009
    Attached adjusted drawings (board and schematic)
  5. tudstudent

    Thread Starter Member

    Jun 6, 2009
    Small bump. Could someone please be so kind to comment on this?
  6. Sensacell

    Senior Member

    Jun 19, 2012
    I see no protective elements on your RS-485 lines.
    This is a risky proposition for any real-world application.

    Our fist version was naked like that, suffered as a result from nasty field failures and unhappy customers.

    Have a look at the attached file, this is what we ended up with, not one failure since.
  7. tudstudent

    Thread Starter Member

    Jun 6, 2009