What would be the total GPIO pins available.

The RasPi Hardware Wiki has information on the GPIOs: http://elinux.org/RPi_Low-level_peripherals#General_Purpose_Input.2FOutput_.28GPIO.29 and http://elinux.org/RPi_BCM2835_GPIOs.

If you're looking to maximise your IO, please check out the board I designed:

https://github.com/Guzunty/Pi/wiki

The standard configuration offers up to 25 IO pins using only four of the Pi GPIO pins via the Pi SPI interface. It can be configured to provide a few more without much work and the other Pi pins can still be used for still more I/O if need be.

You can also program it to use up to 8 of the outputs as zero CPU load pulse width modulated, which may be useful in a model railway configuration. PWM does require one additional Pi GPIO pin to provide a clock.

It is also very easy on the budget, only £3.20 for the bare PCB. Total parts cost is under a tenner.

Perhaps the best bit is that it protects your Pi GPIO from the higher voltages that are present in a model railway layout. If you should make a mistake and allow damaging voltages onto this board, the CPLD at the heart of it is socketed and can be replaced for less than £3.

This board doesn't provide high current drives, but I'm assuming as a model railway enthusiast you'll have the downstream high current stuff already.

HTH,

DC.

You can add lot of I/O whit arduino like Due see the roadtest.

http://www.element14.com/community/roadTests/1144

Looks pretty good depending on its price. When is it going to be available though?

You can expand (and actually protect) the limted GPIO of the Raspi using GPIO expanders that can be interfaced via I2C or SPI, you can find them in DIP packages easy to put on a prototype board and they can handle higher current than the Rpi pins the ones from Microchip are pretty cheap and easy to use, you can even get some free samples from Microchip.

If you use two of the 16bit, you will have 32 GPIOs to play with, I'd suggest you power them from your external power supply and not from the Rpi onboard regulator.

My 0.2

-Jorge

You also have the ability to connect to Arduino with storage and RTC at a pretty good price.

http://wyolum.com/shop/25-alamode.html

Mark

Derek

for the arduino due realy cheap see the link (not the green guy on Zelda)

http://www.newark.com/arduino/a000062/eval-board-arduino-due/dp/47W2961

I think the MCP23017 is going to be my best bet. I am thinking of placing a minimum of 4 chips on a card connected to the Pi. The only issue I am seeing is how to isolate the Pi from the 5V returm signals through the chip. With having 4 chips on the board I was thinking of powering the chips with an external 5V power supply. Will the MCP23017 use 5V to return the signal back to the Pi or will it use what every voltage is powering the chip?

Or would I be best to have a regulator circuit on the board to drop a 5V input down to 3.3V to power the chips?

For higher voltage interfacing, like what is present on a model train layout, I plan to use opto-isolator chips to keep the 5V or 3.3V side away from the 12V to 24V side.

Terry

Terry,

The MCP23017 chips draw almost no current. 4 of them together are much less than a single LED so they can be powered by 3.3v on the Pi.

The optoisoator chips are a nice design but 64 LEDs are quite a load and you may not find opto-isolators that work with only 3-5 mA current available from the 23017 output directly (do let us know). If not, you are likely to need Darlington transistor arrays (e.g.: ULN 2803 A) between your IO expanders and the optoisolators. The Darlington array-connected LEDs can be powered from a separate supply or from the 5V supply of the Pi as long as the Pi supply itself has enough power.

Finally, once you use the Darlington array, I'm not entirely sure the opto-isolators are neccessary. That depends more on the grounds and whether you have inductive loads (motors, solenoids) that could introduce spikes into the Pi power bus.

The PiFace uses MCP23017 and ULN2803's to drive LEDs. I haven't seen a nice schematic for this board but there could be one out there.

Adrian

Adrian,

I was wondering that to, what I might do is use the Darlington arrays on the outputs controlling devices on the layout. I will be using the opto-isolators on the switch machine circuits for sure. Some will be slow moving tortoise switch motors and some will be solenoids, thse devices I wish to try and keep isolated from the Pi circuits.

I am still planning the circuits out so that I can figure out how many units I will need to employ.

Terry