Copyright © Parallax Inc.
Digital I/O Board Kit (#27113)
v1.0 2/19/2010 Page 9 of 13
Relay Selection (Mechinical or SSR)
The Digital I/O Board is capable of using Omron Mechanical Relays (#400-00052) or Sharp Solid State
Relays (#400-00053). You can use either relay in any of the eight available relay slots in any
combination. The slots are mechanically interlocked to prevent you from using both types in the same
position. The Mechanical Relays can switch up to 250VAC @ 12A (24VDC). The Solid State Relays can
switch up to 240VAC @ 8A (AC ONLY). Unlike the mechanical relays, solid state relays do not exhibit
contact noise or arcing. These Sharp Relays even provide zero-crossing detection to reduce switching
noise, however they can only switch AC voltage, not DC.
Relay Power
No matter which relays you use the Digital I/O Board requires a separate 12V @ 1A power supply for the
relays. This power is provided through 2.1mm Barrel Jack at the top left of the board or optionally via
the 2-position terminal block next to it. The terminal block is l and – which the DC barrel jack is
center tip positive, making it compatible with our power supplies (Parallax recommends #750-00007).
Minimal reverse polarity protection is provided by diode D25.
Logic Power
The control logic requires power from the host microcontroller for the inputs to function. This will be
either 3.3V or 5V from your microcontroller provided to the VDD pin on the Digital I/O Board. Your
microcontroller’s ground should connect to the VSS pin. This allows the Digital I/O Board to operate at a
signal level compatible with your microcontroller. The V+ pin allows your microcontroller to get supply
voltage from the Digital I/O Board (reducing the need for an additional power supply). If your
microcontroller’s board has an on-board voltage regulator, you can obtain V+ from the Digital I/O Board
and then send your regulated output back to the VDD pin. V+ will be at the voltage of the relay supply
(typically ~12VDC).
Connecting and Testing (Parallel Interface)
Using the parallel interface (2x10 header) controlling relays is as simple as making a pin HIGH or LOW.
To energize a particular relay make its control pin HIGH. To turn it off bring that pin LOW. If your
microcontroller’s output pins are set to input the relays will also turn off due to internal pull-down
resistors within the ULN2803 IC. Inputs are easily read by reading a HIGH or LOW on their output pins
(labeled IN_1 through IN_8). Note that because the optocoupler inverts the signal, the pins will read
HIGH when that input is inactive and will read LOW when that input is active.
Note: Do not use the parallel and serial circuits at the same time. Use only one type of interface.
Connecting to an AppMod Header
The parallel interface (2x10 header) was designed to be compatible with the AppMod header on our
popular Board of Education development board. You could use a ribbon cable (not included) to connect
both boards together instantly connecting all necessary power and ground for a fully functioning parallel
control system.
When using the AppMod interface there are two additional considerations. You should use the I/O
declarations in the example code for the AppMod because the I/O pins are now mapped to specific pins
on the Digital I/O Board and these may not be what you expect. Also, the V+ connection allows the BoE
to get power from the Digital I/O Board. If you do not want this to happen you must remove the 2-pin
shunt from JP2 above the parallel interface. This disables the V+ pin on that header, but still allows the
BoE to provide 5V to VDD on the Digital I/O Board.