GPCIO Support Boards:
In order to verify the correct operation of the uMODBUS(m) code, designed for use within GPC model(B), it will be necessary to construct a number of simple input and output boards for connection to the circuit during tests.
These are outlined below and simply allow the the Digital Inputs to be altered though PCB links, the state of the Digital Outputs to be monitored on LED's and the level of the Analogue Inputs to be varied on a number of multi-turn potentiometers.
Digital Input Board:
As explained above this board is intended to allow the state of the digital inputs to be altered though the position of the 8 PCB links.Here each input has an in line 1K resistor connected to the PIC's I/O line and a 10K pull down resistor to circuit ground.
The logic state on the I/O can be placed at +5V (Logic 1) when the link is made and effectively 0V (Logic 0),though the action of the pull down resistor,when the link is removed.
As the PIC's I/O lines may be programmed as digital inputs or outputs the 1K is intended to prevents damage to the PIC device if the following set or circumstance came about:
If the I/O line is configured as an output, while the link is in place so that +5V is presented to the PIC I/O line (Logic 1) and the PIC is trying to drive the output Low (Logic 0) a high current will flow which would result in possible damage to the PIC.
Here if this set of circumstance should occur the 1K simply acts as a current limit.
The board requires +5V and 0V to operate.
Because links are used on the circuit several of the tracks have been cut rather then drilled, in order for the cuts to be visible the unpopulated and populated boards are shown below.
Digital Output Boards:
Once again the board deals with a single byte and utilises the popular ULN2803 octal transistor driver device. An attempt has been made to make each of the devises outputs accessible in order to allow the connection to incandescent lamps, relays or solenoids as required though the inclusion of sockets on each of the devices output lines(RL0 - RL7).
Because it is possible to attach the device to something other then the on board LEDs, pull down resistors have been included on the devises inputs to prevent
false triggering during periods when they are allowed to float.
The board requires +12V and 0V to operate.
Here some board space is saved by mounting the pull-down and pull-up resistors upright (See as built).It is probably worth mentioning that the tracks beneath the LEDS are also cut, as it may not be that obvious from the layout below.
Analogue Input Board:
This circuit is simply 3 x 10turn 10K pots presented in such a way to allow easy connection to the GPCIO, the board requires +5V and 0V to operate.
Keypad Link Board:
In order to allow program operation to be changed on the fly on the GPC(b) a keyboard interface was included. In circumstances where a keypad would be an overkill and only a basic selection is to be made, this simple linking board was devised for development with the intention or replacing it with a rotary switch for the final build.
Once again as links are employed in the design, the populated and unpopulated boards are shown below: