View source for Sd84 plug-in

For board specifications see http://www.robotshop.com/ca/PDF/devantech-sd84-servo-controller-specifications.pdf

The rhd-plugin divides the 84 channels into 8 A/D inputs, 36 digital inputs (except for 8 inputs used by A/D), 24 digital outputs and 24 servo channels.

The following RHD variables are defined:

'''read:'''
* version, 8 values, 2 bytes (currently 84,4) for each CPU;
* sd84ad, 8 values, each channel returns a 10bit value in the range 0..1023. Channel AN1..AN8.
* sd84in, 36 values, 8 values are not set, as they are used as A/D inputs. Channels CH1..36".
* steeringChangedTo, one value set by this plugin to inform the speed control plugin to change speed for the driving wheels.

'''write:'''
* sd84out, 24 values, values (0 or 1) are output to channel CH37 to CH60.
* sd84servo, 24 values, values range about 700 to 2200 for pulse width in usec - center should be 1500. Channels are CH61 to CH84.
* steeringangleref, one value, is the desired steering angle, and  - if set - will control the the two servoes at CH67 and CH68.

=== Method ===

Two of the servis are used for an ackerman steering - servo 6 and 7 on channel 67 and 68.
The steering is taken from a RHD variable steeringangleref