Description

This register holds a known double precision floating point value to confirm that the doubles registers are aligned with the addressing convention of the master and that the word order is the same between the master and the controller. The master should read from this register the value 123456789.123 decimal. If this is not the value read then try reading locations 1 above or 1 below the indicated register address. If these alternate addresses produce the proper 123456789.123 value then the indicated offset will have to be used for all references to the Modbus Register Map. Note that if a reduced resolution data type version of the Modbus Memory Map is being used where double precision values are reduced to single precision then the value produced may lose resolution after 7 or so figures and produce a value very close to but not exactly 123456789.123. This is just a limitaiton of the reduced resolution. The objective is to make sure byte and word order is correct. A close value confirms these orders are correct.

Escapes

Reading the Double Alignment Check Constant does not generate any escapes.

Examples

This block list aborts motion when a sensor is detected to position the motor at a detected location:

This alternative block list for the same task avoids the abruptness of aborting by noting where the sensed position is located, decelerating, and returning to that location.

Even though it involves a second move to return to the detection location this approach can be much faster since the search speed can be faster. However with faster speed comes less precision since the controller is generally only sampling at 1 kHz permitting in this case a user unit position uncertainty of 0.002.

This uncertainty can be eliminated by using the hardware capture function of the controller that associates the sensed event with a specific location: