Introduction to DDA. ADLINK Technology PCI-8136M

4.7 Introduction to DDA

This section will introduce the DDA operating theorem that creates all those various motion function. The DDA is for Digital Differential Analyze. It is a method to accomplish motion control under non-realtime OS.

Software

1 2 3 4 5 6

Motion

Library

FIFO (64)

6 5 4 3 2 1

Hardware

DDA

Encoder

Feedback

Refer to above figure, the software (Motion Library) solve motion command and generate a string of displacement data. Each data indicates quantity of puls e in 4ms(default).

For example: The trapezoidal curve at above figure represents a motion command that last 1 sec. The Motion Library solves this command and generates a string of displacement data. {10, 20, 30, … ,

470, 480, 490, 500, …. 500, 490,480,…., 300,200,100} The total number of data is 1000ms / 4ms = 250.Data

The data is transmitted via PCI-Bus into FIFO in PCI-8136M. The size of

FIFO is 64 for each axis. Every 4ms or less, the DDA engine takes a data from FIFO and send these pulses to motor driver with equal time interval.

The DDA engine is in charge of generate pulse in a smooth way. If DDA takes a data of 500 from FIFO, it generates a pulse every 8µs in the following 4ms.

Its means that the DDA engine will divide the total cycle time to send the total pulse counts with equal time interval. Every 4ms or less cycle time, the DDA take another data from FIFO and repeats the same operation.

Why does DDA help to accomplish motion control under non-realtime OS?

The key reason is that all displacement data in FIFO is pre-calculated by software when CPU is leisure, and DDA get data from FIFO, which is always non-empty.

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• Operation Theorem