September 1997, Issue 86

PC/104 Quarter:
Motion Control with PC/104

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COMMUNICATION

"Embedded control communications protocol" can mean designing so that total computer control, part of it, or none at all (nodes only) is onboard the main system.

While communications plays a significant role in all of these cases, in the first example, communications may be useful only for loading or saving premade programs. But if you have nodes only, communications becomes the heart and soul of the control's capability.

A node typically defines a small, compact computer since a node takes instructions from a host at another location. The degree or intensity of the communications, the interface (e.g., Ethernet, RS-232, RS-485, etc.), and the communications protocol must be selected so the system performs as needed.

If, when using node control, the ability to synchronize axes is necessary, then an off-the-shelf protocol that yields high-speed data transfer has a definite advantage.

When using node control, you need to answer the following questions:

To answer, you need to understand exactly how fast the communications required by the embedded application should be.

While plotting might look like an intensive operation, the real-time requirement is loose. If the plotter has to wait while more data is transferred, no loss of integrity is noted. The data-transfer rate can range from as low as 300 bps to 9600 bps.

On the other hand, a labeling unit at 1200' per minute typically has a real-time requirement of less than 250 µs for I/O handling. Assuming 1.5" labels are needed, it takes only 0.0085 s to produce each label. Using a x10 rule to ensure all label data is appropriately transferred, the maximum allowable data-transfer time is 625 µs.

If each label contains ten characters, the minimum transfer rate is 160 kbps, yielding a new character every 62 µs. The data rate automatically eliminates the RS-232 protocol and requires a minimum of RS-485 or Ethernet interfacing.

Wake Up, Address, Command Byte, Bytes to Follow, Check Sum A, or one-byte ACK or NAK are returned to the host, verifying safe receipt of the message.

If multiple axes are used, messages can be acknowledged by pulling a common I/O line low (NAK) or leaving it high (ACK). The line shift would occur in the same timing frame as the ACK and NAK bytes, but all units might respond at the same moment, increasing the band requirements.

While this transfer mechanism isn't necessarily the most efficient, it gives a high probability that the embedded controller's messages are being transferred intact, which is exactly what we're looking for.

Let's consider how to invoke move from a remote host without using up valuable time. The five basic types of move are immediate, point-to-point, time, spline, and stop.