Drive communications is still a very weak area for most drive systems. Once communications between the drive and drive master is established, it is necessary to direct signals from the master to specific parameters in the drive.
Normally, commands and set points sent from the drive master to the drive and status signals sent from the drive to the drive master are grouped together to optimize communications.
The parameters for the command bits, speed set point and torque set point are spread among the thousands of parameters in the drive and must be directed to the correct one. The process of connecting the communications parameter to the drive parameter is commonly referred to as linking parameters. It is as simple as indicating which parameter # is the source for a command or set point parameter. Getting these parameters right may involve hours of reading the user manual and experimentation.
There are many other common problems with linking these signals. Often the signal is scaled differently in the communication module than in the drive regulator. For instance, the speed reference in the master may be 28,000 for top speed of 1750 RPM. This may be auto-magically scaled to 1750.0 RPM in the speed set point parameter.
In some drives, there is an update rate parameter which controls how frequently the links are updated. When this is the case, the default setting may be on the order of 300 milliseconds which is far too slow for web handling applications – even for slow speed lines.
For safety, we expect that communications are checked with built-in or application programmed watch dog timers. Risk assessment will determine how to shut down the line if communications fails.