| 摘要 |
A programmable controller has a scanner module that couples a main processor through two multidrop serial data channels to a relatively large number of remotely located I/O interface racks. The main processor executes a control program to examine input status data coupled from I/O devices on a controlled system through the I/O interface racks and determines output status data which is coupled through the I/O interface racks to control output devices on the controlled system. The scanner circuit encodes a stream of output status data by generating an output mask work and the adapter circuit encodes a stream of input status data by generating an input mask word, the mask words being appended to streams of status data that are coupled to and from each rack in serial data messages. I/O status data that is selected for transmission over the serial data links is limited to data that has changed since the last time its associated I/O rack has been scanned, and refresh data that is periodically coupled to or from each rack, whether such status data has changed or not.
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