User's Manual

Table Of Contents

the REQ bit set on in the header. In this manner, the LRP830 DeviceNet

board only forwards intended commands to the antenna for processing.

Generic protocol repeats the same command over and over again until the

scanner clears the poll message buffer or places a new command in the

buffer. As a result the antenna receives many unintended commands and not

every command can be processed. This is because typical DeviceNet scan

rates are much faster than the time it takes for the antenna to process a read

or write command. Section 6.2 specifies a timeout value of 2 seconds for the

Block Read 0x05 command example. When using a command timeout

value of 2 seconds, the antenna could only process 1 out of 20 block read

commands with a scan rate of 100ms (with REQ on).

The antenna only processes one command at a time, but it is the nature of

the DeviceNet poll protocol to repeat messages at regular intervals. So with

the GENERIC protocol, the antenna cannot process every command con

tained in every poll message, therefore most commands are ignored. If the

scanner were to only send each command out once, there is a good possibil-

ity of lost commands. This is especially true with A-B PLC's where the in-

terface between the SLC/5 CPU module and 1747-SDN module, for

example, is asynchronous. Generic protocol, due to ease of setup, is in-

tended mainly for initial interface setup and test purposes.

164 LRP830-Series Long-Range Passive Reader/Writer