Watlow PPC-2000 - Manual (Page 244)

Chapter 5: LogicPro and Modbus Reference PPC-2000 User’s Guide
216 Watlow Anafaze Doc.# 30002-00 Rev 2.3
It is necessary to begin any host read command (Modbus
function 04) with register 33051. Read requests that do not
start at 33051 are not supported. It is not necessary to
continually read the entire queue at once. A recommended
technique is to read the first 8 registers of the queue on a
periodic basis. If there are more than 7 changes in the queue,
then subsequent changes would be shifted down so that the
next time the queue is read the host will pick up the changes.
System Error Log
The System Error log indicates how many times particular
failures have occurred since the last time the error log was
cleared. The first register contains a total count of system
errors. The last register contains a number of unidentified or
illegal error codes logged. Refer to Table 5.62 on page 217. The
counts stop incrementing at 255.