Rockwell Automation 57C442 Data Highway Plus Interface Module - Manual (Page 13)

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2.2 Electrical Description
The Data Highway Plus Interface module contains an 8 MHz
microprocessor that shares 32K of dual port memory with the AutoMax
rack backplane. This memory is accessible to both the on-board
microprocessor and AutoMax Processors in the rack. A block diagram
is shown in Appendix B. The module contains a watchdog timer which
is enabled when power is turned on to the module. The microprocessor
must reset the watchdog timer within a specified time or the
microprocessor will shut down and the status LED labeled ”OK” on the
faceplate will turn off.
At power-up, the on-board microprocessor runs diagnostics on the
microprocessor, EPROM, RAM and dual port memory, as well as
performing system-level diagnostics. As each test is run, a number is
written out to the seven-segment display. If there is a fault during these
tests, the microprocessor halts, the watchdog times out, and the seven-
segment LED displays the code of the failed diagnostic. See Appendix
D for a list of error codes.