Electrical Safety
It is very obvious from the comments posted in this thread that the people driving the use of the RaspberryPi as a PLC training tool have not conducted any professional, electrical and electronic engineering research into the design criteria required for an educational
PLC or are prepared to acknowledge the legally required European Union safety compliance to IEC61508 and IEC 611313. A PLC’s functional criterion is an industrial control device used to automate electrically driven machinery. The objective of any real world PLC design and training criterion is to write and test PLC programs compliant to IEC 61508 and IEC 611313. Being able to simulate watchdog timer failure in both software and hardware to IEC 61508 and IEC
611313 in the classroom educational environment is also paramount to any professional
PLC training course. Therefore; for classroom training purposes, a PLC used to emulate real world functionality requires three levels of watchdog compliance and the provision to connect the PLC to real world electrically independent failsafe simulation circuits using a low voltage power supply, usually 24 volts AC and DC. First Level – An IEC61508 and IEC 611313 complaint machine language operating system containing a software watchdog timer programmed in pure machine language and burnt in a nonvolatile ROM. The purpose of the CPU watchdog timer is to monitor all
CPU memory functions for checksum errors and watchdog failsafe on any error generated. Second Level – An IEC61508 and IEC 611313 complaint PLC will also contain an external electronic hardware watchdog circuit independent of the First Level operations.
The second level watchdog’s purpose is to monitor the PLC input and output (I/O) electronics. In the event that any I/O device malfunctions, the secondary watchdog deenergises all PLC outputs. Third Level – An IEC61508 and IEC 611313 complaint PLC program and PLC associated electrical interlocking circuits. This
PLC or are prepared to acknowledge the legally required European Union safety compliance to IEC61508 and IEC 611313. A PLC’s functional criterion is an industrial control device used to automate electrically driven machinery. The objective of any real world PLC design and training criterion is to write and test PLC programs compliant to IEC 61508 and IEC 611313. Being able to simulate watchdog timer failure in both software and hardware to IEC 61508 and IEC
611313 in the classroom educational environment is also paramount to any professional
PLC training course. Therefore; for classroom training purposes, a PLC used to emulate real world functionality requires three levels of watchdog compliance and the provision to connect the PLC to real world electrically independent failsafe simulation circuits using a low voltage power supply, usually 24 volts AC and DC. First Level – An IEC61508 and IEC 611313 complaint machine language operating system containing a software watchdog timer programmed in pure machine language and burnt in a nonvolatile ROM. The purpose of the CPU watchdog timer is to monitor all
CPU memory functions for checksum errors and watchdog failsafe on any error generated. Second Level – An IEC61508 and IEC 611313 complaint PLC will also contain an external electronic hardware watchdog circuit independent of the First Level operations.
The second level watchdog’s purpose is to monitor the PLC input and output (I/O) electronics. In the event that any I/O device malfunctions, the secondary watchdog deenergises all PLC outputs. Third Level – An IEC61508 and IEC 611313 complaint PLC program and PLC associated electrical interlocking circuits. This