The automatic monitoring system of safetyensuring power supply in a power plant （4×300MW） has been retrofitted by using PROFIBUSDP field bus and PLC. The design of network and control software for the said monitoring system has been emphatically presented

and the debugging process of the safetyensuring power supply system being expounded. The retrofitted monitoring system has been put into operation for a period of more than one year

any abnormality having not occurred. The practice shows that the retrofitted monitoring system for safetyensuring power supply to be highly reliable

to have convenience and agility for operation

as well as to be sensitive in monitoring and simple for maintenance.