rich_p
Member
Hi Everyone!
In the past I've always used hard wired push buttons into PLC inputs to start and stop machines. The start button a N/O contact and the stop button N/C. I've always assumed the stop button is a n/c contact so that it fails safe.
Anyhow now I've been asked to do a way with the hard wired push buttons and put some buttons on a HMI to start/stop some conveyors. Is this safe? Just wondering how and if others do this? Note: EStop is still hardwired ;o)!
Do you use a seperate address for the start and stop pb's or do you just set/reset a bit? What happens if the HMI fails? Should you always use an 'HMI alive' flag that if fails switches off plant?
Cheers
In the past I've always used hard wired push buttons into PLC inputs to start and stop machines. The start button a N/O contact and the stop button N/C. I've always assumed the stop button is a n/c contact so that it fails safe.
Anyhow now I've been asked to do a way with the hard wired push buttons and put some buttons on a HMI to start/stop some conveyors. Is this safe? Just wondering how and if others do this? Note: EStop is still hardwired ;o)!
Do you use a seperate address for the start and stop pb's or do you just set/reset a bit? What happens if the HMI fails? Should you always use an 'HMI alive' flag that if fails switches off plant?
Cheers