Experimental Physics and
Industrial Control System

Emmanuel Mayssat <[email protected]> · Fri, 14 Sep 2007 17:26:20 -0700

On Fri, 2007-09-14 at 17:20 -0600, Jeff Hill wrote:
> I suppose that via the Ethernet PLC driver the IOC could periodically
set a
> field in the PLC. Periodically the ladder logic might clear this field
and
> also force shutdown if the field remains cleared for too long? That
might
> work w/o needing any special hardware. There would of course need to
be N
> fields in the PLC - one independent field for each participating IOC. 

I thought about it and probably need to think about this solution some
more. When you say no additional hardware you are right, but you still
need to buy the features to enable the PLC to be fully network-enabled. 
Also you may need to buy licenses for the modbus libraries and/or
purchase modbus hardware. You are talking about a price tag of $5K+

Indeed Siemens PLC, cheaper in their base version than allen Bradley's,
do not have the capability to "read" from the network. They can only
write to networked computers (FTP transfer of registers to an IOC).

So far, we are ok with that. This design actually guarantees that the
PLC and the control system (computers and IOCs) are completely
independent and fail-safe.
So far, we "communicate" with the PLC only through switches/klixons.
This lead me to immediately think about implementing a mechanical switch
per IOC.
But that switch somehow needs to be activated...

Nevertheless, I do not want to discredit your solution. Clearly price
aside, it would work. So maybe I should revisit the possibility of a
fully network-enabled PLC. 

Regards,

--
E

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System