Siemens CPU S7-315 DP to OP7

Hi
i Have an OP7 [6AV36071JC200AX1] and a CPU s7-315 DP [6ES73152AF020AB0]
I'm communicating between the two via DP, i need to set up the CPU so that if communications is lost between that and the OP the CPU will go into STOP mode
Theres the PG on the network PC with which im programming the CPU [using Step7 and DP communication cable], other than that its just the CPU and OP7 connected with a DP cable.
I've opened up Block OB87 but i dont know how to wirite the program etc
So,is there anyone who can shed any light on how to get the CPU to go into STOP, if the communication to the OP is lost?
Mant Thanks

First of all, why do you want your PLC to stop? IMHO it is always best to keep the PLC running and set all outputs to what you consider safe.

If you remove OB86 from your program, the PLC should stop. But again, I don't think this is something you want.

I've been told the OP has an intermittent comms fault
[it'll drop comms then back again]

I've had some experience with siemens programming but not alot, i figured i could either make a log of comms error or make it stop the CPU [is a spare CPU so it wont affect anything if it stops]
Easier to make it stop the CPU if theres a comms failure, then i can confirm if its the OP or something else

I would rather use SFC51. It allows you to check the status for each DP slave.