Sorry for the incorrect information in my previous post (#4), I was in Step 7 mode where the CPU stops if the error OB is not present. In a Step 5 system the CPU can continue if there is no error OB present in the CPU, it depends what is programmed in DX0
To force the CPU to stop you to need to program a STP command in OB23 and OB24 and then examine the ISTACK when the CPU goes into stop. This should lead to you the access causing the timeout error. Note that OB23 is for peripheral accesses, OB24 is for the process image update. I read this in the 928B manual - you need to refer to the manual for your particular CPU