PEU error in S5 PLC

[rainman 0]

Hi all, I have 2 sets of IM316 Interface modules for S5-100U, one sitting at the last slot of my S5-100U and the other one sitting beside a 8 digital inputs module. However I'm missing the connecting cable between the 2 interface modules. According to the ISTACK descriptions, PEU means "expansion module not connected","module unknown","module in the wrong slot". Can I say that the PEU error is caused by the missing cable? The troubleshooting guide for PEU error says "check power for the expansion module". When I plug out the interface module, the PEU error is gone and the CPU is able to go to the RUN mode. Under S5-115U, PEU error has different definitions, "missing terminator" in the central unit. Sorry for the newbie question. Thanks.