The PROFIBUS-Tester is a testing tool which supplies an immediate overview of the signal conditions along the line and which enables fault localization in PROFIBUS networks. Most field buses use a twisted-pair cable according to RS485 specifications as their physical medium. Bugs in the application software are often suspected when field bus systems are set into operation. However, experience shows that an incorrect design and incorrect parameters of the bus cabling system can be blamed for the majority of all practical problems.