After more than ten years of operation, upgrade programs for the BESSY II electron storage ring demand a revision of the Personnel Safety Interlock system to ensure compliance with current legal requirements for personnel safety. During the last years, a variety of PLCs approved for safety relevant applications (Safety PLCs) appeared on the market. Applications based on Safety PLCs in combination with appropriately chosen peripherals and software could achieve highest security levels as defined by the relevant standards (IEC 61508 resp. DIN EN 62061 and ISO EN 13849). The paper discusses the application of the relevant standards, and how the upgrade procedure of the BESSY II Personnel Safety Interlock will benefit from the flexibility that Safety PLCs offer.