Software must not have the PTM Enable bit Set in the PTM Control register on a Function associated with an Upstream Port unless the associated Downstream Port on the Link already has the PTM Enable bit Set in its associated PTM Control register. PTM support by a Function is indicated by the presence of a PTM Extended Capability structure. It is not required that all Endpoints in a hierarchy support PTM, and it is not required for software to enable PTM in all Endpoints that do support it. If a PTM Message is received by a Port that does not support PTM, or by a Downstream Port when the PTM Enable bit is clear, the Message must be treated as an Unsupported Request. This is a reported error associated with the Receiving Port (see Section 6.2 ). A Properly formed PTM Response received by an Upstream Ports that support PTM, but for which the PTM Enable bit is clear, must be silently discarded.

如果与上行端口相关联的功能的PTM控制寄存器中未设置PTM使能位，则软件不能在其中设置PTM控制寄存器中的PTM使能位，除非链接上的关联下行端口已经在其关联的PTM控制寄存器中设置了PTM使能位。支持PTM的功能通过PTM扩展能力结构的存在进行指示。并不要求层次结构中的所有端点都支持PTM，也不要求在所有支持PTM的端点中启用PTM。如果一个不支持PTM的端口或者当PTM使能位被清除时，一个下行端口接收到了PTM消息，则该消息必须被视为不支持的请求。这是与接收端口相关联的报告错误（参见第6.2节）。对于支持PTM的上行端口而言，如果PTM响应的PTM使能位被清除，则必须默默地丢弃它。