Method and an apparatus for dynamically reconfiguring a system bus topology

摘要

A method and an apparatus is presented for configuring a system bus topology dynamically. In a preferred embodiment, the system bus is a Small Computer System Interface (SCSI) bus that connects a "daisy" chain of disk drives. Two types of disk drives are used: single ended (SE) "Ultra" drives capable of 20 MHz operation and LVD (low voltage differential) "Ultra Plus" drives capable of 40 MHz operation. LVD disk drives can also function in the slower SE mode. The first drive in the chain of drives may need to be connected by a cable over three feet long. This introduces signal degradation that is often overcome by introducing redrive circuitry to boost signal quality. This is an expensive solution and a much easier solution is presented: install a jumper between the last drive in the chain and the first drive. However, if LVD bus mode is used, then this jumper solution does not work and the jumper must be removed. Disk drives in a server system are "hot swappable," which means they can be changed at run time without shutting down the system. A method and an apparatus is provided for dynamically testing for the appropriate mode of bus operation based on the currently installed disk drives and adjusting the jumper setting accordingly.