Address switch with conflict indicator
Original Publication Date: 2000-Oct-01
Included in the Prior Art Database: 2003-Jun-19
Many computer peripheral interconnections use a simple addressing scheme to differentiate between different devices. A typical small computer system might have up to 16 SCSI storage devices on the same bus. Larger computer systems could have up to 128 fibre channel arbitrated loops (FC-AL) storage devices. Each storage device must have a different address if the bus is to operate correctly. The address settings of SCSI storage devices, and groups of FC-AL storage devices, are usually controlled by rotary switches. FC-AL products have a 7-bit address setting for the storage devices. In a typical system there will be up to 16 storage devices in each enclosure. The enclosure address switch sets the most significant 3 bits of the address. The least significant 4 bits of the address are used to differentiate between the 16 storage devices within the enclosure. If two enclosures within the same FC-AL loop have the same address switch setting, there will be a bus conflict. The FC-AL addressing scheme is quite sophisticated, so in this case the host computer will instruct all the devices to talk on the bus using a method called "soft addressing". However the preferred method is to have all devices at unique addresses so that all operations are done using "hard addressing". Our recent experience with installation of FC-AL products is that users can find this difficult and misleading. People who are experienced with using FC-AL storage devices still make the mistake of connecting two enclosures together with their address-switches set to the same value. This leads to complications when they try to configure their system. The novel method is to add an indicator close to the address switch. When the enclosure is not connected to any other enclosures then the LED is not illuminated (see figure 1). If the enclosure is connected to one or more other enclosures with the same switch setting, then the host computer will instruct all of the storage devices in the enclosure to operate with soft addressing. This is detected by the microprocessor that controls the SCSI enclosure services (SES) operations, and it turns on the LED to indicate a problem (see figure 2). When the user changes the switch to an unused address, the problem is resolved and the host computer then instructs all the storage devices in the enclosure to operate with hard addressing. The LED can be a bi-colour one, so the SES microprocessor can change it from a fault indication colour to a good indication, perhaps from yellow to green.