The basic SCSI bus can connect up to seven devices and 1 host adapter on it, the bus uses 3 address lines which gives you a possibility of 8 addresses. Each device is assigned a SCSI address from 0 to 7 where 7 is reserved as the host adapter. The remaining 7 addresses are used for any device connected anywhere on the bus. Each address is assigned a priority, where 7 (the host adapter) has the top priority and 0 beeing the lowest priority.
Each device can be assigned any address (except for address 7) and is user selected via DIP switches or jumpers. No two devices can have the same address on the same SCSI chain, internal nor external. Usually you can assign any address number to a device as long as the software or drivers for the specific device allow it. Some devices will not let you the freedom of selecting the address, in these cases you must make sure that the predifined address on the device does not conflict with other devices already in use. For example some systems will force you to assign the address 0 on the boot hard drive, this is true for most host adapters that emulate the Western Digital WD1002 controller
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