I feel that the very basics of SCSI need to be easily found and read in a simple format.
Posting this page so that it’s easily found, and people can more easily get a quick answer on how to correctly and properly use SCSI on the Macintosh.
Rules:
1. The SCSI bus on most Macs is shared, internal and external. It’s all one and the same. The “bus” is any part of the SCSI on the Mac, including the external cables, the port on the back of the Mac, and the internal port (if any) and internal cables. It all makes up the “bus”.
2. No two devices can have or share the same SCSI ID number.
3. Use SCSI ID numbers 0 through 6, in any physical order on the bus. It was general practice for the internal hard disk to be SCSI ID 0.
4. NEVER hot-plug or hot-unplug any SCSI device. They are not a hot-pluggable and carry voltage in the connector. Plugging or unplugging a powered-on device into either an unpowered or powered-on Mac could very likely cause the SCSI chip to blow.
5. SCSI is not plug and play. It’s more like plug and pray (it works without issue).
6. Termination. See next…
Termination:
A SCSI bus is like a water pipe. The SCSI cable carries water (data) between the well (device/disk) and the faucet (computer). Just like a hose or water pipe, it needs to be sealed to prevent leaks. The terminator seals the water (data signal) leaking out either end of the bus/cable. It prevents a data mess!
A terminator is either a block that connects to your SCSI connector, or is a SCSI device/drive that contains always-on termination.
1. Ensure you always have only a maximum of two total terminators on your SCSI bus, combined inside and out.
2. Always have at least one terminator or terminated device present if you have anything at all connected to your SCSI bus.
3. Never place more than one terminator or terminated device on either the internal or external part of your SCSI bus.
4. The terminator always goes only at the very end of your physical device chain. It is always the last thing, because it stops the leaks after it.
5. If you have no internal drive/device, and only external drive/device, you only install one total terminator, because in 90% of Macs the internal SCSI bus will auto-terminate if there is nothing connected there. If you want to be extra cautious, you could place a termination block on the inside of the machine either on a ribbon cable connected to the internal connector or directly on the connector, if you have that kind of terminator.
6. Some devices, such as a BlueSCSI and some models of ZuluSCSI, have an “always on” terminator. Be mindful of these devices and pay attention where you place them on your SCSI bus. These devices are generally meant to be alone and do not expect other devices to exist.
7. If you only have internal drives/devices, no external termination should ever be needed or installed. Only have one internally-terminated device, at the very end of your bus (last device). The first device is the one closest to the connector, the last is furthest away. With one internal device, it needs to be terminated.
In regards to BlueSCSI, ZuluSCSI, SCSI2SD, and other such devices, pay close attention to power requirements of these devices. Some draw their power from the SCSI bus itself, and that’s fine, provided they are by themselves in the bus. If you add more than one or have multiple different kinds, you will have to externally power these devices to avoid problems.
A word of caution related to Iomega Zip drives. Many listings on sites such as eBay indicate the Zip drive is the SCSI version or SCSI-compatible. Most times they actually aren’t, and could ruin the SCSI chip in your Mac (or other computer), if connected. The telltale sign is the model number (SCSI is —Z100S— and the non-SCSI or dangerous one is —Z100P—). In fact, I see numerous listings even right now identifying a parallel port Zip drive as being SCSI.
The dangerous Zip drives also sport a port that shows what looks like a printer icon. The SCSI models show an icon that is the same as used on most Macs (tilted square with a horizontal line on the right). eBay sellers are very unwise in their listing practices and refuse to correct their listings.
Posting this page so that it’s easily found, and people can more easily get a quick answer on how to correctly and properly use SCSI on the Macintosh.
Rules:
1. The SCSI bus on most Macs is shared, internal and external. It’s all one and the same. The “bus” is any part of the SCSI on the Mac, including the external cables, the port on the back of the Mac, and the internal port (if any) and internal cables. It all makes up the “bus”.
2. No two devices can have or share the same SCSI ID number.
3. Use SCSI ID numbers 0 through 6, in any physical order on the bus. It was general practice for the internal hard disk to be SCSI ID 0.
4. NEVER hot-plug or hot-unplug any SCSI device. They are not a hot-pluggable and carry voltage in the connector. Plugging or unplugging a powered-on device into either an unpowered or powered-on Mac could very likely cause the SCSI chip to blow.
5. SCSI is not plug and play. It’s more like plug and pray (it works without issue).
6. Termination. See next…
Termination:
A SCSI bus is like a water pipe. The SCSI cable carries water (data) between the well (device/disk) and the faucet (computer). Just like a hose or water pipe, it needs to be sealed to prevent leaks. The terminator seals the water (data signal) leaking out either end of the bus/cable. It prevents a data mess!
A terminator is either a block that connects to your SCSI connector, or is a SCSI device/drive that contains always-on termination.
1. Ensure you always have only a maximum of two total terminators on your SCSI bus, combined inside and out.
2. Always have at least one terminator or terminated device present if you have anything at all connected to your SCSI bus.
3. Never place more than one terminator or terminated device on either the internal or external part of your SCSI bus.
4. The terminator always goes only at the very end of your physical device chain. It is always the last thing, because it stops the leaks after it.
5. If you have no internal drive/device, and only external drive/device, you only install one total terminator, because in 90% of Macs the internal SCSI bus will auto-terminate if there is nothing connected there. If you want to be extra cautious, you could place a termination block on the inside of the machine either on a ribbon cable connected to the internal connector or directly on the connector, if you have that kind of terminator.
6. Some devices, such as a BlueSCSI and some models of ZuluSCSI, have an “always on” terminator. Be mindful of these devices and pay attention where you place them on your SCSI bus. These devices are generally meant to be alone and do not expect other devices to exist.
7. If you only have internal drives/devices, no external termination should ever be needed or installed. Only have one internally-terminated device, at the very end of your bus (last device). The first device is the one closest to the connector, the last is furthest away. With one internal device, it needs to be terminated.
In regards to BlueSCSI, ZuluSCSI, SCSI2SD, and other such devices, pay close attention to power requirements of these devices. Some draw their power from the SCSI bus itself, and that’s fine, provided they are by themselves in the bus. If you add more than one or have multiple different kinds, you will have to externally power these devices to avoid problems.
A word of caution related to Iomega Zip drives. Many listings on sites such as eBay indicate the Zip drive is the SCSI version or SCSI-compatible. Most times they actually aren’t, and could ruin the SCSI chip in your Mac (or other computer), if connected. The telltale sign is the model number (SCSI is —Z100S— and the non-SCSI or dangerous one is —Z100P—). In fact, I see numerous listings even right now identifying a parallel port Zip drive as being SCSI.
The dangerous Zip drives also sport a port that shows what looks like a printer icon. The SCSI models show an icon that is the same as used on most Macs (tilted square with a horizontal line on the right). eBay sellers are very unwise in their listing practices and refuse to correct their listings.
Last edited:
