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Hot Crossed Plugs

In last week’s quiz, we asked: "Into which of the following ports should you never plug a device while the Macintosh is turned on." Of the over 2,200 responses, 64 percent chose the correct answer, which is SCSI, with 29 percent being fooled by the so-close-it-hurts wrong answer of ADB. A few percent guessed incorrectly at the serial port and Ethernet, but we’re pleased very few people guessed at USB and FireWire. Let’s look at the answers, along with a few possibilities we kept out of the possible answer set to avoid confusion. Keep in mind that details vary between specific Macintosh models, so check your owner’s manual for the final word (Apple makes all old manuals available online, in case you’ve lost yours). TidBITS Talk also covered additional details of interest.


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One brief aside: The question has come up as to whether or not a PowerBook is "turned off" when it’s in sleep mode. Apple seems to consider a sleeping PowerBook sufficiently turned off most of the time, but SCSI devices are a notable exception, particularly with older PowerBooks.

SCSI — You can plug all sorts of things into a Mac, but SCSI is the only quiz answer we offered that we can guarantee is not safe to plug in or unplug while your Mac is running. There is a difference between "safe" and "possible," of course, and we’re not denying that it’s possible to plug or unplug SCSI devices without powering down. According to Apple, "always turn off the Macintosh and all peripherals before attaching or detaching any cables or devices or changing SCSI ID numbers."

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Here are the problems – special thanks for these details goes to Sentient Software’s David Shayer, who has developed several disk recovery programs. SCSI connectors weren’t designed to prevent pins from touching anything other than the matching pin, so it wouldn’t be hard to connect the wrong data lines or cause a short. If you’re adding a device to a SCSI chain, it will probably go on the end of the chain, which means you’d have leave the SCSI chain without termination while providing termination to the new last device. Next, every SCSI device must have a unique ID number on the chain from 0 to 7. If two devices are assigned the same ID number, a SCSI ID conflict results and the Mac may refuse to boot or display other problems. However, if the Mac is on and you attached a hard disk that caused a SCSI conflict, it’s possible that the Mac OS would write out the master directory block to the wrong device, trashing the correct one and causing data loss. Finally, unlike USB and FireWire, SCSI doesn’t automatically load drivers for new devices, though you can get around this using utilities like Robert Polic’s free SCSIProbe.

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If you’re really interested in plugging and unplugging SCSI devices more safely, TidBITS sponsor APS Technologies sells the APS SCSI PowerPlug for precisely this purpose. It provides a switch that electronically isolates the device you’re adding or removing from the SCSI bus and interrupts the term power line inside the PowerPlug so termination remains active. Two versions are available, the SCSI PowerPlug II with active termination for use at the end of the SCSI chain, and the SCSI PowerPlug NT without termination for use anywhere in the chain other than the end. Both cost $50 and are available from APS’s catalog (call 800/374-5688 or 816/483-6100), though not their Web site.


Further, Apple recommends that you don’t turn SCSI devices on or off while other devices in the SCSI chain are running, and some SCSI devices must be powered up before you turn on the Mac. Despite this recommendation, people with SCSI-based scanners, for instance, are used to turning their scanners on only if they plan to use them to preserve the light bulb and reduce power consumption. Damage is unlikely from toggling power to a SCSI device while the Mac is on, though doing so could confuse the Mac and potentially require a restart. Remember that if you want to shut off an external hard disk while the Mac is turned on, it’s extremely important to dismount the disk by dragging it to the Trash before cutting power. Otherwise, you risk significant data corruption.

ADB — Okay, we’re sorry: this was our trick answer. Many long-time Macintosh users know ADB devices shouldn’t be plugged in or unplugged while the Macintosh is running because you risk blowing out the ADB controller if you cross pins that carry power – and the only official way to fix that is to get an expensive new motherboard. However, although this advice is generally true and certainly bears heeding, it’s not always true. Apple’s PowerBook G3 Series – one of the last machines to ship with ADB ports – enabled hot-swapping of ADB devices, as noted in the developer documentation linked below. (The manuals that came with those machines also imply swapping ADB devices was not a problem.) Also, some machines like the PowerBook 5300 included circuitry which would protect the PowerBook by shutting down ADB if devices drew too much power.

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Despite recommendations against plugging or unplugging ADB, it is something that many people do at least occasionally. If you’re in that category, be very careful with the plugs so as not to cross any pins.

Serial — Serial devices (including LocalTalk networks) can be plugged in or unplugged at any time. However, even though there’s no physical danger to your Mac, some software might expect a particular device to be available at all times – such as a fax modem that’s set up to receive faxes automatically, or a serial printer shared on a local network. And, of course, if you unplug a LocalTalk network, no services on that network will be available, which might cause the Mac to complain, or running applications to complain or even crash, depending on what you were doing.

The GeoPort serial interface on some Macs has a ninth pin which is powered, and there was some question about whether or not plugging or unplugging a device could damage the Mac or the device. However, Apple notes that if the GeoPort Telecom Adapter starts emitting a clicking noise, you could "unplug the adapter for a few seconds" to resolve the problem, which certainly implies that it’s acceptable to plug and unplug that specific GeoPort device.

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Ethernet — All flavors of Ethernet from 10Base-T and 100Base-T to the older thin Ethernet (10Base-2), thick Ethernet (10Base-5), and Apple’s proprietary AAUI built-in Ethernet can be safely plugged in or unplugged at any time – although, of course, you will interrupt any services using Ethernet. Also, some old-style Ethernet networks set up in a daisy-chain topology might stop working temporarily, experience noise, or display other problems if you suddenly disconnect machines from them. However, there’s no physical danger to the Mac or the network. As with LocalTalk networks, the loss of services on an Ethernet network may confuse or even crash applications on the Mac.

USB — USB devices are designed to be hot-swappable, both with regard to their connectors and the way USB drivers can load automatically. That’s one of the reasons Apple switched to USB from ADB for things like mice and keyboards – and they even brag about it. As with SCSI, if you’re disconnecting a USB-based hard disk from your Mac, make sure to dismount it first or risk data corruption.


FireWire — Like USB, it’s safe to connect and disconnect FireWire devices from your Mac at any time – it’s one of the features of the technology which Apple promotes aggressively, and both the design of the connector and the way that FireWire drivers can load automatically support this. However, at the risk of sounding like a broken record (now there’s an analogy that isn’t long for this world), if you want to disconnect a FireWire-based hard disk, make sure to dismount it first.


Audio Input/Output — Although Apple recommends turning off all equipment when connecting audio input or output devices, there’s little danger from plugging in or unplugging audio devices from microphone jacks, audio inputs, or speaker jacks on your Mac. The main thing to remember is to turn down the levels on all equipment before making connections, so you don’t inadvertently overdrive or blow out a speaker. It is possible to damage these ports by connecting inappropriate equipment – we know a bassist who blew out audio inputs on a Power Mac 8500 by connecting the ampl output from his 300 watt amp. However, these instances aren’t the Macintosh’s fault: we’re sure most ports don’t like to be hooked up to household electrical current either.

Video — Video is a tricky issue, which is why we didn’t include it in the quiz. Although Apple’s documentation always recommends connecting monitors and video equipment with the power off, the manuals don’t warn of any potential damage (as they do with SCSI, for instance). Some Macs won’t boot without a monitor or video adapter connected, and Timbuktu Pro (which is commonly used to control headless Macs) requires that there be at least a video adapter present so the Mac knows what resolution to provide. Plus, although we’re not willing to try this, it would seem that unplugging one monitor and plugging in could cause problems if the two operate at different refresh rates or resolutions. It’s easier to shut down, and those of us who run headless Macs as servers are accustomed to pressing the Power key, then the Return key to shut the Mac off from the keyboard.

If you want to connect an external monitor to a PowerBook, either shut the PowerBook down or put it to sleep first, or else it won’t recognize the presence of the other monitor.

PC Cards — PC Cards, the credit-card-sized expansion cards generally used with PowerBooks, can be inserted and removed while the Mac is turned on, although the Mac may not be able to recognize the card if the necessary drivers haven’t been loaded at startup. To eject a card, drag its desktop icon to the Trash or choose Put Away from the Special menu to eject it gracefully (on some models you may also have to press an eject button on the case).

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Backup, Backup, Backup — We’ve tried to pass along the recommended approaches to working here, but as we implied with the comment about the difference between "safe" and "possible," even we don’t always follow our own advice here. On TidBITS Talk, Alex Hoffman jokingly accused me of being wimpy for recommending that people not hot-swap devices, but the fact is that I hot-swap ADB devices all the time, plus turn SCSI devices on and off while the Mac is turned on. I ignore my own advice in this regard because much of my old hardware has essentially no value, and I back up religiously. Sure, I’d be sad if I blew the motherboard on my SE/30 by fiddling with an ADB connector, but I could restore its entire set of services to another machine within 15 minutes based on the previous night’s backup.


So if you want to take the risk of frying some hardware, at least make sure you have a current backup first. You’ll still be sorry if you lose a motherboard, but at least your data won’t be jeopardy as well.

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