Choosing and Using Peripheral Buses

Out with USB 2 and in with USB 3; out with FireWire and in with Thunderbolt, then Thunderbolt 2. But which should you choose to use, and how?

Just as we have got the hang of plugging in existing printers and other peripherals, every time that Apple revises its Mac range, so the new models seem to sport new ports with new strengths and foibles.

Although we should still see support for past standards such as FireWire and Universal Serial Bus (USB) 2 for a while to come, and there are adaptors to hook up the latest Macs with older peripherals, you should look now to using newer, faster connections, like USB 3 and Thunderbolt.

A bus is simply a set of connections to transfer data between electronic systems or sub-systems. These can be internal, such as the Serial ATA bus that connects internal hard disks, or external, such as a printer hooked up via a USB cable. Serial buses transfer the data as a stream over a small number of wires, as in USB, FireWire, and SATA. In contrast, parallel buses use many wires to transfer the data in parallel, an older approach which results in bulky and expensive connectors and cables, as in Parallel ATA and PC Cards.


Classic Macs supported a range of old peripheral buses, including SCSI for connecting storage devices, and Apple Desktop Bus (ADB) for input devices such as keyboards and mice. The latter ambled along at a leisurely pace, shifting data at a maximum of 1.25 kilobytes per second (kB/s).

Against these the advent of FireWire 400 (known generically as IEEE 1394) was a revelation: using a cable much cheaper and simpler that SCSI, it can transfer data at a maximum of 49 MB/s. In practice users found its performance similar to that of high-speed USB 2.0, although the latter has a maximum rate of 60 MB/s, and half the speed of successor FireWire 800 (IEEE 1394b) at 98 MB/s. For comparison, gigabit Ethernet delivers peak transfer rates of 125 MB/s, and fast WiFi IEEE 802.11n can achieve 75 MB/s.

To move your data faster than those older standards permit, turn to external SATA (eSATA), USB 3, or of course Thunderbolt.

eSATA is most widely available in its 300 MB/s form, but the newer 600 MB/s variety is becoming more popular. That should perform similarly to USB 3, with its maximum transfer rate of 625 MB/s, but all are paled by comparison against Thunderbolt with each of its two channels reaching 1.25 GB/s – a total of 2500 MB/s – and now Thunderbolt 2 which can aggregate both channels to double the rate on the resulting single channel.

Thunderbolt is effectively a hybrid of PCI Express used for internal expansion card buses, with DisplayPort for displays.

Connectors and cables

Peripheral buses differ not only in their performance, but also in the connectors and cables that they require. Whilst you have probably become quite used to those commonly encountered with older peripherals, newer ones can readily confuse.

Ports, plugs, and cables vary most in the early phase of the lifetime of any peripheral bus, although they can also become more complex later one, when a wider range of peripherals try to accommodate to the bus, and when attempts are made to eke out better performance.

At the moment the only novel peripheral bus which is likely to cause cable chaos is Thunderbolt, as it incorporates circuitry into each plug on a cable, and thus can be prone to problems such as overheating and cable failure. There have been reports of some batches of black Thunderbolt cables being particularly vulnerable to overheating, so for the moment you will probably do best sticking with reputable brands such as Apple.


Most modern peripheral buses can deliver power down the cable to peripherals, and that has generally improved in more recent standards. For instance USB 2.0 is required to provide 500 to 900 mA of current at 5 V, but ‘charging devices’ may provide up to 5 A; that is 25 W of power. iPads draw more current when charging than most other devices, and some Mac USB 2 ports provide insufficient to permit normal charging, although slow trickle charging should still occur when connected to such ports.

USB 3 brings wider support for the delivery of 1.5 A (giving 7.5 W power) as standard.

eSATA, being intended for storage devices, does not itself provide power over the bus, but a powered variant known as eSATAp or eSATA/USB is offered by some manufacturers. That provides a hybrid USB 2 port with 5 V power, and one or two optional 12 V lines, but is not a general standard and is being eclipsed by USB 3.0.

Thunderbolt carries power down the present copper versions of its cable, and can supply up to 9.9 W in all, with maxima of 0.55 A and 18 V.


Peripheral buses also differ in their topology, the manner in which you connect multiple devices to them.

USB has generally used a star layout, in which Macs have multiple USB ports, to which you can connect external hubs which can in turn accept further connections. However peripherals such as displays and keyboards may be designed to act as hubs themselves, allowing you to connect additional USB devices to them.

Prior to USB 3, sporadic issues have been produced or resolved by connecting devices direct to your Mac, or to a USB hub, which may work best with external power supplied, or without that. Early experience with USB 3 is that you can encounter similar issues, although it is unclear whether they result from inadequate power provision or engineering shortcomings.

The most common USB 3 issue is that of backward compatibility: for example, there are reports that M-Audio USB 2 devices can play up when connected direct to a Mac’s USB 3 port. These can resolve if you connect the USB 2 device to certain USB 2 or 3 hubs, but selecting the right one is currently little more than guesswork.

Thunderbolt uses a daisy-chain layout, so that you connect your Mac to your Thunderbolt display, and then connect that to an external RAID system, for instance. When buying Thunderbolt peripherals, check that they have two ports to permit this. There is something of a black art developing over the exact order in which you should connect these peripherals. Apple recommends connecting its Thunderbolt display direct to your Mac’s Thunderbolt port, and adding further displays then storage systems beyond, in that order.

However if you connect a Mini DisplayPort display on the far side of an Apple Thunderbolt display, the former will not work. It may work if you interpose a non-display peripheral such as a RAID system between the two displays, though. Apple’s full documentation on how to get the best from Thunderbolt is here. Currently Thunderbolt supports a maximum of 6 devices on each daisy-chain.

eSATA works point-to-point, so you can connect just one eSATA device to each eSATA port. If you need more ports you can install a port multiplier, but those are unusual.

Better buses should mean faster performance, less fuss, and no trouble. Although there is abundant evidence as to the improvements in performance delivered by USB 3 and Thunderbolt, these are still early days still, and most users still have many more USB 2 peripherals.

Tools: Connectors

Finding the right electric string to connect peripherals can be fraught. Even simpler buses such as FireWire and Thunderbolt have become quite messy.

FireWire connectors come in three main flavours:

  • miniature 400,
  • full-size 400,
  • rectangular 9-pin form for 800.

Adaptors are available, as are long cables up to the 4.5 metre maximum length.

Ports on a MacBook Pro: from the left, Power, 2 Thunderbolt 2, USB 3, stereo audio output. Courtesy of, and © Apple.

Thunderbolt cables use the same plugs and sockets as Mini DisplayPort, but with quite different cables, and you cannot use an ordinary display cable in a Thunderbolt daisy-chain. Currently all Thunderbolt cables are made of copper wire, so are limited to a maximum of 3 metres in length, but fibre-optic versions may extend that in future.

eSATA is even more limited, to a maximum of 2 metres, but more uniform unless it carries power.

A range of standard USB 2 connectors. By Viljo Viitanen, Wikimedia Commons.
A range of standard USB 2 connectors. By Viljo Viitanen, Wikimedia Commons.

USB connectors and cables cause greatest confusion, and the advent of USB 3 is only worsening this. Full-sized USB 2 connectors are either slim rectangular ‘type A’, or squarer ‘type B’ often found on printers and scanners, but there are four different varieties of smaller connectors, resulting in most users having a drawer full of different cables for cameras, GPS, and so on. To maintain high speed transfer over the cable, USB 2 cables can be no longer than 5 metres, although longer cables still work at lower speeds.

Main standard USB ports. By Milos, Wikimedia Commons.
Main standard USB ports. By Milos, Wikimedia Commons.

USB 3 ports and plugs should have distinctive blue plastic internals to distinguish them, although there are already many exceptions. Because you should be able to interchange all full-size USB 2 and 3 plugs and ports, this may be the only way of telling them apart unless you can see that USB 3 has an extra five pins. As with Thunderbolt, copper cables are limited to no more than 3 metres, but optical versions might go further.

Tools: Diagnostics

So you have hooked up your external hard disk, graphics tablet, or other peripheral, and it does not work at all, or plays up. Aside from the simple logical steps of checking that the cables are connected properly, changing port, cable, and performing a USB hub shuffle, what can you do to diagnose the problem?

Details reported on serial ATA (SATA) ports, which are all internal.
Details reported on serial ATA (SATA) ports, which are all internal.

The first point of reference is to open System Information, the bundled utility you can access through the About This Mac dialog, or direct in the Utilities folder. Each internal and external bus is listed under Hardware, allowing you inspect what OS X believes to be present.

Details reported on external Thunderbolt ports.
Details reported on external Thunderbolt ports.

Select Thunderbolt and you can see the firmware version of your Thunderbolt cable (it will be interesting to see how long it will be before that causes issues), what is connected to each port on your Mac and any peripherals fed from a Thunderbolt port, and their full details.

Part of the report on USB ports.
Part of the report on USB ports.

USB diagnostics have improved considerably, for instance showing the current available and required at each connection. The latter allow you to check that no device is trying to draw more than can be delivered by the port that it is connected to. Software driver and other components can be inspected by selecting the Extensions and Frameworks items, and Preference Panes where relevant, listed under Software. However System Information cannot run bus diagnostic routines.

If your Mac came with a hardware diagnostics disk, that can run through checking the function of its ports. There are also more specialist utilities that allow you to go deeper into some types of expansion bus and the devices connected to them: USB Prober, formerly part of the suite of tools supporting Apple’s Xcode SDK, is one of the few that still works, but is antiquated now and may lack support for USB 3.

Updated from the original, which was first published in MacUser volume 28 issue 25, 2012.