I love IO-Link. I much prefer it to analog. That said, there are always trade-offs.
-You get values in some kind of engineering unit, rather than having to figure out what 6.7mA translates to.
-You know if you're connected. (Yes, 4-20mA gives you this to.)
-It's only 3 wires to get as much data as possible. Power is two of those wires.
-Depending on the device and the controller, you can change parameters on the fly.
-Depending on the device, you can have bi-directional communication.
-Depending on the device, you can get diagnostic data as well as process data.
-Even without real-time diagnostic data, you can do things like check the device number, helping to make sure you've got the right thing plugged in.
-Generally easier to set up than devices that take an IP Address.
-One IP address can have many IO-Link devices, saving addresses and switch ports.
-Network war independent (i.e. masters are available on EtherNet/IP, Profinet, etc.)
-Devices are generally cheaper than comparable Ethernet devices.
-Generally uses standard M12 4-pin connectors.
-You can add an analog to IO-Link or IO-Link to analog adapter. (Although this is a bit of the worst of both worlds.)
-You can add a few digital I/O if you need to.
-There's an inductive wireless option for some admittedly niche applications.
-You need the IODD file, or else enough documentation to figure out how to set it up as a generic device and parse the data.
-IO-Link is a "bag of bits" protocol. The IODD helps, but you still end up mashing bytes together, and endian matters.
-There's no where to put a meter. On a 0-10V, it's very easy to hook up a meter and see what you're reading. On a 4-20mA it's only slightly harder. You can't do that with IO-Link. (It should be noted that there are tools you can hook up to a laptop to check on the device, but that's orders of magnitude harder than checking voltage with a meter)
-Sometimes there's not an IO-Link version of a device, so you're stuck with analog anyway.
-Can be more costly than analog, and is almost certainly more costly than digital I/O, such as photo-eyes. Yes, you get the parameterization and diagnostics, but money is money.
-Point to point only, so you need an IO-Link port for every device. (You can also argue this is a good thing, since you don't get the complexity that comes from networks or fieldbuses when at the device level.)
-Speed is generally "fast enough", but may be a factor in some applications.
Does anybody know if power and ground can be bussed with IO-Link? In other words do they wink the 24V line or something to start transmission? All of the IO-Link modules I've seen have connections for all 3 wires which leads me to believe the power is not exactly passive.
I'm pretty sure that the power can be bused, and it's just power. I think the main reason that the modules all have 3 points is just because most of them are anticipating 3 wires off of 4-pins on an M12. However, the 1734-IOL 4-port IO-Link module only has two terminals for power (and 2 for common), and I believe they're internally jumpered.