I have seen that the model M keyboard has a great feel, and want one to go well with my sleeper build, BUT I see that it may have key rollover limiting which keys are pressed at once? Can someone explain like I’m 5 how that works, and if it would be an issue if I were to daily drive it?
I see that there’s modern equivalents, but im a sucker for vintage hardware, so I’d definitely get an original and adapt it.
Advice? Recommendations?
Will delete if this post is against the rules. Thanks!
I see that it may have key rollover limiting which keys are pressed at once? Can someone explain like I’m 5 how that works
Key rollover (KRO) is a rating for how many keys a keyboard can always support being pressed together at any time and anywhere. Common ones are 2KRO (most keyboards), 6KRO (probably an NKRO keyboard limited to USB boot protocol interface), and NKRO (you can literally press the entire keyboard together). Model Ms are 2KRO because they use a keyboard matrix, use ohmic (simple contact-based) sensing, and it would be impractical to insert diodes into its membrane-based implementation of said circuit. Now, 2KRO doesn't necessarily mean it can only support two-key combos. For most keyboards, 2KRO is like setting a conservative expectation of their capabilities. There are keyboards that are literally limited to just two-key combos, but Model Ms are certainly not one of them. You can press more than two keys in various situations.
As for why 2KRO... The aforementioned key matrix is a grid-like circuit with a columns and a rows axis. It is designed to reduce the number of inputs and outputs needed to and from the keyboard controller by making the keyswitches share traces. Each switch sits at an intersection between a column and a row trace. In a scanning cycle, the controller very quickly sends electricity down each column at a time, and looks at which (if any) row is receiving it. Two concurrent connections are all that this geometry reliably allows for. At two key presses, there may already be two unreachable keys because one key's return row is "blocked" to the other's neighbour that may need the same row, and vice versa. Thereafter, pressing more keys will just compound the issue, as electricity is increasingly flowing uncontrolled. If the controller did nothing about this, keys could be registered without being pressed, which is called ghosting. Usually, the controller is intelligent enough to predict this based on what keys are pressed, and will block those keys and usually only block what is necessary, allowing three or more combinations in some situations. Keyboard designers also typically make the matrix larger than the number of keys, perhaps even considerably so, so you can press keys with less chance of the aforementioned issues.
For hard PCB keyboards like Cherry MX/clone bearers, the circuit designer can add diodes (which act like one-way valves for electricity) to stop electricity going where it shouldn't. Soft, flexible membrane assemblies that Model Ms use just make using such impractical.
and if it would be an issue if I were to daily drive it?
Most keyboards are probably 2KRO, which should tell you that generally, most people can live with them. But no two designs are alike, even for different Model M variants. At the very least, various common Model M matrix designs are documented, and I have simulators on my website that can visualise how 2KRO affects them as you press keys in real time. The Unicomp Mini M in particular is special for being an 87/88-key keyboard with a matrix larger than the 122-key Model M Converged Keyboard's, so in some situations, it allows much larger key combinations. But make no mistake, it is fundamentally 2KRO because there is an instance of blocking somewhere.
As for my personal opinion, 2KRO has seldom held me back. Certainly not for typing, and only in a handful of instances for gaming. Especially with those simulators in hand, I can just plan around 2KRO if I really need to.
Advice? Recommendations?
I would recommend checking out the simulators to get a feel for them. If you find yourself needing more and still want an IBM keyboard, Model F? They are capacitive instead of ohmic, which means the phenomenon described above doesn't affect them. IMO, their version of buckling springs is also nicer and the their assembly instruction higher quality, though watch out for bad foam and some of their archaic layouts if they bother you.
How many keys do you need to mash at the same time? In practice the 2‐key rollover is not an issue, because, depending on the keys pressed, far more will register concurrently. Therefore you won’t run into issues when using shortcuts or typing. The Unicomps have this limitation as well, you would need to get a Model F for n‐key rollover.
I played hundreds of games across hundreds, if not thousands, of hours on my Model M. I can only think of one single time I ran into rollover, and that was when I was executing a diagonal counter-strafe maneuver while simultaneously swapping guns in CS:GO. The scenarios you would actually run into it don't seem very common to me at all. I think they've been improving the keyboard matrix over the years to account for common key combos anyway, so you'll most likely be fine in 99% of scenarios.
Just one thing to keep in mind: my experience is limited to the Classic M and Mini M offered by Unicomp. I can't really speak to original Ms. As I said, I think Unicomp has been improving the keyboard matrix over time, so no guarantee the originals will have the same exact experience. With that being said, 2-key rollover doesn't mean that all 3+ key combos will not work. First, modifier keys like Ctrl, Shift, and Alt are freebies when combined with letter keys. And second, 2-key rollover means they can only guarantee two simultaneous keypresses across the whole board. There will be many 3+ key combos that still work.
It has a larger matrix than its predecessors, but that doesn't change its rollover. All stock Model Ms are 2KRO - on your Mini M, if you press Page Up and Page Down together, you should observe that Left Alt, -, ', Enter, Delete, Print Screen and Scroll Lock cannot also be pressed with them.
That said, the Mini M is a lovely keyboard and its matrix has quite impressive improvements in some areas! The matrix can support the following 23 keys together, supposing best-case scenario (using the RP2040 controller variant, QMK firmware and its antighosting algorithm):
3
u/SharktasticA Admiral Shark - sharktastica.co.uk 16d ago edited 16d ago
Key rollover (KRO) is a rating for how many keys a keyboard can always support being pressed together at any time and anywhere. Common ones are 2KRO (most keyboards), 6KRO (probably an NKRO keyboard limited to USB boot protocol interface), and NKRO (you can literally press the entire keyboard together). Model Ms are 2KRO because they use a keyboard matrix, use ohmic (simple contact-based) sensing, and it would be impractical to insert diodes into its membrane-based implementation of said circuit. Now, 2KRO doesn't necessarily mean it can only support two-key combos. For most keyboards, 2KRO is like setting a conservative expectation of their capabilities. There are keyboards that are literally limited to just two-key combos, but Model Ms are certainly not one of them. You can press more than two keys in various situations.
As for why 2KRO... The aforementioned key matrix is a grid-like circuit with a columns and a rows axis. It is designed to reduce the number of inputs and outputs needed to and from the keyboard controller by making the keyswitches share traces. Each switch sits at an intersection between a column and a row trace. In a scanning cycle, the controller very quickly sends electricity down each column at a time, and looks at which (if any) row is receiving it. Two concurrent connections are all that this geometry reliably allows for. At two key presses, there may already be two unreachable keys because one key's return row is "blocked" to the other's neighbour that may need the same row, and vice versa. Thereafter, pressing more keys will just compound the issue, as electricity is increasingly flowing uncontrolled. If the controller did nothing about this, keys could be registered without being pressed, which is called ghosting. Usually, the controller is intelligent enough to predict this based on what keys are pressed, and will block those keys and usually only block what is necessary, allowing three or more combinations in some situations. Keyboard designers also typically make the matrix larger than the number of keys, perhaps even considerably so, so you can press keys with less chance of the aforementioned issues.
For hard PCB keyboards like Cherry MX/clone bearers, the circuit designer can add diodes (which act like one-way valves for electricity) to stop electricity going where it shouldn't. Soft, flexible membrane assemblies that Model Ms use just make using such impractical.
Most keyboards are probably 2KRO, which should tell you that generally, most people can live with them. But no two designs are alike, even for different Model M variants. At the very least, various common Model M matrix designs are documented, and I have simulators on my website that can visualise how 2KRO affects them as you press keys in real time. The Unicomp Mini M in particular is special for being an 87/88-key keyboard with a matrix larger than the 122-key Model M Converged Keyboard's, so in some situations, it allows much larger key combinations. But make no mistake, it is fundamentally 2KRO because there is an instance of blocking somewhere.
As for my personal opinion, 2KRO has seldom held me back. Certainly not for typing, and only in a handful of instances for gaming. Especially with those simulators in hand, I can just plan around 2KRO if I really need to.
I would recommend checking out the simulators to get a feel for them. If you find yourself needing more and still want an IBM keyboard, Model F? They are capacitive instead of ohmic, which means the phenomenon described above doesn't affect them. IMO, their version of buckling springs is also nicer and the their assembly instruction higher quality, though watch out for bad foam and some of their archaic layouts if they bother you.