To be honest I never tried maxing out a midi cable, but I guess you’ll be fine as long as you’re not sending continuous controller data on every channel.
Reliably? 0 … Zero … Nada …
The cable is not the limiting factor.
Technically you can send 3,125 bytes per second on a MIDI cable (that is the speed a MIDI interface) and considering that the typical MIDI message is 3 bytes, it can more or less send one message per millisecond. It’s variable because o other messages, e.g. MIDI clock or active sensing. That ought to give you an idea of what you are facing in terms of timing. 
I use several MIDI splitters (one port is split into more ports, exact copies of the original) so that each device gets their own cable. Each of the out ports from my Midex8 goes directly into a MIDI patch bay (with 16 ports in and 20 ports out) and each out port from there is connected to a 1/4 splitter box (small and very fast). So essentially all of my equipment is constantly connected, and available with simple commands or via button pushing.
The same thing goes for keyboards, drum machines and other controllers coming in. They are summed by MIDI mergers (this can be a little trickier depending on the devices) and those are routed to the Midex8. Some devices just don’t play nicely with others! 
I’ve used an entire rack (6-9 or so synthesizers) for live performances before (granted it was a long time ago, but MIDI is still MIDI) and I don’t recall any complaints about poor timing, or irregular beat/melodies.
So, I think you’ll be fine with one or two cables.
Unless you are planning on maybe twisting pitch benders, spinning mod wheels, and send CC messages like crazy… 
wait until you try to send MIDI clock …
Just so you know, you can enable sending MIDI clock in Cubase on a per port basis (as it is global), CTL+Click on the transport to access the settings. For example, a couple of my MoogerFoogers will synch to tempo via MIDI port, but have to have the clock. Or, my Voyager can sync its LFO to MIDI clock.
In the config you tell cube who the source is. So you could tell cubase to use the motu as midi clock
I don’t think you’ll need midi clock for your application. I use it for syncing up the arps on my MO6, the click on my DTX etc.
Yeah, MIDI is one of those interfaces that was created from the concept of this-should-be-enough. Of course, nowadays EVERYTHING is included in the “musical instrument digital” part, and the “interface” piece is stuck in the past and there really is no viable alternative. Many companies have created possibilities, but none that stuck, like MIDI.
I think in general, you should be good. Like I said, and to add to JMCecils comment about the clock, it was included on our live performances. (Though there were problems frequently.) Back then there were no “computer” sequencers, only hardware sequencers. The MC-4 was awesome (I still have one packed up, and in decent condition too!) which didn’t even have MIDI, and some of the follow up ones, like MC-500 (also have one of those in box, as a matter of fact). With MIDI, a couple of drum machines synced to it, and a bunch of MIDI synths (some of which were not even synced, but with approximate tempo set internally). Those were the times! 
MIDI clock is used to synchronize multiple MIDI devices; wordclock is used to stabilize multiple audio devices. All communication between digital audio devices uses wordclock (it’s embedded in the signal on units that don’t provide a separate output).
I’ve read where big setups, like the ones used on big-name tours, will use a master wordclock unit to help stabilize MIDI sync. To be honest, I’m not sure I completely understand how this works, or why. I assume it only pertains to units that handle both MIDI and audio.
Yeah, that guy explained it really well. I’d alter his explanation slightly: it’s true that a wordclock master signal sets all the units that are slaved to it to the same sample rate, but that is just a baseline of sorts – it’s real function is to make sure all the units are sampling at the same exact time. Even the best wordclocks have some timing errors, or drift (also known as jitter) to them, and it’s not even a matter of technology, but physical law. Some clocks, however, are better than others (you pretty much get what you pay for here): the clock is generated by the vibration of a crystal, and some units use cheaply made crystals, and some use very precisely made and thus more expensive ones.
Yeah, people mix terms all the time.
I’ve seen a few devices that has MIDI and word clock or other sync protocols, e.g. SMPTE. These devices don’t mix data in any way, e.g. between MIDI and SMPTE, but rather makes sure the onboard protocols are synchronized properly and across devices.
Here are some additional resources on the sync topic Sweetwater and another forum HomeRecording.com.
Just want to make sure you’re aware of the difference between word clock and midi clock…
I’ll start with word clock…
If you want to integrate more than 1 digital audio device in your system, you will need word clock.
If you have, for instance, your Motu AV, connected to a digital mixer, and they are both running on internal clock,
ie both are clock MASTERS, you will definitely hear clicks and pops over time. This is because they are clocked independently. THe way to avoid these clicks and pops is to make 1 device a master, and the other a slave. This is achieved by connecting a word clock cable [generally BNC to BNC] between the word clock OUT of what you designate as the master to the word clock IN of the slave device, and selecting Word Clock slave on the Slave!!. Which one as master/slave??? Depends on how “good” the clock is. There are heated arguments as to the quality of different word clocks - better stereo imaging, 192 Khz vs 96kz v 48k/44.1k… At a minimum, either as Master and the other as Slave should end your clicks and pops. If you have more digital devices, a master synchronising clock with multiple clock outs should be used to feed all the digital devices in your system, guaranteeing a common reference.
For example, my system has 3 computers - Comp 1 has RME MADI and Raydat interfaces [over 100 channels of audio in and out], Comp 2 has RME 9652 [26 chnls i/o], and Comp 3 has MOTU 2408. An SSL MADI AX converts analog outputs from various synths feeding into Comp 1. THere is a DAT machine, an AKAI S5000 with 8 digital outs, a Roland XV5080 with 8 digital outs. THe AES [digital stereo] output of Comp 1 feeds a TC Finalizer 96k, which returns to the Computer to print a stereo mix. There are other digital devices in the system.
All are clocked via a Swissonic WD8…the system works extremely well, and other than pilot error from time to time, I have been very pleased with its operation for the last number of years.
NB…the only thing transmitted from the Word Clock Out, is digital audio synchronising data.
On the other hand,
Midi Clock is part of the MIDI protocol, and is transmitted with other messages through the 5-pin DIN - also know as MIDI-cable.
Midi Clock is tempo-related. A device sends 24 Midi Clock messages per quarter note. If a project has a tempo of, let’s say, 120 bpm, that means there are 2 quarter notes per second, i.e. 48 Midi Clock messages.
These MIDI clock messages are generally used to :
1] Synchronise two MIDI sequencers - you could have a software and a hardware sequencer, and synchronise them, so that they would consistently run relative to each other.
2] Control audio devices that have “timing” functions, eg delay units…Midi Clock output from a Master [eg your Cubase program] will “tell” such a device what the bpm is, and it will calculate delays accordingly. Changes in tempi on the Master will be observed on the slave device.
SO!
There is no direct relationship between Word Clock and Midi Clock…they are two completely different protocols,
Digital audio systems will work quite well without Midi clock, and trying to put word clock into a delay device will not give it tempo information!!
The answer to your first question is down to “How much” info is on the Midi line. Some years back, I would have had perhaps 30+ Midi instances [Midex 8 + 2 Yamaha 256’s etc] - ie 30 Midi cables worth, each going to and from a seperate device - in use, so as to avoid any clogging. This has been much reduced in the last few years, with plug-ins inside the DAW, and with VSL Ensemble PRO running on the slave computers via ethernet.
Sorry that this has moved away from your opening question, but you seem to be following where it’s going, so hope this helps.
With regard to wordclock, what is important is setting up your devices correctly. As andyoc pointed out, you need to determine which device you want to be the source of your set-up’s wordclock – a.k.a. the “master” – and then setting all the others to slave (sometimes called “external”). The master device should be whichever one is most likely to output the best wordclock signal – usually the most expensive piece of gear. For most of us, who have an audio interface feeding a DAW, the wordclock signal is going through the same cable the digital audio signal is. Some devices have a dedicated BNC-type wordclock output/input to ensure even better integrity of the wordclock signal (since it doesn’t have to share the same cable with other signals). As Andy said, you can record with a setup where the wordclock isn’t set up correctly, but you’ll almost invariably have pop & click issues.*
*I’ve actually recorded whole songs without issue only to realize later I had both my A/D and Cubase set to Master
I did a bit of reading and I think I now know why big set-ups use wordclock to help enhance stability in MIDI playback. This is because big setups like those in a big live act use SMPTE for synchronization, usually because they’re also syncing lights and video playback to the music (or editing video to the music later on). The problem with SMPTE however is that there’s a bit of drift built into it, so the addition of a wordclock signal helps eliminate this. I think I have that right 
Steve, if you’re just triggering external sound modules and synths etc. with MIDI recorded inside Cubase, you don’t need any sort of synchronization. The purpose of synchronization is to allow devices that have any kind of tempo and/or song position capability to be controlled from ONE central unit, called the master. My understanding is that MIDI clock only provides a way to start and stop the devices, as well as perhaps “return to beginning.” If you want the ability to sync the devices in terms of position within the song, you need a timecode. There are several formats of timecode you can chose from. One of them is MTC, which I used to use a lot to sync Cubase to the sequencer on a keyboard (and vice versa). The pro’s use SMPTE.
Well, I doubt you’ll have any memory/CPU problems on account of MIDI, per se.
Consider MIDI at up to 3,125 bytes per second, and in comparison, a basic 44.1kHz 16-bit stereo audio channel which has a constant flow of 176,400 bytes per second. That’s 56 times the amount of data for one channel of audio, and that’s if you to max out the MIDI throughput.
Try adding some Pitch Bend, Modulation and After Touch data to your MIDI tracks. That’ll give you a marker on when MIDI limitations become noticeable. I think we did some test back in the old world, but unfortunately those memories have long since expired.
Excellent. I am glad things turned out positive for you. You need anything you know where to find us, mate!
Gee Steve, you have seriously gone MIDI crazy…I have read every post here with great enthusiasm for you and my limited understanding. I was about to try and sum it up as Electrobolt said initially but he brought it out again and that is: you are just not yet exceeding those 3,125 bytes/sec. Sounds like you can keep pushing it