Hello folks...I see a lot of questions on this topic, so I thought I would chime in. Having had about 35+ years of experience in electroacoustic music, as well as having designed a few devices, plus having used a plethora of modular setups over the years, I'd like to offer some how-to-get-going advice to people starting in this direction.

1) Don't start cold. Before designing a modular system to YOUR spec, see how others have been designed over the years by others. Especially note semi-modular patchable designs; the ARP 2600 comes to mind immediately, as many schools still use these as a primary tool for learning analog synth programming basics. After looking at (or better, using) a few of these, you'll notice that there are certain patterns to the layout of the panels, and this all relates to signal flow. Making this as efficiently directional as possible, instead of a patchcord hodgepodge, results in a more instrument-like...well...instrument. Which leads to...

2) Generators / modifiers / controllers / processors. These are the four basic 'food groups' of modules. Some modules can fit into a couple of these categories (or more), but it's how YOU define their uses that determines where in there these sorts wind up. So, let's look at these:
a) generators. Pretty straightforward. Things that create a waveform that is the 'raw meat' of your sound. Oscillators, certainly, but also signal inputs, noise sources, sample-based modules, and so on. If it MAKES noise, it fits here.
b) modifiers. Now these are things that ACT ON the waveform and modify how it behaves. Filters, ring modulators, VCAs if you use them for AM, waveshapers, and the like. If it changes the output of the generator(s), it's a modifier.
c) controllers. The obvious things here are controllers themselves: sequencers, keyboards, and the whole gamut of such widgets. But also various modules, especially modulation sources like LFOs, EGs, etc. If it makes something do something, it fits in this club.
d) processors. The 'summation'. When everything that's been through a-c above gets to the end of its journey, it arrives at d. Mixers go here, plus effects, output stages. But also, processors are scattered throughout any good synth design. A multiple is a sort of processor (passively splits something), as are submixer stages for various other subsets of modules, either AF or CV.

3) Why is that important? Well, it's because you, optimally, group things according to those categories...and when you do, you begin the basics of sensible signal flow. "But I want a VCO way the hell over here!", you say? Well, you could do that, sure. But at the same time, if you get adept at reading your patchcord jungle, you will come to notice 'unusual' patches that require a cord to go 'way the hell over' there, and therefore you'll pay attention to it, because, obviously, you set that special patch aspect up for a significant reason. The cord there becomes an 'arrow' that tells you 'hey...look at this a bit more carefully than the rest of the spaghetti'.

4) Flow directions. Once you get your groupings sussed out, then you need to decide how they fit together and play together. Again, study some existing, tried-and-true designs that had a lot of work go into them and which are considered 'classics'. And if you do, you notice something of a rule of thumb: up and left/right and down. Huh? Well...consider...
Your human input, as control signals, are probably best coming in at the 'bottom' of the layout. You want them at hand. Knobs to grab, wheels to turn, keys to tickle. Then from there, the control signals from those control other things to augment the control. And now, we're heading upward, building up the control signal structures. Some of these branch one way, and triggers, gates, etc go to other control things, stuff to modulate, while your CVs head on upward to (where I like to put them) the upper part of the layout, where your generators live. So now, we're all the way up and all the way left. Now we have to go rightward and downward.
The generator signals, influenced by modulators coming from that middle-leftward zone, and jiggered by processing to cook 'em down, arrive at the modifiers...where you find filters to tamper with timbre, VCAs for amplitudes, waveshapers to mangle stuff, etc. And with this stuff here, your controlling modulations are simply moving across the middle...left to right...to have at the CVs inputs herein. Once we're done here, then it's a simple move on down to the final modifiers, namely your effects processing and mixing, as well as other end-stage trickery...which, like the controllers at the beginning, are nicely at hand in the bottom-right quadrant where you can easily get at 'em. It feels right, it looks right, it's not QUITE so maddening to program...perhaps...and cables all seem to flow around the front panel's user interface in a way that doesn't seem look like some sort of connect-the-dots puzzle on mescaline.

Anyway, that's how I tend to work with these things, employing much the same sort of flow structure on an instrument as I might with a studio environment...which, after all, is what the innovators such as Don Buchla, Bob Moog, Peter Zinovieff, and the like had in mind: an 'in-one-box' solution to the electronic music studio setups of the 1960s and before. Not saying these are hard 'n' fast rules, but they work for me, and have for quite some time. Give it a try (which Modulargrid makes oh-so-easy) and I think you'll be pleasantly surprised.

L


This is very helpful. My rack has been growing from 6U to 12U and I'm going to need to think more about module placement. What you've suggested here is what I've also discovered for myself. Controllers lower left, oscillators/filters up top, effects down right (with output).