There are today quite many different modular systems, e.g. eurorack modular systems, Buchla systems, and classical Moog systems. These are mainly compatible with each other, the most obvious difference between them is found in the patching, in the connectors, apart from different sizes of the front panels. This, however, is easily solved with adapters, or cables with banana jack in one end and 3.5 mm phono in the other end, or with a patch/multiple with different connectors. Other differences might be in the signal levels, both when it comes to audio and to CV signals. These differences are also quite easily solved by using attenuators and amplifiers to adjust the signals to the right voltage levels.
Other differences that might be present between different systems and between different analogue synthesisers might be found in the pitch CV and in the Gate/Trigger signals. Volts per octave (V/oct) is the standard pitch CV signal found in Eurorack systems. It was originally created by Bob Moog in the 1960s. V/oct is logarithmic, which mean that when the pitch CV increases with 1V this gives a pitch increase of one octave. The other pitch CV standard is Hertz per volt (Hz/V). This standard is linear wich means that the pitch increases proportional to the pitch CV, where a difference of one V might be the difference of an octave (i.e. between A1 and A2) but higher in the frequencies the difference might be about one tone). These two pitch CV standards are not compatible with each other, however no damage will be done if modules with different standards are connected to each other. There are different gear and modules that convert between V/oct and Hz/V.
When it comes to Gate/trigger signals there’re mainly two different standards: V-Trigger and S-Trigger. V-Trigger is the standards signal in Eurorack systems. The V-Trigger is a positive going trigger signal, that is when the key is pressed the trigger signal is high, or when the clock pulse comes the signal is high. Doepfer has set the high level to +5V. S-Trigger on the other hand is inverted compared to the V-Trigger, and when the key is not pressed the trigger is high, while it goes low (0V) when the key is pressed. Some old analog synthesisers (Yamaha) has a S-Trigger that goes from 0V when no key is pressed to -V when the key is pressed. It is fairly easy to convert between the different types of triggers, an inverting amplifier with adjustable off set is all that is needed.
For more information about CV/Gate check out this wikipedia page.
According to Doepfer audio signals should be 10V peak to peak, around 0V, in the A-100 system, which I consider to be standard for all Eurorack systems. Audio signals above that are possible, since the power supply gives +/-12V, even if this might provide saturation and distortion.
When it comes to control voltages, e.g. from a LFO or an ADSR, these are from -2.5V to +2.5V (5V peak to peak) for the LFO, and from 0 V to +8 V for the ADSR. It is possible to use other signal levels but this might result in saturation in the signal path, like a filter sweep in a VCF that is way beyond the desired audio range, or that a VCA distorts. By adding attenuators either at the output of the modulation source or at the CV input of the next module solves these problems easily.
Trigger or Gate Signals in the A-100 system, are typically positive going triggers from 0V to 5V. It is most probably possible to use a positive going trigger that exceeds +5V but (!), this might destroy a digital circuit (like a logic gate) at the input. An attenuator or voltage divider will solve this problem. Usually a lower trigger level than +5V will trigger a module, but if a trigger voltage is too low a simple non-inverting amplifier will suffice to get all triggers going.
When DIY, the above standards should be considered to avoid problems. However, for most things it’s not that critical (apart for when it comes to pitch CV). Always add an attenuator in the signal path, either at the output or at the input, to be able to adjust signal levels, and you’re good to go. One thing to consider though is the power supply. The power supply of the typical Eurorack module is a 10 pin bus, that has the following connection in pairs of pins: +12V, GND, GND, GND, -12V. There is also a 16 pin version that also has Gate, CV, and +5V like: Gate, CV, +5V, +12V, GND, GND, GND, -12V. Usually cabinets and power rails comes without the +5V option. This is normally not a problem as most modules that need +5V comes with a voltage regulator built in. Nonetheless, it is easy to add a +5V option to your modular system: +5V option.
There are many excellent designs, schematics, ideas, and modules out there! Some of these follow the Eurorack standard, some don’t. A rule of thumb is, if the module that you’ve found on the net is powered with +/-12V, it’s good to go. If you just add attenuators on the inputs and the outputs (just to be sure) you’ll be fine! However, if the module is powered by +/-15V there might be other problems that you need to consider if you run it on +/-12V. Some modules will work equally well on a lower power supply, others might need to be adjusted, resistor values changed and so on, to work. If the module run on +/-9V or maybe +/-5V, you most probably need attenuators on it’s inputs and an amplifier on it’s output. You also need to check so there are no components that’ll be destroyed by +/-12V, e.g. opamps, logic gates, capacitors. If the module is run by a single supply there’s most probably a virtual GND as well as GND (0V), this might cause problems in a dual supply setup. Furthermore, the signal levels in the single supply module might be less than in a dual supply module, why attenuators at the inputs and an amplifier after the module might be needed. Also, the DC off set might be different compared to a dual supply module.
For more information about the Eurorack standard check Doepfer’s webpage.