In your case, it would be better to connect the DAD to port A, the ADI 648 to port B and to create a stream on port C.

Well, they do have a life of their own. All four available matrices can, for example, be clocked individually. Routings shouldn't change for no reason - if you've made yourself familiar with the manual, it should be quite straight forward.

It is possible.

What happens with the 648 in Room 2 and Room 4?

I haven't used System Link, however I will try to integrate it.

1. In the router transparency settings, "activate MADI receiver" and "pass subframes". The MADI receivers should always be switched on, else the matrices won't work. Passing or creating new subframes doesn't matter in the RME world, however when other manufacturers' signals are involved, it may be of use to keep their frame architecture.

2. Decide who's clock master. A MADI Router can only be clock master within its matrices and at the word clock output (it is clock agnostic when complete MADI streams are routed, they always keep their own clock). I would therefore suggest to make the 648 at Coax C the clock master for everything, and clock the MADI matrices as follows in the "Matrix Clocking" setting for all four matrices:
Source: Coax C, Rate: 44.1 or 48k (I'll only cover single speed here because I assume that it's what you are using), S/MUX: Off, Channels: 64.

3. Clock all hardware devices to the MADI Router. If the 648 at Coax C is up and running, clocked to internal, it sends out a MADI signal to the Coax C input. In order to ensure that all devices are set up and synchronized correctly (four computers, DAD, and the remaining 648s), we do a preliminary STREAM routing from Coax C to all of those outputs. First move the output cursor to each port and then apply the routing by moving the input cursor to Coax C each time. Now all of your devices should receive a valid MADI signal.

Configure the devices and software (see VST system manual) to be properly synced to the incoming MADI signals. The TC&Lex should be synced to ADAT or WCL IN if you connect the 648 WCL OUT to them.

4. Change the stream routing to match your needs.
Move output cursor to Coax A, assign input Fibre A. No need for a matrix here.
Move output cursor to Coax C, assign input Fibre A. No need for a matrix here either.
This way, both 648 and DAD get the same signal, but the DAD will only care about the first 16 channels and you use the matrix on the 648 to only care about ch. 17-64.

5. Now the matrices:

I don't understand that. The TCs and Lex can only RECEIVE their input signal from ONE computer at each port, or do you have a set of TCs and Lex for each room that are connected to the other two ADI648? Of course the returns from TC/LEX can be sent to all four computers.

Anyhow, the procedure to split and distribute signals with matrices is like this:

Think output first, then assign source signal to it. Just like you set up the stream routings above, you move the output cursor to Computer 1 and think about which signal should be routed to it. You want 16 Channels from Coax A and 48 Channels from Coax C. So you move the output encoder to Fibre 1 and then rotate the input Encoder until MX.1 pops up (Matrix 1). Within the matrix, same concept, select output 1-8, assign Coax A 1-8, GANG one step with the out encoder to continue the next 8 channels, then assign output 17-24 from input Coax C [whatever channels you set up in the 648 matrix], GANG all the way to 64, done. The manual describes the procedure in detail.

This way, Computer 1 receives 1-16 from DAD and 17-64 from 648, while both devices receive the output signal from the Computer 1.

Now concerning your daisy-chain for VST, you'll have to draw a proper plan. Basically, each of the four computers needs its own matrix, so MX1 for Fibre A (already have that), MX2 for Fibre B, MX3 for Fibre C and MX4 for Fibre D.

As the Steinberg manual also states, you should try with two devices first and make sure it is running before you blame the MADI Router ;-).

In order to change the cursor size in the matrices (default is 8 channel, but for VST you only need two channels), you hold the ROUTE button and turn the IN encoder to set it to two channels.

Then, thinking output first again, and assuming that you have set up your VST Link to send and receive on Ch. 63+64 in each of the four workstations, in Matrix 1: output 63/64 from Fibre D 63/64, Matrix 2: output 63/64 from Fibre A 63/64, Matrix 3: output 63/64 from Fibre B 63/64 and Matrix 4: output 63/64 from Fibre C 63/64. Since Matrix 1 goes to Fibre A, Computer 1 will now receive the outgoing VST Link signal from Computer 4. Since Matrix 2 goes to Fibre B, Computer 2 will now receive the outgoing VST Link signal from Computer 1. And so on.

Once you understood how this works it is actually quite simple and I'm sure you'll be able to patch whatever TC or LEX to the correct channels of the work stations. However if you want the 648 to also receive a matrix (for example to set up 4 stereo reverbs on the Lex960 and giving each Computer acccess to one machine on the same channel), you must do a workaround, for example by using the 648 matrices.

Best regards,
Max

Correct. The four matrices are in between the inputs and outputs ("a layer above the outputs") and can be assigned to any number of physical outputs. Each of them is a little 768 to 64 audio channel patchbay.

You cannot split signals with one matrix because each matrix stands for one new MADI signal which consists of one or more channels from different MADI streams. Splitting of streams is usually not very important anyhow because the receivers will choose which channels they need. In other words: for a real "split" procedure, one signal of 64 channels would be routed to two matrices, and within each matrix only the important channels are "extracted". Each matrix is then sent to a different output. But you might as well send the complete source stream instead and let the final receiver do the channel selection itself.

I think it can all be set up the way you need it.