Combining 8 digital signals
I have troubles combining the 8 signals into 1 to bring it to the 74221 


Maybe something like this. You didn't say what logic levels you are using or how fast so I chose to work with 74LS logic, but you should be able to do this with a faster, lower-power family just as well:







simulate this circuit &ndash; Schematic created using CircuitLab

C1, R1, and D1 are a positive edge detector.

The LS14 is a Schmitt-trigger inverting buffer, generating a negative pulse for each rising edge in the input. 

Q1 is a kind of upside-down open-collector inverter (because I didn't find a Schmitt-trigger non-inverting buffer in the LS family). Q2 shows how these can be connected in wired-OR configuration.

The final buffer symbol represents the B-input of a '221 monostable multivibrator, which is the one sensitive to positive-going inputs.