Japanese version is here.
See following figures if you would like to connect Domain Nodes via SIO.
Note: DS3000 has only one SIO line, other Domain Nodes (except DN3500/3550/4000/4500/5500/Series 4xx, DN460/660/DSP160) has 2 SIO lines.
Original document: "Domain Personal Workstations and Servers Technical Reference (APN: 008778-A01)"
[Power Supply] [SIO Line]