S0-connector (upper sub d connector):

  sub-d iae (rj45) function
2 sr1-  5 (b1)     \ tx
3 sr2+  4 (a1)     /
4 sx1+  3 (a2)     \ rx
5 sx2-  6 (b2)     /

lower sub-d connector (isdn feature connector)

IOM-Bus           Serial Interface
1                 RTS
2 dclk
3 fsc
4 IDP1
5 signal GND      signal GND
6                 RX
7 IDP0
8 Vcc (optional)  CTS
9                 TX