Siemens S5-135U/155U System Manual page 255

If you unplug the jumpers 12-14 and 4-6, the two-wire line R is not preset
and the break state cannot be recognized clearly.
With the following jumper setting you can switch over the data direction on
the two-wire line R:
Jumper 16-18: setting for full duplex operation. Data can only be received
on the two-wire line R (default).
Jumper 18-20: setting for half duplex operation. Data can be transmitted or
received on the two-wire line R (special driver required).
With the following jumper setting you can switch the two-wire line B as an
input or an output.
Jumper 17-19: two-wire line B is switched as an output. The line X(B) can
be used as an input. X(A) is connected to ground (default).
Jumper 15-17: two-wire line B is switched as an input. The line X(B) cannot
be used.
With the following jumper setting you can either place the signal /PS3 or the
internal transmitting frequency (TxCint) on the two-wire line B. The
two-wire line B must be switched as an output.
Jumper 2-4: the signal /PS3 can be transmitted via the two-wire line B
(default).
Jumper 4-6: the internal transmitting frequency TxCint can be transmitted
via the two-wire line B.
System Manual
C79000-G8576-C199-06
CPUs, Memory Cards, Memory Submodules, Interface Submodules
5-115