Siemens S5-135U/155U System Manual page 247

The TTY submodule feeds in the current (20 mA) via jumpers in the
connector of the standard connecting cable. The 24 V required for generation
of loop current is taken from the power supply of the PLC. In the quiescent
state, with a correct loop current connection, there should be a flow of 20 mA
(= logic 1). When the current is interrupted there is a logic 0.
The following applies to the TTY signals:
Data Transmission A maximum of 9600 bps is permissible for data transmission with the TTY
Rate
submodule.
Pin Assignments
Shown in the figure are the pin assignments of the 25-pin subminiature
of the TTY
D-type connector in the front plate of the TTY submodule:
Submodule
Pin
13 13
25
14
1 1
: Input
: Output
System Manual
C79000-G8576-C199-06
CPUs, Memory Cards, Memory Submodules, Interface Submodules
Logic 0 is represented by: no current
Logic 1 is represented by: current (20 mA)
Designa- Current
tion direction
1 Shield
9 24 V
external
10 + TxD
12 + 20 mA
13 + RxD
14 – RxD
16 + 20 mA
19 – TxD
21 – 20 mA
24 – 20 mA
Remarks
This connection is changed over between
24 V internal and 24 V external with
jumper J3 (see next page).
Current source, transmitter
Current source, receiver
Current return
Current return
5-107