Siemens SINUMERIK 840D sl Function Manual page 189

05.08
Test signals
"3–terminal concept" (see Chapter 7.1.3 "Connecting sensors – actuators using
the 3–terminal concept"):
S If an input signal ($A_INSE)is, for example, evaluated through two channels ,
the associated test output signal can be implemented using one channel. It is
extremely important that the input signal can be forced/changed and checked in
both channels.
S In the same way, the assigned test input signal for two–channel output signals
($A_OUTSE) can be implemented in one channel if it is connected according to
the following rule:
The test input signal may only return an "OK" status ("1" signal level) if both
output signals function (i.e. both monitoring channels have output a "0").
A simultaneous test in both channels allows the correct functioning in both
channels to be checked using one feedback signal.
Trigger/test
The timer or event controlled triggering of the forced checking procedure is acti-
vated in one channel by the PLC.
If errors are detected, the PLC user program should respond by initiating an exter-
nal "STOP D/E" and switching the external SGAs into a safe state.
Notes to avoid errors
1. A "2–terminal concept" in which a single–channel net (useful) signal is to be
subject to a forced checking procedure using a single–channel test signal is
not permitted. In this case, the two–channel SPL structure would be worthless
and crosswise data comparison would have no effect.
The following are permitted:
S A "full 4–terminal concept for sensors" (two–channel test signal for a two–
channel useful [net] signal),
S the "3–terminal concept for sensors/actuators" recommended above
S a "2–terminal concept for sensors without test signals", if the two–channel
net (useful) signal to be tested automatically changes its level dynamically
as a result of the process – e.g. for the input signals of a protective door,
S a "2–terminal concept for sensors without test signals", if the sensor is a
safety–related component, e.g. light curtains,
S a "2–terminal concept for actuators without test signals", if the actuator is a
safety–related component, e.g. safety valves,
S a "2–terminal concept for actuators without test signals" if the feedback sig-
nal can be checked using other useful signals – e.g. for a valve that indi-
rectly switches a BERO via the process and this is available for evaluation,
S a "2–terminal concept for actuators without test signals" if the function of the
mechanical system can be checked using other useful signals – e.g. for a
brake that is checked using a separate brake test.
© Siemens AG 2008 All Rights Reserved
SINUMERIK 840D sl/SINAMICS S120 SINUMERIK Safety Integrated (FBSI sl) – 05.2008 Edition
Connecting Sensors/Actuators
7.1 Safety–related input/output signals
7-189