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Version 17
Safe-E-
Stop™
User Guide
3.
Operating Principle
3.1 General
The MSD has two safety relays that are wired in series with an external hard-wired E-Stop circuit that provides a
machine with the ability to run. The MSD safety relays are changeover types that are held energized in normal
(non-e-stop/run) operation. If the MSD power were removed, the relays would revert to their unpowered state and
the machine would be brought to a halt as though an E-Stop had been pressed.
The MSD is a complex safety PLC. In the event of any fault it will de-energize its safety relays; in its normal
operating mode, it will maintain these safety relays in an energized state.
A normal operating mode can be maintained in the absence of any online PSD, as well as with one or more PSDs
in a linked and normal operating state.
PSDs can come and go as required without affecting the MSD E-Stop safety relays, provided that the PSDs join
and leave the system by the deliberate act and process of linking and leaving the active group. (Patent Pending)
When a PSD is part of the active group, it will trigger an E-Stop if it has its E-Stop switch pressed or if it loses the
RF link to the MSD either because a) it goes out of range, b) it shuts down due to low battery, c) it has a fault, or
d) it has its RF link blocked by interference.
Both the MSD and PSDs feature fully redundant hardware and safety-critical software. They individually and
jointly meet the safety level of SIL3 according to ISO61508.
The MSD polls the PSDs in turn and assigns a PSD an active slot if one is available. The linked PSDs return the
poll after each MSD poll, forming a closed loop safety system.
3.2 Radio Transmission
The connection between the MSD and PSDs is performed by means of radio communication. With regard to the
actual radio frequency that is used, there are several radio frequency bands available.
A specific RF frequency band and channel is selected prior to delivery of the system.
Depending on the frequency band, a defined number of RF channels are available.
The MSD and PSDs must operate on the same RF channel in order to be able to communicate.
Some systems use two RF channels in a frequency diversity mode to increase immunity to RF interference.
3.2.1 Continuous Transmission
Once the units are linked, transmission between MSD and PSD must be continuous. The MSD uses this as part
of the information required to maintain its E-Stop safety relays in a closed state. If the MSD does not receive a
valid telegram from a linked PSD after a short time, it turns off the E-Stop safety relays.
In order to ensure optimum communication between the MSD and PSD, operate the PSD with line-of-sight to the
MSD antenna as far as possible. Avoid total shielding of the signal path by extensive metal and other thick solid
obstructions.
3.2.2 Radio Interference
Signals from other RF-emitting sources might interfere with the radio communication between the MSD and PSD.
If the radio link reliability is affected by interference, changing the RF channel or even the RF band may be
necessary.
