12
9M02-8978-A002-EN
Version 14
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 normally open types: if the MSD
power were removed, the relays would open 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 open its safety relays; in its normal
operating mode, it will maintain these safety relays in a closed 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 linked 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.
