The PLC includes considerable internal Auxiliary Relays (M), the function of those are to store up plenty ON/OFF status,
which provided data for the processing demand. The operating method of an Auxiliary Relay is the same way to
operate the External Output Y, but the contact of Auxiliary Relay can not directly drive an external load.
The identifier number of an Auxiliary Relay M uses the decimal method, and the functions can be divided into three
different types:
(1) General Auxiliary Relay
When the input power is cut off during the PLC operation, the contents of this kind relay will be cleared. After the
power resumes, the contents of this kind relay are all OFF.
(2) Latched Auxiliary Relay
When the input power is cut off during the PLC operation, the contents of this kind relay will be latched. After the
power resumes, the contents still remain. That means, if the content of a latched relay is ON before power cut, its
state will keep ON at the moment when the power is back.
(3) Special Auxiliary Relay
Each special relay has its own specific function. Some special relay has only the readable contact of its status, but
without the writable coil to be driven by the program. Do not use special relays which are not defined. For the details
of special relays, please refer to the section 2-14 “Special Relay and Special Register”.
M0~M1999, M4000~M8191
Tot.=6192Pt.
M2000~M3999, Tot.= 2000 Pt. M9000~M9511, Tot.=512Pt.
The auxiliary relays in the VS1,VS2,VSM and VS3 series share exactly the same ranges.:
Below show some examples:
Suppose there is a platform that, when the motor rotates forward and backward, it drives the platform to move left
and right. In addition, the left limit switch (X0) and the right limit switch (X1) are connected to the left and right
limitations of the moving range. If the operation of the motor should finish the uncompleted movement after the
power is returned, that must use a latched auxiliary relay.
M2000
M2001
X0 X1
X0
M2000
X1
M2001
Self-holding circuit, use M2000 to retain the platform moving to the right.
Self-holding circuit, use M2000 to retain the platform moving to the left.
Also, now there are many mechines using human-machine interface (HMI) as operation interface. In such cases,
many setups to PLCs by HMI may need the latched function.
18
2-3 Auxiliary Relay (M)
General Auxiliary Relay
Latched Auxiliary Relay
Special Auxiliary Relay
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