[awemawson]

I've come across the expression 'back contact' several times in the Mitsubishi Meldas 330V instruction manual.An example is reading the status of the three key operated switches, which is at address Y238 on the PLC interface. Y's are normally outputs and X's are inputs, but it says 'note they are back contacts'

Can anyone explain to me what they mean?

[Al_The_Man]

I have integrated Mitsubishi controls and have never run across that expression, unless it refers to the rear contact of a multi contact sw, why is the switch a Y function (output) switches are usually X (input)?
But in the case of a PLC it is normal to only input once, as the contact can be changed to NC or NO in the logic.
Do you have a better reference, manual no etc? I could look it up.
Would this be the 3 key switches for program/parameter/TO lock-out?
Al.

[awemawson]

Al, thanks for your reply - I've done a search through the manuals that I have as text .pdfs (rather than images) and have come up with the following incidents, the last one of which probably answers my question but I'd appreciate your comments:

Mitsubishi CHC Meldas 330HM-V/335M-V BNP-A2940*-E


Page 339 in text box PLC-I/F <SET DATA> refering to the Key inputs as 'Back Contacts'



Mitsubishi 300 series Connection Manual BNP-B3484D-E

Appendix 2 Page 203 a table with many points indicated as 'Back Contacts'
Appendix 2 Page 206 ditto
Appendix 2 Page 208 ditto
Appendix 2 Page 211 ditto
Appendix 2 Page 213 ditto
Appendix 2 Page 215 ditto
Appendix 2 Page 218 ditto
Appendix 2 Page 220 THIS MAY HOLD THE CLUE!! it says:

Data protection key,servooff, external deceleration, block start interlock, cutting start
interlock, interlock
The above signals are assigned pins on the optional card (2nd DIO card) in the
M320L/320M system. These are back contact (significant with input open) signals and so
the input signals must be shorted to a common ground (+ 24V) if they are not to be used.
Equivalent processing is undertaken when the optional card is not available.

[Al_The_Man]

I believe I see what it refers to, you notice that they also have an * next to the input.
This indicates they are all active low.
Whereas the rest are active high.
Never seen it referred to Back Contact?
Al.

[awemawson]

Al,

Have you any further thoughts as to why it should be a 'Y' for an input (those three key operated switches on Y238) and not an X - it makes no sense to me unless they are sensed on some other X input and put out on a 'Y' - but why ????

Andrew

[Al_The_Man]

I will pull out my old Mits books and see, what you may be seeing is that although there are 3 or 4 switch protection options, many MTB's use one switch and OR them all so one switch does the action of all three simultaneously, the Y address would be the result of the switch action for ladder use?
I will have to check whether Y238 is a free or dedicated output?
Al.

[Al_The_Man]

I see it now, I had to jog my memory, there are either 3 or 4 DATA PROTECT keys, according to what controller and each operate the different lock out functions.
These dedicated outputs must be turned on or off in the ladder for each to register in the controller.
When power is applied these are a 1 (data not protected) so a rung must be written in the ladder for automatic turn off or via a key switch etc.
Al.

[awemawson]

Ah thanks - that explains the milk in the coconut !

Whilst you have your MITs books out - do you have any definitive spec of the DIL switches on the front of the MC161-1 CPU card? I know some of the combinations such as 1&2&3 on to search for installed cards, and 2 on to stop the user PLC, and 3 on to prevent parameter download from the eeprom - but there are obviously more combinations. Do you have any mits publications that go a bit deeper than the one downloadable from the meau.com site. I feel I'm skating over the surface and not getting down to a low enough level being a frustrated ex component level engineer !!!

Andrew

[Al_The_Man]

I will see what I can find, I had to refer to the later PLC manuals as I do not have much on the 300 series, I started with the 500 and the 50 when I hooked up with Mits.
Al.