R
Rufus
<clip>
> > But there have been several times when I wished I/O modules had a polarity switch.
>
> Why? Is this because you got to site and found the information you had been given was inverted?
>
Actually, it is when a system has been updated with new devices.
In one case (output), engineering found a better, more reliable, higher performance, (or whatever) vacuum valve that was "on" (sucking) when the output was high, in the other when the output was low (it was more of a bypass function).
In another case (input) we were using a reflective sensor to detect paper/no paper. When we found we could mount a through-beam sensor which would be more reliable, we found that it
returned the opposite polarity true signal when paper present in the detection area.
It might even be that we use the same logic in spite of the devices used, and it would be just a matter of wiring as to which device is used. The question is, is this invert table just another bit of programming or is it the province of the device wiring person?
I guess it's a matter if you want to have different programs for each device which differ only by polarity.
I believe have a different viewpoint from most PLC-users on the list. For many PLC users, it's either a single machine, or multiple identical machines, spec'ed and programmed nearly from scratch. In this case, it's not a hindrance that
I/O addresses have fixed, absolute locations, and polarity is decided in the machine design.
In my world, we build equipment based on multiple subassemblies that can be at any point in the sequence. This means that even though the logic for each submachine remains the same, the wiring positions change (and potentially the polarity, if the individual "point size" device changes)
It's like Mexican fast food. There's only about 6-10 different ingredients, but the amount and order of application of each ingredient give you a different end product. Our machines are similar.
Actually, it was this need that prompted me to write my own RLL-to-8051 assembly language converter about 15 years ago.. I had come from a
programming background and wanted to use logical names for input and output signals and all the PLC's allowed me was symbols like I0.3 or Q110.7, which meant nothing to me. Not to mention, when I changed code I would have to redefine all my symbols or have some symbols define some distance
(numerically) from the original symbols. If I had two of the same device I would basically have to rewrite the same control logic for each additional device.
I want to program a machine function one time, and hook up the correspondence table to real world points when the configuration is done. The closest I've seen to this is Step 7 software
which has function blocks encapsulating functions,
but the left and right sides of the block had all the interfacing signals to the logic, I's, O's, numerics, etc.
I've got to cut this short...
_______________________________________________
LinuxPLC mailing list
[email protected]
http://linuxplc.org/mailman/listinfo/linuxplc
> > But there have been several times when I wished I/O modules had a polarity switch.
>
> Why? Is this because you got to site and found the information you had been given was inverted?
>
Actually, it is when a system has been updated with new devices.
In one case (output), engineering found a better, more reliable, higher performance, (or whatever) vacuum valve that was "on" (sucking) when the output was high, in the other when the output was low (it was more of a bypass function).
In another case (input) we were using a reflective sensor to detect paper/no paper. When we found we could mount a through-beam sensor which would be more reliable, we found that it
returned the opposite polarity true signal when paper present in the detection area.
It might even be that we use the same logic in spite of the devices used, and it would be just a matter of wiring as to which device is used. The question is, is this invert table just another bit of programming or is it the province of the device wiring person?
I guess it's a matter if you want to have different programs for each device which differ only by polarity.
I believe have a different viewpoint from most PLC-users on the list. For many PLC users, it's either a single machine, or multiple identical machines, spec'ed and programmed nearly from scratch. In this case, it's not a hindrance that
I/O addresses have fixed, absolute locations, and polarity is decided in the machine design.
In my world, we build equipment based on multiple subassemblies that can be at any point in the sequence. This means that even though the logic for each submachine remains the same, the wiring positions change (and potentially the polarity, if the individual "point size" device changes)
It's like Mexican fast food. There's only about 6-10 different ingredients, but the amount and order of application of each ingredient give you a different end product. Our machines are similar.
Actually, it was this need that prompted me to write my own RLL-to-8051 assembly language converter about 15 years ago.. I had come from a
programming background and wanted to use logical names for input and output signals and all the PLC's allowed me was symbols like I0.3 or Q110.7, which meant nothing to me. Not to mention, when I changed code I would have to redefine all my symbols or have some symbols define some distance
(numerically) from the original symbols. If I had two of the same device I would basically have to rewrite the same control logic for each additional device.
I want to program a machine function one time, and hook up the correspondence table to real world points when the configuration is done. The closest I've seen to this is Step 7 software
which has function blocks encapsulating functions,
but the left and right sides of the block had all the interfacing signals to the logic, I's, O's, numerics, etc.
I've got to cut this short...
_______________________________________________
LinuxPLC mailing list
[email protected]
http://linuxplc.org/mailman/listinfo/linuxplc
