Functionality of the XOR Instruction?

I would Like someone to give me a useful example of and XOR function in a common plc application.

Lets say you had a bank of inputs (say push buttons) if you one-shot each input into a XOR the bit you punch inverts it's status.

Nick

Refer to this thread for a mention of using XOR to do a flip-flop.

A 'Useful example of an XYZ function in a common PLC application' would be any instance where a basic binary function is useful in achieving a goal.

There are a limited selection of binary functions (bitwise or wordwise); AND, OR, and XOR (disregarding the complement functions where they exist logically of NAND and NOR). Each is a valid construct depending on the situation.

Generalize the truth tables for each, and see what might apply in a given situation.

In pure binary terms, AND conditions requires that both input be true to achieve a true output... OR conditions require that either input term be true to achieve a true output... XOR conditions require that the input terms be opposite to achieve a true output.

So, if you require an output to be true based on A 'AND' B, then both must be true for the output to be true.

If you require an output to be true based on either A 'OR' B being true, then either can be true for the output to be true.

If you require an output to be true if and only if A 'XOR' B is true, than one must be true, and the other must be false for the output to be true.

XOR instructions are useful in almost as many Boolean logical operations as are AND and OR, and essentially combine a branch of: (A 'AND' NOT_B) OR (NOT_A 'AND' B)

Bitwise, the instruction is useful to detect mutually exclusive events as being faults, as perhaps with a valve...

Assume a two position valve has a status contact back to a PLC.

Command_Valve_Open 'XOR' Status_Valve_Open will be false, if the valve is operating properly, but if you give Command_Valve_Open, and the Status_Valve_Open input remains false, you have 1 XOR 0, which would indicate that the valve failed to open.

XOR is a common instruction, even if not provided as an actual instruction in the PLC instruction set, it is achieved often by using a branch of (AB Mneumonics here):
BST XIC Input_A XIO Input_B NXB XIO Input_A XIC Input_B BND OTE State_Mismatch

randy741985 said:

I would Like someone to give me a useful example of and XOR function in a common plc application.

Click to expand...

I cant take credit for this but its very nice

S7 Flip Flop in STL


It doesn't convert to S7 ladder, but it converts to S7 FBD.

XORing a bit array with itself will clear that array. Used that on old plcs that didn't have an array clear

xor(a(xor(b(xor(a,b))) will swap the values in word a and b without using any additional memory locations.

The instrucitons AND OR NOT and XOR are at the very heart of solving a ladder diagram. The processor works with 16 bit or 32 bit words, it does not work with individual bits. So what seems to be a simple bit instruction such as XIC -] [- or OTE -( )- requires that your processor execute some binary gymnastics to determine what the state of a single bit in a word is or to set/clear just one bit in a word without changing any of the other bits. So whether you realize it or not, everytime you use a bit insturction, the program actually compiles to a series of AND, OR, NOT, and XOR instructions. AND, OR, NOT, and XOR do much of the behind the scenes work of almost all of other instructions.

Xor

Another use; compare two numbers that change at a certain point in time. A trimsaw with ten saws with prox movement detection and a timing "window" on an Encoder of 2344 to 2420 can be monitored and an error message sent to the operator if you can see a problem. You xor the valve output word with the prox input word. If they are not the same or xored to equal zero at that point in time, you have a problem. We monitor gates, saws and operator buttons in this fashion. Any I/O word can be compared in this way.

Some examples above are artificial since they may be solved more naturally with CMP (Bruce99), AND (GeoffC).
The main feature of XOR is inverting bit by XORing with 1.
This is very important operation within microprocessor, since inverting word bits is step 1 of creating a binary complement (used for negative value representation).
In user-level program it also may be used for inverting bits.
Actually, XORing a bit with 1 is the simplest way to alter this bit.
Example: a pulse of a single button may alter On/Off or Run/Stop status of a device.

Cmp?

Not all PLCs have a CMP instruction. The poster was asking the applications of XOR in programming, not what is natural or artificial in programming.

randy741985 said:

I would Like someone to give me a useful example of and XOR function in a common plc application.

Click to expand...

Gray code to Binary code conversion....

CRC calculations.

Component error

I've used it with safety related components that have two contacts that should be closed at the same time. It is simple way to detect if one contact is broken. You also need Off delay timer to filter short differances between the contacts.

Does anyone find it discouraging/irritating that randy741985 has not deigned to reply to this thread?

Bruce99,
You made me curious!
Which PLC does not have CMP?