Terry Woods
Member
- Join Date
- Apr 2002
- Posts
- 3,170
This is a long, double-post explanation of XIO and XIC. It goes through the full derivation of the concept and ultimately ends with a "cut-to-the-chase" short cut. So, if you don't have the patience, go to the end of Post #2. Otherwise, it would be worthwhile to find out what "they" were thinking and how it all came to be as it is.
XIO... XIC... What an incredibly dumb idea that was!
XIO...
When viewed by itself, this acronym is ambiguous...
"Examine If ON"?
"Examine If OFF"?
"Examine if OPEN"?
Only when both of the complimentary acronyms are known, does the meaning become apparent... sort of...
XIC can only mean "Examine If Closed"
Since XIO and XIC are supposed to be opposites...
if XIC means "Examine If Closed" then...
XIO, the opposite of XIC, must mean "Examine If Open".
OK... so the literal meanings of the acronyms, XIO and XIC, are understood.
Now... what is the practical meaning of those acronyms, or, for that matter, what is the practical meaning of the literal translations?
I won't even go into what symbols represent XIO and XIC; it only causes confusion.
Let's just jump into the concept. The concept involves the state of a particular, "imaginary", double-throw relay.
In their infinite wisdom, the originators of the XIO/XIC scheme envisioned the acronyms being applied to the state of a pair of contacts in an imaginary, single-pole, double-throw relay. In this kind of relay, there are two outputs. One output comes from the normally closed contact (--|/|--), the other from the normally open contact (--| |--).
If we assume that the common terminal is always Hot (ON), then, when the relay is OFF, the output from the normally closed contact (--|/|--) is Hot (ON). The output from the normally open contact (--| |--) is Not-Hot (OFF).
When the relay is ON, the ouput from the normally closed contact (--|/|--) is Not-Hot (OFF) and the output from the normally open contact (--| |--) is Hot (ON).
Now... looking at it a bit backwards, it can be seen that, if the normally closed contact (--|/|--) is Hot, then the relay must be OFF. If the normally open contact (--| |--) is Hot, then the relay must be ON.
Ultimately, the purpose in this madness is to determine the state of the signal that controls the relay. That is...
Is the signal to the relay ON, or...
Is the signal to the relay OFF.
So now... how is the state of that signal ascertained?
The scheme says that you can determine the state of the signal to the relay by "examining" the state of the output signals from the contacts.
Now remember,...
...if the relay is OFF...
the normally closed contact (--|/|--) is Hot (ON), and the normally open contact (--| |--) is Not-Hot (OFF).
...if the relay is ON...
the normally closed contact (--|/|--) is Not-Hot (OFF), and the normally open contact (--| |--) is Hot (ON).
It should be immediately apparent that the state of the relay signal can be determined by "seeing" the ON/OFF state of either contact... as long as you know which contact you are looking at.
That is...
...if the normally open contact (--| |--) is Not-Hot (OFF) then the relay signal is OFF.
...if the normally open contact (--| |--) is Hot (ON) then the relay signal is ON.
On the other hand...
...if the normally closed contact (--|/|--) is Hot (ON) then the relay signal is OFF.
...if the normally closed contact (--|/|--) is Not-Hot (OFF) then the relay signal is ON.
Basic Electrical Wiring, using normal relay logic, requires a "hot path" from the left rail (HOT) to the output in order to turn on the output.
.
In the example above, if the relay is OFF then the output from the normally closed contact is Hot (ON); the output will turn ON.
.
In this example, if the relay is ON then the output from the normally open contact is Hot (ON); the output will turn ON.
In the first example, if Indicator Light #1 goes ON then it can do so only because the normally closed contact is Hot (ON). If the normally closed contact is Hot (ON) then that can only be because the relay is OFF.
In the second example, if Indicator Light #2 goes ON then it can do so only because the normally open contact is Hot (ON). If the normally open contact is Hot (ON) then that can only be because the relay is ON.
Indicator Light #1 will go ON only if the relay is OFF. Indicator Light #2 will go ON only if the relay is ON.
That is...
If the relay is OFF, then Indicator Light #1 will be ON and Indicator Light #2 will be OFF.
If the relay is ON, then Indicator Light #1 will be OFF and Indicator Light #2 will be ON.
Now here are a couple of interesting things to notice...
First, in the examples above, the contacts controlling the lights have two different names...
The first is called "Relay N.O. Contact" and the second is called "Relay N.C. Contact".
It should be clear to see that power to the lights is being controlled by two separate entities, namely, a normally open contact and a normally closed contact. Even though these two contacts are within the same relay, they are in fact, two separate signals. Each signal is used to drive a particular output. Each signal "asserts" a particular status. That is, when the signal from the normally closed contact is Hot (ON) then the signal causes Indicator Light #1 to turn ON thus asserting that the relay is OFF. When the signal from the normally open contact is Hot (ON) then the signal causes Indicator Light #2 to turn ON thus asserting that the relay is ON.
Second, since the ultimate aim of the scheme is to determine the state of the relay, is it not apparent that the state of the relay can be determined by monitoring the status of only one of the contacts?
That is, because the results are mutually exclusive... both signals can not be Hot (ON) at the same time.
...if the normally open contact is Hot (ON) then the relay must be ON.
...if the normally open contact is Not-Hot (OFF) then the relay must be OFF.
...if the normally closed contact is Hot (ON) then the relay must be OFF.
...if the normally closed contact is Not-Hot (OFF) then the relay must be ON.
Notice that the state of the relay can be determined by examining the status of only one of the contacts.
If we choose to use the contact that provides the "positive" indication then we would choose the normally open contact. If the normally open contact is Not-Hot (OFF) then the relay is OFF. If the normally open contact is Hot (ON) then the relay is ON. The state of the contact signal corresponds directly to the state of the relay.
We can determine the state of the relay simply by examining the state of the normally open contact.
So, in general terms, we can say if the state of the normally open contact is Hot (ON) then the state of the relay is ON and thus we turn ON the output that indicates that the relay is ON. This is a "positive" assertion; that is, the signal to the relay is ON.
On the other hand, if the state of the normally open contact is Not-Hot (OFF) then the state of the relay is OFF and thus DO NOT turn ON the output that indicates that the relay is ON. This too is a "positive" assertion; except, in this case, the assertion is that the relay is NOT ON, i.e, the relay is OFF.
So....
Let's say that you need to know the state of the relay in order to control something... a motor, a valve, whatever.
If the relay is ON, then turn ON a valve. If the relay is OFF, then turn ON a Light.
In terms of the valve, it is simple to examine the state of the normally open contact.
.
If the normally open contact is Hot (ON), then the relay is ON and so turn ON the Push Valve, otherwise, DO NOT turn ON the Push Valve.
Now... let's say we have a Light that indictaes that the relay is OFF. How do we control the light?
CONTINUED in Post #2
XIO... XIC... What an incredibly dumb idea that was!
XIO...
When viewed by itself, this acronym is ambiguous...
"Examine If ON"?
"Examine If OFF"?
"Examine if OPEN"?
Only when both of the complimentary acronyms are known, does the meaning become apparent... sort of...
XIC can only mean "Examine If Closed"
Since XIO and XIC are supposed to be opposites...
if XIC means "Examine If Closed" then...
XIO, the opposite of XIC, must mean "Examine If Open".
OK... so the literal meanings of the acronyms, XIO and XIC, are understood.
Now... what is the practical meaning of those acronyms, or, for that matter, what is the practical meaning of the literal translations?
I won't even go into what symbols represent XIO and XIC; it only causes confusion.
Let's just jump into the concept. The concept involves the state of a particular, "imaginary", double-throw relay.
In their infinite wisdom, the originators of the XIO/XIC scheme envisioned the acronyms being applied to the state of a pair of contacts in an imaginary, single-pole, double-throw relay. In this kind of relay, there are two outputs. One output comes from the normally closed contact (--|/|--), the other from the normally open contact (--| |--).
If we assume that the common terminal is always Hot (ON), then, when the relay is OFF, the output from the normally closed contact (--|/|--) is Hot (ON). The output from the normally open contact (--| |--) is Not-Hot (OFF).
When the relay is ON, the ouput from the normally closed contact (--|/|--) is Not-Hot (OFF) and the output from the normally open contact (--| |--) is Hot (ON).
Now... looking at it a bit backwards, it can be seen that, if the normally closed contact (--|/|--) is Hot, then the relay must be OFF. If the normally open contact (--| |--) is Hot, then the relay must be ON.
Ultimately, the purpose in this madness is to determine the state of the signal that controls the relay. That is...
Is the signal to the relay ON, or...
Is the signal to the relay OFF.
So now... how is the state of that signal ascertained?
The scheme says that you can determine the state of the signal to the relay by "examining" the state of the output signals from the contacts.
Now remember,...
...if the relay is OFF...
the normally closed contact (--|/|--) is Hot (ON), and the normally open contact (--| |--) is Not-Hot (OFF).
...if the relay is ON...
the normally closed contact (--|/|--) is Not-Hot (OFF), and the normally open contact (--| |--) is Hot (ON).
It should be immediately apparent that the state of the relay signal can be determined by "seeing" the ON/OFF state of either contact... as long as you know which contact you are looking at.
That is...
...if the normally open contact (--| |--) is Not-Hot (OFF) then the relay signal is OFF.
...if the normally open contact (--| |--) is Hot (ON) then the relay signal is ON.
On the other hand...
...if the normally closed contact (--|/|--) is Hot (ON) then the relay signal is OFF.
...if the normally closed contact (--|/|--) is Not-Hot (OFF) then the relay signal is ON.
Basic Electrical Wiring, using normal relay logic, requires a "hot path" from the left rail (HOT) to the output in order to turn on the output.
Relay
N.C. <-- "Normally Closed"
Contact
--|/|-------( ) Indicator Light #1 (Relay is OFF)
.
In the example above, if the relay is OFF then the output from the normally closed contact is Hot (ON); the output will turn ON.
Relay
N.O. <-- "Normally Open"
Contact
--| |-------( ) Indicator Light #2 (Relay is ON)
.
In this example, if the relay is ON then the output from the normally open contact is Hot (ON); the output will turn ON.
In the first example, if Indicator Light #1 goes ON then it can do so only because the normally closed contact is Hot (ON). If the normally closed contact is Hot (ON) then that can only be because the relay is OFF.
In the second example, if Indicator Light #2 goes ON then it can do so only because the normally open contact is Hot (ON). If the normally open contact is Hot (ON) then that can only be because the relay is ON.
Indicator Light #1 will go ON only if the relay is OFF. Indicator Light #2 will go ON only if the relay is ON.
That is...
If the relay is OFF, then Indicator Light #1 will be ON and Indicator Light #2 will be OFF.
If the relay is ON, then Indicator Light #1 will be OFF and Indicator Light #2 will be ON.
Now here are a couple of interesting things to notice...
First, in the examples above, the contacts controlling the lights have two different names...
The first is called "Relay N.O. Contact" and the second is called "Relay N.C. Contact".
It should be clear to see that power to the lights is being controlled by two separate entities, namely, a normally open contact and a normally closed contact. Even though these two contacts are within the same relay, they are in fact, two separate signals. Each signal is used to drive a particular output. Each signal "asserts" a particular status. That is, when the signal from the normally closed contact is Hot (ON) then the signal causes Indicator Light #1 to turn ON thus asserting that the relay is OFF. When the signal from the normally open contact is Hot (ON) then the signal causes Indicator Light #2 to turn ON thus asserting that the relay is ON.
Second, since the ultimate aim of the scheme is to determine the state of the relay, is it not apparent that the state of the relay can be determined by monitoring the status of only one of the contacts?
That is, because the results are mutually exclusive... both signals can not be Hot (ON) at the same time.
...if the normally open contact is Hot (ON) then the relay must be ON.
...if the normally open contact is Not-Hot (OFF) then the relay must be OFF.
...if the normally closed contact is Hot (ON) then the relay must be OFF.
...if the normally closed contact is Not-Hot (OFF) then the relay must be ON.
Notice that the state of the relay can be determined by examining the status of only one of the contacts.
If we choose to use the contact that provides the "positive" indication then we would choose the normally open contact. If the normally open contact is Not-Hot (OFF) then the relay is OFF. If the normally open contact is Hot (ON) then the relay is ON. The state of the contact signal corresponds directly to the state of the relay.
We can determine the state of the relay simply by examining the state of the normally open contact.
So, in general terms, we can say if the state of the normally open contact is Hot (ON) then the state of the relay is ON and thus we turn ON the output that indicates that the relay is ON. This is a "positive" assertion; that is, the signal to the relay is ON.
On the other hand, if the state of the normally open contact is Not-Hot (OFF) then the state of the relay is OFF and thus DO NOT turn ON the output that indicates that the relay is ON. This too is a "positive" assertion; except, in this case, the assertion is that the relay is NOT ON, i.e, the relay is OFF.
So....
Let's say that you need to know the state of the relay in order to control something... a motor, a valve, whatever.
If the relay is ON, then turn ON a valve. If the relay is OFF, then turn ON a Light.
In terms of the valve, it is simple to examine the state of the normally open contact.
Relay
N.O. <-- "Normally Open"
Contact
--| |-------( ) Push Valve
.
If the normally open contact is Hot (ON), then the relay is ON and so turn ON the Push Valve, otherwise, DO NOT turn ON the Push Valve.
Now... let's say we have a Light that indictaes that the relay is OFF. How do we control the light?
CONTINUED in Post #2