NetLinx Programming Overview
9
NetLinx Programming Language Reference Guide
Axcess/NetLinx Incompatibility
According to previous versions of each of their language reference manuals, Axcess and NetLinx each
give the operator
NOT highest precedence while giving AND and OR lowest. As demonstrated in the
following code, however, the two systems behave differently. In reality, Axcess gives the operator
NOT
lowest precedence.
DEFINE_VARIABLE
C D E
DEFINE_CALL 'GO' (A,B)
{
C = !A && B
D = B && !A
E = !B && !A
}
DEFINE_PROGRAM
PUSH[1,1]
CALL 'GO' (0,0)
PUSH[1,2]
CALL 'GO' (1,0)
PUSH[1,3]
CALL 'GO' (0,1)
PUSH[1,4]
CALL 'GO' (1,1)
Axcess RESULTS
A B !A && B B && !A !B && !A
0 0 1 0 1
1 0 1 0 1
0 1 1 1 0
1 1 0 0 1
NETLINX RESULTS
A B !A && B B && !A !B && !A
0 0 0 0 1
1 0 0 0 0
0 1 1 1 0
1 1 0 0 0
The problem applies whether A and B are channels, variables, or expressions, and for OR as well as AND.
To solve the problem, AMX always recommends the use of
(!A) && B instead of !A && B; however,
and this is critical, some programs out there are taking advantage of the logic flaw. Where the Axcess
programmer intended the truth table of
!(A && B) he/she may have coded !A && B and gotten the
desired result. If these systems are converted to NetLinx Masters, the logic will not work as desired.
Please be aware of this difference as you support programs being converted from Axcess to NetLinx.
When it occurs, Axcess-like operation can generally be achieved by including all the conditions to the
right of the
NOT in a single set of parentheses. For example:
IF (SYSTEM_POWER && ![VCR,PLAY] || [VCR,RECORD])
becomes:
IF (SYSTEM_POWER && !([VCR,PLAY] || [VCR,RECORD]))
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