84
This execution condition is then restored after executing the right-hand in-
struction by using the same TR bit as the operand of a LOAD instruction
Instruction 1
00002
00000
Instruction 2
Diagram B: Corrected Using a TR bit
00001
TR 0
Address Instruction Operands
00000 LD 00000
00001 OUT TR 0
00002 AND 00001
00003 Instruction 1
00004 LD TR 0
00005 AND 00002
00006 Instruction 2
In terms of actual instructions the above diagram would be as follows: The
status of IR 00000 is loaded (a LOAD instruction) to establish the initial ex-
ecution condition. This execution condition is then output using an OUTPUT
instruction to TR 0 to store the execution condition at the branching point.
The execution condition is then ANDed with the status of IR 00001 and in-
struction 1 is executed accordingly. The execution condition that was stored
at the branching point is then re-loaded (a LOAD instruction with TR 0 as the
operand), this is ANDed with the status of IR 00002, and instruction 2 is ex-
ecuted accordingly.
The following example shows an application using two TR bits.
Instruction 1
00003
00000
00002
TR 1
00005
TR 0
00001
00004
Instruction 2
Instruction 3
Instruction 4
Address Instruction Operands
00000 LD 00000
00001 OUT TR 0
00002 AND 00001
00003 OUT TR 1
00004 AND 00002
00005 OUT 00500
00006 LD TR 1
00007 AND 00003
00008 OUT 00501
00009 LD TR 0
00010 AND 00004
00011 OUT 00502
00012 LD TR 0
00013 AND NOT 00005
00014 OUT 00503
In this example, TR 0 and TR 1 are used to store the execution conditions at
the branching points. After executing instruction 1, the execution condition
stored in TR 1 is loaded for an AND with the status IR 00003. The execution
condition stored in TR 0 is loaded twice, the first time for an AND with the
status of IR 00004 and the second time for an AND with the inverse of the
status of IR 00005.
TR bits can be used as many times as required as long as the same TR bit is
not used more than once in the same instruction block. Here, a new instruc-
tion block is begun each time execution returns to the bus bar. If, in a single
instruction block, it is necessary to have more than eight branching points
that require the execution condition be saved, interlocks (which are described
next) must be used.
When drawing a ladder diagram, be careful not to use TR bits unless neces-
sary. Often the number of instructions required for a program can be reduced
Inputting, Modifying, and Checking the Program Section 4-6
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