201
Controlling Bit Status
Section 4-4
4-4-2
DIFFERENTIATE UP and DIFFERENTIATE DOWN
DIFFERENTIATE UP and DIFFERENTIATE DOWN instructions are used to
turn the operand bit ON for one cycle at a time. The DIFFERENTIATE UP
instruction turns ON the operand bit for one cycle after the execution condition
for it goes from OFF to ON; the DIFFERENTIATE DOWN instruction turns ON
the operand bit for one cycle after the execution condition for it goes from ON
to OFF. Both of these instructions require only one line of mnemonic code.
Here, IR 01000 will be turned ON for one cycle after IR 00000 goes ON. The
next time DIFU(13) 01000 is executed, IR 01000 will be turned OFF, regard-
less of the status of IR 00000. With the DIFFERENTIATE DOWN instruction,
IR 01001 will be turned ON for one cycle after IR 00001 goes OFF (IR 01001
will be kept OFF until then), and will be turned OFF the next time DIFD(14)
01001 is executed.
4-4-3
KEEP
The KEEP instruction is used to maintain the status of the operand bit based
on two execution conditions. To do this, the KEEP instruction is connected to
two instruction lines. When the execution condition at the end of the first
instruction line is ON, the operand bit of the KEEP instruction is turned ON.
When the execution condition at the end of the second instruction line is ON,
the operand bit of the KEEP instruction is turned OFF. The operand bit for the
KEEP instruction will maintain its ON or OFF status even if it is located in an
interlocked section of the diagram.
In the following example, HR 0000 will be turned ON when IR 00002 is ON
and IR 00003 is OFF. HR 0000 will then remain ON until either IR 00004 or IR
00005 turns ON. With KEEP, as with all instructions requiring more than one
instruction line, the instruction lines are coded first before the instruction that
they control.
4-4-4
Self-maintaining Bits (Seal)
Although the KEEP instruction can be used to create self-maintaining bits, it is
sometimes necessary to create self-maintaining bits in another way so that
they can be turned OFF when in an interlocked section of a program.
To create a self-maintaining bit, the operand bit of an OUTPUT instruction is
used as a condition for the same OUTPUT instruction in an OR setup so that
the operand bit of the OUTPUT instruction will remain ON or OFF until
00000
00001
DIFU(13) 01000
DIFD(14) 01001
Address
Instruction
Operands
00000
LD
00000
00001
DIFU(13)
01000
Address
Instruction
Operands
00000
LD
00001
00001
DIFD(14)
01001
00002
00004
00003
00005
R: reset input
S: set input
KEEP(11)
HR 0000
Address
Instruction
Operands
00000
LD
00002
00001
AND NOT
00003
00002
LD
00004
00003
OR
00005
00004
KEEP(11)
HR
0000
