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Reference Manual
00809-0100-4696, Rev AA
September 2004
E-15
Rosemount 848L
Rung L1 is equation 1. It latches on when the start button is true and neither
motor is running. It unlatches when the NC stop button is pushed or the
pressure stays low for too long.
• The expression is RS(AND(NOT(IN(3)),
NOT(IN(4)),IN(2)),OR(NOT(IN(1)),EQ(3));
Rung P is equation 2, which generates a 2 second start pulse from the rise of
the run latch or the rise of the low pressure condition. This pulse toggles the
latch and starts the selected motor.
• The expression is TP(OR(RISE(DO(1)),RISE(EQ(1))),20);
Rung T is equation 3, which is a TON that is run by the on state of the low
pressure and the run latch.
• The expression is TON(AND(DO(1),EQ(1)),100);
Rung L2 is equation 4. The latch determines which motor to start and run. It
toggles when equation 2 generates a pulse. The pulse duration is more than
one evaluation cycle, so rise functions are required.
• The expression is
RS(AND(RISE(EQ(2)),NOT(EQ(4))),AND(RISE(EQ(2)),EQ(4)));
Rung Out1 is output equation 1. This is basic alternate motor control.
• The expression is
AND(EQ(1),OR(EQ(2),IN(3)),NOT(EQ(4)),NOT(OUT(2)));
Rung Out2 is output equation 2. This is also basic alternate motor control.
• The expression is AND(EQ(1),OR(EQ(2),IN(4)),EQ(4),NOT(OUT(1)));