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Reference Manual
00809-0100-4696, Rev AA
September 2004
E-9
Rosemount 848L
Hand-Off-Auto The HOA switch has one contact that is closed in the Hand position (input 1)
and one contact that is closed in the Auto position (input 2). Both contacts are
open in the Off position. The local hand controls are Stop (input 3) and Start
(input 4). These require the auxiliary contact on the starter (input 5). Auto
control is done in some DCS function blocks that generate a Run signal which
is linked over an H1 fieldbus to a DO block in the 848L. This is the equivalent
of a toggle switch, so the 848L logic breaks it up into start and stop signals. A
pulse timer is required to extend the rise of the DCS Run signal until the
confirm contact can pull in. An operator will hold the start button in until
something happens. The DCS logic needs to know when the HOA switch is in
the Auto position and also the state of the auxiliary contact, so DI blocks are
configured for them. The ladder diagram looks like this:
| |
| DO1 5 T |
+--|^|-----|/|------------------------( )--+ EQ1
| |
| |
| 1 3 4 Out1 |
+--||-----| |--+--||--+------+--------( )--+ OUT1_EQ
| | | | |
| | 5 | | |
| +--||--+ | |
| | |
| | |
| 2 DO1 T | |
+--||------||--+--||--+------+ |
| | | |
| | 5 | |
| +--||--+ |
| |
First, instantiate two DI blocks and a DO block. Set the DO channel to 9. Set
the Auto DI channel to 2 and the Contactor DI channel to 5. Use appropriate
configuration for the other data in the blocks, such as Tag.
Rung T is equation 1. This is necessary because the Out1 equation has 70
characters, not because it is needed in two or more equations.
• The expression is TP(AND(RISE(DO(1)), NOT(IN(5))),30);
Rung Out1 is output equation 1. The first expression and the second are basic
motor control expressions with an additional selector contact.
• The expression is
OR(AND(IN(1),IN(3),OR(IN(4),IN(5))),AND(IN(2),DO(1),
OR(EQ(4),IN(5))));
To test this, use the usual buttons and relay along with a selector switch and
manual operation of the DO block.