Emerson manual Hand-Off-Auto, Rosemount 848L, Reference Manual

Reference Manual

00809-0100-4696, Rev AA September 2004

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.

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