4-4
FIGURE 4.4 FUNCTIONAL DIAGRAM OF SCANNER
The second method for protection, also shown in figure 4.4, is the ‘close gate’ lines to
the decoder circuits. The control circuit will allow these gates to open only if the series
protection line is complete to ground. One of the contacts on each relay is connected in series,
and will complete the series protection circuit only if all the relays on the line are in the clear
position. Thus, no relay on the A line can be closed if any other relay on the A line is already
closed; and similarly no B line relay can be closed if any other relay on the B line is already
closed. These series protection lines are brought out to terminals on the rear panel. Thus if
more than one scanner is used in a large system and these terminals are connected together, all
units in the system will be protected.
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