Simplex 2001 Nac card

[Retired_STR-SG (Brad Corey)]

@Owenmullen

I don’t understand this

1. It reads the voltage as 25.8 in alarm and Normal
2. The trouble led goes off when a 10k resistor is attached but the voltage drops to 16.1
3. I cannot tell if polarity changes do I need a certain multimeter?
4. I also got a march time card and I want to know how to get that to work and also cont. coding
5. And the card in the photo is the one I am using

OK, let’s take this one at a time.

1. It seems to me that the relays on the NAC module are not turning on. You should see the voltage and polarity change when the system changes from normal to alarm condition. That is the first item we have to figure out. Look on the NAC module for jumper wires or 0 ohm resistors being installed or missing between solder pads 15 to 16 and 29 to 30 on the NAC module. In an unmodified system those jumpers should be there. (see picture) Also check the 556-880 control module to see if diode D25 is in or cut out. It will be there in a standard system and removed in a modified system. If these components are missing it indicates the system is modified. (Sorry no picture but D25 is a large diode near the PC edge connector.) One of the modifications that requires these components to be removed is adding a march time module.
   https://cdn.thefirepanel.com/legacy/3047_db3a952c246f2d4e2a113ca81d24002b.jpg

2. The voltage across a NAC will be lower than the power supply voltage when the circuit is in normal condition. The supervision circuit has additional resistors that are in series with the EOLR to make a voltage divider circuit. Other components on the card use these voltage differences to determine if the field wiring is normal, open, or shorted. The 2001 NAC modules were designed for a 15K ohm EOLR, but a 10K will probably work. When a NAC is in alarm the supervision circuit is disconnected and the signal power supply is connected to the circuit in the polarity that allows the horns to sound.

3. I don’t know what meter you are using but all will indicate a polarity reversal. On an analog meter the pointer needle will go in the opposite direction when the leads have not been moved but the polarity has changed. A digital meter will display a - in front of the digits to indicate negative polarity.

4. Let’s hold off on the march time module until we get the basic panel situation figured out. Adding the MTM will take adding some wires and modifying the control and signal modules.

5. I figured the 2001-2076 (556-874) was the NAC module supplied with the system. Most systems of this type were supplied with that card.

Here are some basic diagrams on how a NAC works.
The first diagram is when the system is in normal standby condition. The field wiring is connected to the supervision circuit that checks for the wiring integrity. This is the circuit that lights the card TROUBLE LED and sends a trouble signal to the CONTROL MODULE.
Notice that each horn contains a diode. When the circuit is in normal condition these diodes are reverse biased so the supervision current cannot flow through the horns so has to flow through the EOLR.

This diagram is what happens when the system goes into alarm. The field wiring is connected to the signal power supply in a polarity that allows the horns to sound.
In this diagram the polarity is reversed and the diodes in the horns are forward biased. This allows the signal power current to flow through the horns so they make noise.