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Detailed Description of Converting Dry Contacts to Analog Signals

Supplemental Information For Tank Farm Application

Before converting a dry contact to an analog signal it is important to first determine what signal levels you will need.  Since the TMZ accepts a 4-20mA signal then we can choose any current values within 4mA and 20mA.  For this application we chose 'NOT FULL' to be equal to 20mA, and 'NEAR FULL' to be represented at 4mA. 

These two current levels will now indicate that the tank is ‘NEAR FULL’ or ‘NOT FULL’. The ‘NEAR FULL’ signal lets the DCS operator know that the tank should not be filled any further while the ‘NOT FULL’ indicates that the tank needs additional filling.

Using resistors in conjunction with a 24Vdc power supply, based on Ohm's law, allows a discrete or relay signal to be converted to an analog signal. Using ohm’s law the two resistors chosen for the desired analog signal levels were 1.5K Ohms and 6K ohms.  If the tank level is low, then the circuit includes both resistor values thus creating a 20mA signal to the TMZ. This 20 mA reports ‘NOT FULL’. 


(24V/1500Ω) + (24V/6000Ω)

16mA + 4mA = 20mA

Otherwise, when the contact switches only the 6K ohm resistor is in the circuit and a current signal of ~4mA indicating ‘NEAR FULL’ (this occurs when the dry contact switch is open).

These two signal levels 4mA and 20mA will be converted to a logical “1” and “0” for MODBUS transmission.  Those TMZ specific programming details will be covered later.

Wiring Details

The bulk 24dc power supply, two resistors and contact terminals on the level transmitter need to be wired as a current input to the TMZ utilizing +I and COM terminals (See Figure 1).  

Figure 1. Top of TMZ highlighting connections


This wiring configuration creates a “failsafe” situation.  For example, should the level switch accidentally fail to open, or the 24Vdc become short circuited, then that tank is reported ‘NEAR FULL’ (4mA signal). A detailed wiring schematic can be found in Figure 2.

Figure 2. Tank Wiring and Setup

TMZHLFace3 400

  1. The float cage signal is converted using resisters (6000Ω and 1500Ω)and a regulated bulk 24Vdc power supply, connected to the TMZ, which is then connected to the RS-485 port (MODBUS) of the new WNM.  
  2. The TMZ converts the analog signal to a MODBUS output that is connected to the WNM's RS-485 port.
  3.  The WMN, also powered by the existing bulk24Vdc power supply, transmits signals wirelessly from the new tanks across the river to theWNM radio network connected to the DCS’ MODBUS network.

Details on Configuring the TMZ Transmitter to Communicate Data via MODBUS

Use the Model TMZ programming cable and free programming software to configure the Model TMZ for each tank. Assign a MODBUS address to this storage tank TMZ. Configure the baud rate, parity, etc. to match that of all other MODBUS devices.

Configure the Model TMZ ‘Custom Curve’ feature such that current less than 5mA is ‘0.0’ (‘NOT FULL’) and current above 19mA is ‘1.0’ (‘NEAR FULL’).  

(Note: This was chosen as the customer preferred the valve state for filling the tank to be 0 for open, and 1 for closed).  

For example, a simple four point custom curve like the following will work:

Figure 3. Four Point Custom Curve

Point      X Data   Y Data   
 3 19 
 4 20 

WNM Configuration and MODBUS Network Wiring

Configure the Model WNMs as point-to-point radios and choose all other settings as required.  Moore Industries Application support pre-configures the WNMs prior to shipment from the Moore Industries factory, making the WNMs ‘ready-to-use’ upon arrival.   

Wire the Model TMZ and WNMs to the twisted pair MODBUS network.  See TMZ and WNM instruction manuals for RS-485 wiring diagram. 

Have questions, or want help with your application?

To learn more about this application, read the Tank Farm Application and Monitoring Solution page.

If you have questions or need help on your own application, send us a message using E-Help.

If you want product information, visit the catalog.


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