Knowledge Base:  
Tech Note 113: CMCP420VT(T) Compliance
Last Updated: 05/10/2017

CMCP420VT(T) ComplianceIn General: The output compliance of the CMSS420VT-x has been increased from the original black terminal model such that; a single LED or LCD display and, one external load of 250 Ohms can be used in the same loop when the voltage supplied to the loop is 24Vdc. When the supply voltage is reduced to +22 Vdc, one display and one additional load of 100 Ohms or, one load of 250 Ohms without any display, can be used. The unit will operate with a supply voltage down to +18Vdc with a single 100 Ohm load and no display.

Note: The DC Supply Voltage range is: +18 to +36Vdc (See the application recommendations discussed above and note the limitations for Vs = +18Vdc)

Specific compliance: RLmax for any given loop supply voltage can be calculated from the following equation as follows:

RLmax = (Vs 14 Vd) / .020

(Note: The equation for the old black terminal unit was approximately (Vs - 14 - Vd) / .032)

RL = Load Resistance

Vs = DC Supply Voltage at the +V terminal.

Vd = Display Voltage: 5.0 Vdc for LED, 2.0 Vdc for LCD, 0.0 Vdc for no display.

The recommended RL for most installations is:

Either 100 or 250 Ohms @ Vs = +24 Vdc at the +V terminal, with or without an LED or LCD display.

Use the above RL equation to determine the required minimum supply voltage for a given loop load. Re-arranged to determine supply voltage, that equation becomes Vs = ((RL x .020) +14 +Vd)

Ex: To drive 600 ohms with an LED display in the loop would require a minimum supply voltage of +31.6 Vdc. Vsupply min = (600x.020)+14+5 = 31.6 Vdc.


STI uses 23 mA for certification max current purposes. In practice the unit is limited to 21 mA. Anything over that indicates there is probably a problem with the unit.

The older style (Black terminal) unit cycled its output current between 32mA and some lower value during power up. The latest version (Green Terminal) does not does not do this. It goes to approximately 21.5 mA at power application and then settles to the current reading. This takes a few seconds but it does not latch unless the RL and supply values from the equation are exceeded.

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