The firmware in the SL1600 will only recognize 16 zones. It will only accept the SLM4 module. The SL1624 controller has firmware that will recognize up to 24 zones and it will accept the SLM12 and the SLM4 module. To create 24 zones on an SL1624, remove the SLM4 module that came with the unit and install a second SLM12.Check the firmware Version number to verify that the firmware is 1.08 or higher. If the firmware version is 1.05 then this model of SL1600 will only accept and recognize the SLM4.
To check the firmware version
1) Turn the dial to the Advanced Functions Positions.
2) Press the UP or DOWN Adjust Value button until ABOUT shows in the display.
3) Press the NEXT button and the firmware version number will be displayed.