If you are using the CXN/851N in digital 'pre-amp' mode in conjunction with the CXA-60/80, then the CXN/851N’s audio output may be too low for the CXA’s APD (auto power down) detection to recognise the audio signal, causing the amplifier to switch itself off after 30 minutes. You can disable the Auto Power Down function (APD) to avoid this happening. A guide to disabling APD is located here.
If you are using an integrated amplifier such as the CXA60/80, then instead of using digital pre-amp mode on the CXN/851N, you may use the control bus function to control the volume of the amplifier. More information regarding control bus is located here.