On page 268 of NET+50/20M Hardware Reference I read: “Important: The NET+50 chip does not support HDLC DMA”. But it looks like the HDLC driver in Net+OS uses DMA. Surely, something is wrong here. Does it work or not? To use DMA for HDLC or not to use DMA for HDLC? That is the Hamlet question. 
I received a reply from Ray Henningson, NetSilicon Support Center: “…apparently there is error in the documentation. The Net50 supports DMA for HDLC. According to the SW people, it is the only way we support HDLC. The statement in the manual is an error.” Thank you, Ray!