On Dec 13, 2012, at 2:29 PM, John Wilson wrote: From: "Rob Jarratt" <robert.jarratt at ntlworld.com> No. It uses a TCP connection. Each buffer received from the device driver is sent along on the TCP connection with a 2-byte length prefix, and that is it. So no DDCMP underneath. OK cool. Little-endian? [Paul Koning] If you want to really make your head spin, imagine DMP/DMV emulation (DDCMP multipoint). I've never really understood this. Is it a broadcast net, or what? I'm pretty sure I have a pair of DMVs in this mess somewhere... For the gory details, read the DDCMP spec. It's gory, all right. Physically is's a broadcast net. Logically there is a single master and N-1 slaves. The master polls each of the slaves. The master can send at any time; the slave can send only in response to a poll. I think in theory it can be full duplex, I'm not sure if in practice that made any real difference. Performance sucks, but if you needed lots of connected stations without using up lots of hardware on the PDP11 side, multipoint is an answer. Ethernet, of course, is vastly superior, but that came later. It could be worse. I once spent some weeks debugging a communication problem between a PDP11 and some Harris 2200 terminals. Those used multipoint also, but Bisync, not DDCMP. And the interface was a DL11-E. So: half duplex bisync multipoint over a 9600 baud async line. I think it needed the -E variant because the modem control signals were used for poll control. Yikes. paul