[HECnet] DDCMP sync line support now in SIMH

With help from Mark Pizzolato, I added support to SIMH for my DDCMP sync line interface ("DDCMP framer"). This is a USB device, built around a Raspberry Pico, that implements the lower layers of the DDCMP synchronous line protocol. It allows SIMH (or PyDECnet) to talk to real machines that use those interfaces. My ability to test with real hardware is limited; in particular I don't have any coax cable ("integral modem") devices like the DMC-11/AL. But I did test RS232 mode with my Pro 380. The current SIMH on Github includes the necessary code and documentation to allow the use of this device as an emulated DUP, KMC-DUP, or DMC. The current PyDECnet (on my Subversion server) also supports it, and the two will interoperate as you would expect. The design for the device is open source, it can be found on github: https://github.com/pkoning2/ddcmp . At the moment if you want one of these you'll have to build it; I provide the design files but not boards or devices, and I don't plan to. But of course since it's open others may do so if they wish. I'd be very interested in test results of the coax interface since I could only test that against the device itself and against the documentation. paul