Happy New Year to all, from Pasadena! I'm the original author of the VH simulator and mostly lurk on this list. I wanted to add my thoughts to a couple of points that have been brought up.
I too am of the opinion that for Unibus machines, the DH/DM combination was outstanding. Great throughput, low system (software) load, extensive functionality, though high space and power consumption. For PDP-11s, this was a great mux. I used /70s and /45s with these and they could handle MANY users easily. Conversely, a DZ could bring any of these to its knees. Even worse, when the VAX-11/780s came out, DZ was IIRC the only supported serial mux and could easily crash the system (VMS V1, V2, certainly; can't remember if V3 crashed) under load. Responsiveness was horrible with DZs under load, even when they didn't cause a crash. Ultimately the DHU came out, but it was quite late, as I recall. Able eventually made a good Unibus device that worked well under VMS. DMZ/32? Can't quite remember. I do remember the DHDM was good. For Qbus, in my opinion the DHQ was the best choice, if your software supported it. The DHV was a good second choice. I put together a brief comparison of the Qbus serial muxes: <http://dundasclan.org/retro/simh/dzdh-compar.html> Regarding the VH simulator specifically, I'll note that the simulator attempts to mimic the DMA (output) capability of the simulated device(s) by filling a packet with the DMA buffer, rather than one packet per character (as a DZ/DZV or similar would). The VH simulator does not attempt to mimic the DH/DM, though it does a reasonable job at the DHU, getting one pretty close. Without referring to documentation, I no longer remember the specific differences, but it shouldn't be that hard to add whatever logic is necessary for the remainder of the DH/DM combination. [Clem, did we meet recently in another context (CENIC/AT&T)?] John _______________________________________________ Simh mailing list Simh@trailing-edge.com http://mailman.trailing-edge.com/mailman/listinfo/simh