two issues. first DEC subsetted the modem control lines so running modems - particularly when you wanted hardware flow control like the trailblazers - did not work. second as others pointed out there the buffering was mostly lacking and the interrupt load was terrible on the OS if DEC had used different and better USARTs this would not have been as bad. When it came out there were other options for the HW guys but IMO it was an example of tunnel vision - plus HW guys not really understanding the SW implications of the choice. one of the guys behind the DZ would later become a good friend of mine. I realized Dave never looked at RS-232 the same way SW people did. To Dave the DZ was great because it was two boards to do what he thought was the same thing as a DH - ie cheaper / less power / smaller etc. But the saving was a HW one and not what we needed. that said DEC sold a lot of them on VMS systems. it was us UNIX guys that switched to Able's board as soon as we could. Cheaper and better functionality for the OS. ken once told me he always loved us Unix guys because we got it. he also listened too us and built what we needed when he could. I tried to get him to put the DH/DM on a AT form factor but he said he could not make money at it. Eventually the rocket port guys built and ASIC and pretty much did it Clem