March 04, 2016
3000-L breaks silence with DTC primer
On the verge of four weeks without a new message, the 3000-L mailing list and newsgroup delivered a primer on the control of DTCs on MPE/iX networks. A longtime contributor to the community set up a question about using these venerable devices that connected HP's terminals and other devices to a 3000.
I have multiple HP 3000s sharing one DTC. My problem is, which one controls the DTC? In the event of a power cycle, there is a race between them for which 3000 will download the new configuration. I need system A, my A-Class, to be in charge. However, systems B or C are most likely to download first, leaving me with the manual step of unplugging the network from B and C, power-cycling the DTC, and then waiting for the A-Class to download the configuration. Is there anything that can be done to just leave the A-Class in charge?
Tracy Johnson replied, "It's always been a crap shoot. I'm of the opinion the first HP 3000 to notice the DTC needs downloading will do the job. Which usually means the less busy machine." As it was just the first answer on the newsgroup, there was still a need to do unplugging.
"For example," he added, "if you have a test machine on the side nobody's using, I would simply unplug the test machine's network cable until the DTC was reset. Since there will be no other machine in control, the regular machine will do the download. When complete, I'd plug the network cable back into the test machine. However if both machines are in production and you have users active on both, you'll have to make a decision."
Lalley came back with an answer of his own, at least to eliminate the travel from chair to server.
"On the DTC's I guess it could be possible to do it without leaving your chair, if you TELNET to the console on B and CDTCCNTRL (has to be run from the physical console) to stop the DTC subsystem, then restart it."
Mark Ranft at Pro3k had another idea.
Back in the good old days, we used DTC Switching - a feature that allowed a DTC prompt to appear at a terminal. The user entered 'C HPA' to connect to host HPA. I believe you could set a default system. Initially DTC Switching was only available if you configured and downloaded the DTC using OpenView DTC Manager software running on a PC. Later HP set up NMMGR to allow DTC Switching.
The Communicator for that release may have more details, but I only entered the MAC of the DTC on the one system that was supposed to download the DTC. The other systems would have the DTC name, but not the MAC. This allowed the system to have printers and other devices accessible on the DTC without concerns about which one downloaded the configuration.
Of course, the location of that MPE Communicator took all of 41 minutes to dig up. Barry Lake pointed at HP documentation (now hosted outside HP's baffling website) that covers Enhanced Host-Based DTC Management Functionality in Chapter 3. Plus, a manual on Configuring and Managing Host-Based X.25 Links.
Not too shabby for a mailing list that's more than 20 years old, but still manned by dozens of experts. It's the kind of expertise a good third party support provider offers—as usual, better than HP's today.
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