DTJ Number 3 September 1987 - Digital Technical Journals

Terminal Servers on Ethernet Local Area NetworksFigure 4Communications ServerInternally, the DECserver 100 is radically differentfrom the other two members of theterminal server family, yet still retains the sameexternal characteristics. The DECserver 100 is alow-cost terminal server capable of connectingeight asynchronous ASCII terminals to an Ethernetusing the LAT protocol. This server is a verycompact unit and can be located in a computerroom, a communications closet, or in an officeenvironment. The server has no modem control.Modem control is implemented using an 8-MHzMotorola 68000 chip, with 128KB of RAM , and512 bytes of nonvolatile RAM (NVRAM). Like theEthernet Terminal Server software, the DECserver100 software is down-line loaded from aDECnet load host.Inc. Each PAM interface connects up to eight linecards, each of which is a dual RS232C interfacewith full modem-control capability. The serveralso has a console boot terminator (CBT) modulefor self-test code, bootstrap code, and remoteconsole support. The Ethernet Terminal Serveroffers a user interface similar to that on the DECserver100. Using the LAT protocol, the servercan connect up to 32 terminals (either locally orremotely via modems) to the Ethernet. The EthernetTerminal Server can be located in a computerroom environment or a communicationscloset. The software is always down-line loadedinto the unit from a DECnet load host across theEthernet.Extensions to the OriginalImplementationsThe initial implementations of the LAT protocolwere on the terminal servers described aboveand on VAX/VMS host systems. The serversimplemented only the master end of theLAT protocol, whereas the hosts implementedthe slave end . Follow-on implementationshave added similar support for additionalhost systems: the MicroVMS, RSX- 11M-PLUS,MicroRSX, ULTRIX-32, ULTRIX-32m, TOPS- 10,and TOPS-20 systems.Each system implementation offers access tothe command interpreter as the service accesspoint. Figure 5 illustrates this support.VAX/VMSSYSTEMRSX1 1 -M-PLUSSYSTEMUL TRIX-32SYSTEMTOPS-20SYSTEMMicroVMS MicroRSX TOPS-10SYSTEMSYSTEMSYSTEMIIIIIETHERNETDECserver 100TERMINAL// .\\. .TERMINALSSERVERIITERMINALS\\DIAL-INMODEMSFigure 5Additional LA T Host Support84 Digital TecbnicaljournalNo. 3 September 1986