The coax port was used for connecting to communication controllers attached to a mainframe, to support 3270 terminal sessions. These combination behaved like a true hardware terminal.Īpple once built a combined coax/twinax card. IBM and other manufacturers built ISA cards and provided software for MS-DOS and early Windows versions for accessing AS/400 terminal sessions over twinax. Today, the twinax attachment is still a necessity with old hardware, since it provides console services, for OS installation and running backups without any users keeping files open. Bigger machines could have multiple twinax controllers inside to support hundreds of terminals simultaneously. There were controllers available with supported eight-port bricks as well. Transmission rate was 1 Mbit/s per port, providing almost instant screen updates. Because the terminals ran a subset of SNA, they could be daisy chained, easing cabling requirements somewhat. The whole system was expensive, complicated to run through a building (because of a large minimum bending radius) but rock solid.Īn AS/400 could utilize at least one twinax controller (board) with at least four independent ports for daisy chaining terminals. Terminals were attached through Twinaxial Cables, thick as a small finger with bulky connectors. They're not quite as dumb as, say, a VT100. These terminals included a basic SNA stack and coded logic for local processing of input and screen appearance. Today, they're called dumb which is plainly wrong. Rarely accompanied by the possibility to connect a PS/2 mouse. Historically, all access to the machine involved terminal sessions which were established with, well, terminals: A combination of a CRT display, a controller chip and a keyboard. Such stuff as incremental search as one types a search string while the application narrows possible matches with every additional character from that string is not possible in block oriented mode. The user can only do what the protocol permits locally. On the other hand, block-oriented transmissions have drawbacks in interactivity. Imagine hundreds of users typing data into fields of forms and thus interrupting the CPU for simple cursor movement and character-echoing. Every single keypress will go back to the host, trigger an action, based on which appropriate data will be sent back to the terminal for updating on-screen content.īlock oriented protocols like 5250 (and 3270) were useful when computing resources were scarce. This is in stark contrast to terminals in the Unix world, where the host is handling communications and screen appearance itself. This is quite like a HTML form being displayed and filled out in a browser window, getting also transferred back in its entirety by pressing the submit-button. When everything is finished, a special key is pressed and the form is transmitted in its entirety back to the host for processing. Changes are only made locally, in the memory of the terminal or emulator. However, a huge difference between those is that with the former, the user is presented with a form, to look at and/or fill out. 1 Block- versus character-oriented transmissionsīlock- versus character-oriented transmissions.
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