Re: TN3270 on Terminal Servers


Larry Backman (m2c!ulowell!interlan!backman@husc6.harvard.edu)
16 Mar 88 12:36:00 GMT


In article <Mar.12.17.46.18.1988.5479@athos.rutgers.edu> hedrick@athos.rutgers.edu (Charles Hedrick) writes:
>I know at least one vendor who claims to be working on tn3270 mode. I
>think I can tell you why it hasn't been done yet. First, it needs
>more memory and CPU than a normal telnet connection. You have to
>maintain a screen image and various other state information, and you
>have to do the emulation. Most terminal servers currently use a 68000
>with not much memory. But that's not the big problem. The big
>problem is that you need to have access to somthing like termcap for
>the user's terminal. Terminal servers don't have disks, and they

        []

        From my past, I remember implementing a 3270 emulator under
        UNIX using termcaps. It doesn't work! 3270 terminals need
        5 or 6 modes of screen attributes to be effective (normal,
        hilight,reverse,reverse hilight,blinking,underline,etc.).
        Verry few termvcaps entries have these attributes defined
        completely. Now the vendor has to decide... do I want to
        start mucking with termcaps for each and every terminal type I wish to
        support, or... let the user do it themselves. As I remember,
        neither way was a viable solution.

        Yes you could define a limited subset of terrminals that would
        be supported, but that subset grows rapidly based on user feedback.

        As to memory and CPU. The EBCDIC --> ASCII stuff eats some
        cycles, but the byte stream interpretation was no worse than
        any VT220 application. You have to scan for order codes and
        parse some following bytes accoordingly. Each screen image
        needs roughly 4K of memory, 2 K for the image, and 2K for the
        attributes, not a lot of memory these days.

        As with anything else, it can be done, but its a pain. Is it
        worth it? Count the number of PC based 3270 emulators sold;
        multiply by $500. What do you think?

                                                Larry Backman
                                                Micom - Interrlan



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