Updated 2015-06-25 09:58:50 by SimonK

[Explain. Analyze.]





This appears to be an application which can talk (via IBM's 3270 terminal protocol) to IBM mainframes.

RJ Unable to analyze (no mainframe to test it with), but at the risk of dating myself, I'll explain what little I remember. In the 70s and 80s, the IBM mainframe ruled. They were very proprietary in nature (think of Microsoft marketing strategy - they learned it from IBM) and developed their own protocols for communicating with their computers. It was based on IBM's 327x terminal equipment platform. That platform consisted of a base controller 'serving' up to 32 (256 with SDLC addressing) terminal displays attached to the controller in a star configuration. The base controller provided the network connectivity to the mainframe (usually via a synchronous polled comm link) and it passed information in store-and-forward mode between the addressed terminal and the mainframe host (usually through a Front End Processor (FEP) - a smaller computer collocated with the mainframe whose only job was networking for the host). It was a polled environment, meaning a display terminal (user) can never send until the FEP polls the base controller. So depending on the size of the network and it's configuration, a lot of time can be spent waiting after hitting the enter key. In 3270, only display screen deltas are transmitted and formatting of the screen is done by the base controller following those instructions. So 3270 was one of the first "full screen display" protocols. The EBCDIC character set us used - it contains 256 codes whereas, (unextended) ASCII only uses 128 - and most of those EBCDIC codes are screen formatting instructions in 3270, similar to ASCII escape sequences on VT100 like terminals.

tcl3270 appears to be written to allow an application to emulate an s3278 (monochrome) or s3279 (color) terminal display - complete with graphics capabilities and IND$FILE interfaced file transfer (sort of a mainframe ftp daemon). In order to do so, it establishes a telnet session and pretends to be an IBM 327x controller and display. Please correct any errors here - my memory was the second thing to go.

CL notes that the functionality emphasized to him was that tcl3270 includes Tcl commands for screen-scraping! My question: how can one drive a 3270 withOUT a mainframe? There must be a way ... Through OS/400 trickiness? RJ tcl3270 allows an application (pc, server) to emulate a 327x terminal display for the purpose of obtaining information FROM a mainframe 3270 based application. Screen-scraping. BTW, I am not sure that many of those still exist, but the incredible pre-millenium demand for Cobol programmers to fix the date problem indicates that at least on 1/1/2000, there were lots of them.

AM The remark that IBM were very proprietary is somewhat misleading, IMHO: back then everyone was proprietary. With hindsight you might say that IBM were even more so than the other players, but fact is they were the largest company in this field back then. RJ will stipulate to his long-standing prejudice as concerns Big Blue. He also admits that this same market dominance created standards that simplified his early career in networking.

Category Application [Category communication]