IBM 3790

IBM 3790 was a computer system developed by the IBM Data Processing Division (DPD) announced in 1975, one of the first distributed computing platforms. It preceded the IBM 8100, announced in 1979.

It was designed to be installed in branch offices, stores, subsidiaries, etc., and to be connected to the central host mainframe, using IBM Systems Network Architecture (SNA).

Components

• The 3791 control unit was offered in two models, Model 1 and Model 2. Both models offered an integrated floppy disk unit and a Synchronous Data Link Control (SDLC) communications interface. Either control unit could optionally include an integrated 120 lines per minute (lpm) line printer, and could attach up to sixteen IBM 3277 display stations, up to three 3292 auxiliary control units, and up to four 3793 keyboard-printers. A 1200 baud internal or external modem was optional. The model 1 supported 8.3MB of disk storage and the model 2 up to 26.9MB.
• The 3792 auxiliary control unit had options for attachment of up to two dial-in IBM 2741 communications terminals, up to four 3793 display stations, and a line printer.
• The 3793 printer-keyboard.

Host software

• Function Support Program.
• Subsystem Support Services.
• User Application Support Program.

Lack of marketing success

The 3790 was never very successful. One reason was that it had an almost incomprehensible programming language, The 3790 Macro Assembler, and the customers who purchased it found it very hard to deploy applications on it. The major drawback with the Macro Assembler was that the customer had to compile on an IBM mainframe and then move the compiled and linked object to the target computer for testing. The 1970s were a decade of turmoil within IBM, the company wanted to expand their reach into the mid-size companies, and the 3790 was designed as a departmental processor. The requirement of needing an IBM mainframe development environment defeated the purpose, and the result was lackluster interest in the product. As with so many other IBM Mini Computers that appeared in the 1970s it was overpriced and under powered.

One of the products IBM released to help developers was Program Validation Services (PVS). With PVS, one could test a program in the mainframe environment using scripts. The scripts were cumbersome to create, and one mistake would result in a worthless run. Back then, mainframe time was expensive and hard to schedule. Very few programmers used PVS for anything other than the crudest of testing.

The manual for the Macro Assembler was about 4 inches thick, and it was almost impossible to find out how to do the simplest thing. Another major issue was code design and size. The hardware architecture loaded code into memory on 2k segments, for optimal execution time it was critical to ensure that processing intensive loops did not cross the segment boundary and incur delays swapping segments in and out of memory.

IBM recognized the problems with the Macro Assembler and created an automated program generator, DMS. DMS later became Cross System Product (CSP) when the 8100 became available. DMS was little more than a screen painter. While field values could be checked (number between 1-100, or character rather than number), there was very little logic that could be automatically generated. The user still had to resort to the Macro Assembler to make the program actually do useful work.

External links

• IBM World Trade Corporation. "IBM 3790 Communications System Description" (PDF). Retrieved Feb 26, 2012.