IBM 3770 SNA/RJE Communications Introduction

Introduction to IBM 3770 SNA/RJE Communications


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Introduction to IBM 3770 SNA/RJE Communications

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Back in the 1970s, IBM manufactured a family of keyboard/printer remote job entry (RJE) data communications terminals that included models designated the IBM 3776 and 3777 Communications Terminals. (Less sophisticated 3771 through 3775 terminals were also manufactured.) This entire family of terminals is generally referred to as simply 3770 terminals.

The 3776/3777 terminals were batch or RJE data terminals which used punched cards and typically were configured with a card reader, one or two diskette drives, a card punch, and a line printer. They used IBM's System Network Architecture (SNA) to transmit and receive data to and from an IBM mainframe computer. A terminal usually connected over dial-up or leased lines at line speeds from 2400 bits per second (bps) using Bell 201C modems or at 4800 bps using Bell 208B modems. Some models supported high speed connections of up to 20,400 bps. Today, most 3770 emulation occurs at 9600 bps or faster using V.32 and V.34 modems, or are connected by local area networks to the mainframe.

The RJE subsystems on the IBM mainframes are known as MVS/JES2, VS1/RES/RTAM, and DOS/VS/POWER.

Line drawing of an IBM RJE Terminal. Click for a larger view.
The RJE terminals that originally made up the 3770 family consisted of the following:
  • the 3776 terminal, Models 1, 2, 3, and 4, were medium speed RJE terminals with a line printer of up to 400 lines per minute (lpm) and connection speeds up to 4800 bps.
  • the 3777 terminal, Models 1 and 2, were medium-to-high speed RJE terminals with a line printer of up to 1200 lpm, optional high-speed card reader, and connection speeds up to 20,400 bps.
  • the 3777 terminal, Models 3 and 4, were medium-to-high speed RJE terminals with a line printer of up to 1200 lpm, optional high-speed card reader, and connection speeds up to 20,400 bps; the terminal utililzed the multiple logical unit (MLU) protocol of SNA that permitted the interleaving of up to six independent data streams.
While the 3776/3777 terminals themselves may not have survived through the ages, the mainframe has. Even today, in the name of "backward compatibility", the most modern IBM mainframe computers still offer support for SNA/RJE connectivity. Why? Because SNA/RJE has remained a cost effective way to transmit files to and from mainframe systems, and particularly to distribute line printer output to locations physically removed from the mainframe site.

Today's SNA/RJE communications is provided almost exclusively by emulating the original 3776/3777 terminals. Emulation is performed by software programs on all classes of machines -- from mainframes themselves all the way down to PCs.
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