The IBM 7171 ASCII Device Attachment Control Unit

The IBM 7171 ASCII Device Attachment Control Unit supports a variety of ASCII terminals, printers and plotters. It provides a full duplex asynchronous ASCII terminal interface, with electrical connection via EIA RS-232-C. Flexible communications attachment options allow devices to be connected directly to the IBM 7171 without modems, or via leased and switched lines using line drivers, modems- or acoustic couplers. Data rates may be from 50 to 19,200 bits per second, depending on the capabilities of the communication link and the terminal. Auto-matic baud recognition (autobaud) is selectable on a per-line basis for 300, 600,

2-2 IBM 7171 Reference Manual and Programming Guide

1200, 1800, 2400, 3600, 4800, 9600 and 19200 baud upon receipt of a single car-riage return (CR) character.

The host processor may be an IBM 43xx, 308x, or 3090 running a VM/SP or MVS operating system. The IBM 7171 attaches to the host via a block multiplexer channel and communicates with the host processor channel program.

The IBM 7171 provides support for ASCII terminals, such as the IBM 3101, for use with interactive host applications running under VM/SP or MVS. It emulates a locally attached 3274-10 control unit with IBM 3270 terminals attached. It allows users of ASCII terminals to communicate with standard IBM host interactive pack-ages and editors operating under VM/SP or MVS. This emulation of 3270 termi-nals gives ASCII terminal users access to a wide range of host application programs with no host modifications required.

In addition to full screen ASCII terminal support, a transparent interface is pro-vided which allows connection of devices that are not full screen by nature, such as printers and plotters.

The main functional components of the IBM 7171 are the CPU board, the channel controller board, the channel adapter board and up to eight terminal controller boards. Figure 2-2 shows the interfaces between the hardware components. The channel adapter board interfaces with the host processor channel. Each terminal controller controls up to eight ASCII terminals.

When the IBM 7171 power is turned on, microcoded diagnostics verify that the hardware system is operational. A description of the Diagnostics may be found in Chapter 7, "IBM 7171 Diagnostics."

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Figure 2-2. Functional Hardware Components within the IBM 7171

Chapter 2. System Description and Operation

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2.2.2 Terminal Management

Terminal management within the IBM 7171 for the emulated 3270 terminals is organized around a full screen image buffer, a ring buffer for keyboard input, a buffer for output to the terminal, and Terminal Definition Tables (TDT's) main-tained in IBM 7171 main memory for each ASCII terminal managed by the IBM 7171 Control Microcode.

The keyboard input routine interprets characters received from each ASCII ter-minal according to its associated Terter-minal Definition Table, and separates them into text, local editing function and "attention" functions.

Text characters received from the terminal are entered directly into the screen buffer unless they would alter a protected field. In that case, the terminal's audible alarm sounds and the character is ignored. Local editing functions alter the screen buffer and/or cursor position buffer as appropriate. "Attention" generating func-tions are passed to the host and do not alter the screen buffer.

The output process is invoked whenever changes are made to a terminal's screen image in IBM 7171 main memory by keyboard input or by output from a user'~

host application. The terminal screen is updated to match the screen image in IBM 7171 main memory.

2.2.3 Memory Organization of the CPU Board

The main memory of the CPU board contains the Control Microcode and the sup-plied Terminal Definition Tables in Read-Only Memory (ROM). A part of memory in Non-Volatile RAM, referred to in this document as NV-RAM, is reserved for the user to add his own Terminal Definition Tables. The integrity of the contents of Non-Volatile RAM is maintained when the IBM 7171 power is off.

The memory of the CPU board, which consists of ROM, NV -RAM, and RAM, is organized as shown below.

ROM

• IBM 7171 Control Microcode Diagnostics

Supplied Terminal Definition Tables for several ASCII device types

• EBCDIC/ASCII translation tables.

Non-Volatile RAM (NV-RAM)

Pointers to Terminal Definition Tables (in ROM and NV -RAM) Area reserved for user generated Terminal Definition Tables RAM

• Screen images for up to 64 active terminals

• Work area for IBM 7171 Control Microcode Host and terminal input/output buffers.

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2.2.4 Asynchronous Operation

2.2.5 Error Recovery

The terminal input, host input/output, and terminal output processes are coordi-nated, but operate asynchronously. Also, the user or the host application can gen-erate new screen images more quickly than the terminal is able to display them (for example, by rapid, consecutive page forward requests). The changes occur almost immediately in the screen buffer, but the output process is paced by the terminal line speed. Thus the user may see only the last version of the screen buffer and may not have to wait for all intermediate screens to be displayed.

Recovery sequences are automatically initiated whenever a transmission or data error is detected. If a transmission error is detected on input data (incorrect parity, framing error or overrun), or when the input ring buffer is full, the terminal is marked in an input-error state. Once marked, every character received is ignored and the ASCII BEL character is sent to the terminal causing a "beep" that informs the terminal user of the error state. The terminal user can exit from this error situ-ation by typing in an appropriate reset key sequence. This error recovery sequence occurs transparently to the host application program.

Should connection with the terminal be lost due to modem or phone line errors, the line is disabled and re-enabled to permit the user to dial back in and logon again.

Error situations that a terminal user may encounter at his terminal are described in more detail in section 3.2, "Error Situations" on page 3-11.