Tandem Protocol Support

With async, each terminal is directly connected to the host. With sync transmission, each Tandem terminal can use a unique network ID address. With this mode, each terminal is specifically polled by the host on a multidrop line.

The AM6520 protocol supports the following transmission modes:

The first mode does not use formatting codes. The screen formatting codes are identical for the remaining modes. The only difference between the second and third modes is the transmission method (sync or async).

With the first three modes, the Tandem terminal must be directly connected to the communications controller. A communications controller (or multiplexer) port is required for every terminal. The fourth mode uses a polled, multipoint protocol to connect several terminals to the same port. Each terminal is specifically polled since AM6520 protocol does not support a General poll capability.

Screen Memory

The major difference between the Tandem terminals is the memory size. The Tandem terminal allocates its RAM memory in two different modes:

• Multi-Page Mode - The terminal supports several pages of memory. Each page consists of a 24 row x 80 column screen.
• Single-Page Mode - The terminal builds a single screen buffer consisting of a large number of 80 column rows. This is called ITI mode.

In Single-Page mode, the user can scroll through the screen buffer without interacting with the host. The screen scrolls one line at a time or one 24 line page at a time. In Multi-Page mode, displayed pages can only be changed by the host. The user works with the displayed section of the screen buffer. Prints or transmissions only affect the displayed 24 line section.

Screen Formatting

Tandem AM6520 data streams consist of control codes and data formatting codes. Only the data formatting codes differ from one terminal to another.

Read Commands

The 6530 uses a bit in the attribute to determine which data is sent to the host when an Attention Identifier (AID) key is entered. When the operator (or host application) makes an entry into an unprotected field, the Modified Data Tag (MDT) is set. When the ENTER key is pressed, the terminal scans the screen buffer to determine which fields have the MDT set. The modified fields are then assembled into a block for transmission to the host.

The host can use a Read Command to have all or part of the terminal's screen buffer sent to the host.

Transmissions

The operator enters a Function Key which is sent to the host. The host application issues a Read Command associated with the Function Key to the terminal which causes a transmission to be sent to the network.

Status Line

The 6530 terminal uses a 25th Line to inform the operator of the terminal's status and network activity. The 25th line indicates printer status and operational modes. The host can also write messages to the 25th line.

Print Options

The 6530 supports three printing options:

To view the available options that can be specified for this protocol, please click here. If you need further information on AM6520 protocol or our implementation and support for this protocol, please e-mail us.