What is Character Mode?

  • 7021530
  • 22-May-2003
  • 02-Mar-2018


Verastream Host Integrator


This technical note describes character mode and how it differs from screen-oriented block mode.


Character Mode Defined

In UNIX/OpenVMS VT terminal communication, and sometimes in HP terminal communication, data is sent in character mode, as a constant stream of characters, rather than as a block. For example, if the A key is pressed, the A key is directly sent to the host and the host immediately sends screen updates to the terminal. Even though a VT application may appear to a user as a series of screens, to the terminal it is just a series of characters and escape sequences. There is nothing in the VT datastream to denote what constitutes a screen.

In contrast to character mode is block mode, used with IBM mainframe and AS/400 systems (3270 and 5250). Block mode connections are page-oriented. When the host sends a screen to the terminal, the host informs the terminal that it is transmitting one screen's worth of data. For example, when you type data on a terminal screen, the data entered is local to the terminal; therefore, nothing is sent to the host until an aid key, like the Enter key, is pressed. This makes it easy for the Host Integrator to distinguish one entity from another.

Is it the connection or the applications that are block mode or character mode?

A combination of the host and the application it is running determines block mode versus character mode. VT/ASCII terminals are always character mode. IBM AS/400 terminals, which use the 5250 datastream protocol, and IBM mainframe terminals, which use the 3270 datastream protocol, are typically block mode. HP3000 terminals can be block mode or character mode, but it is the combination of the host and the host application that determines the type of connection that is needed.

What's the difference between scrolling operations in character mode applications and block mode applications?

Scrolling operations, such as page up or page down operations, are in no way unique to character mode or block mode applications. They are important for recordsets, but these types of screen constructs exist in all types of host applications that are accessible by Verastream. The only difference is that character mode applications require more specific recordset configuration since it is more difficult to determine when the host is finished sending recordset data after a query has been made.

To view the real-time data sent and received from a character mode host, use model debug messages (version 6.0 or higher; see KB 7021543) or the Design Tool Datastream Window (version 5.5 or earlier).

Additional Information

Legacy KB ID

This article was originally published as Attachmate Technical Note 10020.