NCR 7454, Hardware User'S Manual

Page: 154 / 180

6-6 Chapter 6: NCR 7454 4x20 Customer Display
Command Descriptions
Structure/Logic Description
The Retail VFD firmware is comprised of operating system type
services such as common subroutines, interrupt service routines, and
data processing routines. Normally, the firmware is always active
refreshing the Retail VFD to keep display data visible. The firmware
will also act on interrupts due to host parallel communication. This
communication is in the form of commands from the host to the Retail
VFD that require the Retail VFD to modify the display appearance.
The Retail VFD firmware uses a micro ‐ controller and interfaces to
parallel links.
Host/Retail VFD Command Interface
The Retail VFD accepts two types of data; display data and command
data. If a byte received from the host is any character except ESC (0x1B)
character, it is processed as a character and displayed on the Retail
VFD. If an ESC character (0x1B) is received, the subsequent byte(s) is
processed as a command. If the subsequent byte is an invalid
command it will be ignored. If an ESC is received, but no subsequent
byte is received within ten seconds, the firmware will ignore it.
Note that each command consists of at least two bytes. The first byte
(0x1B) is a command identifier indicating the next byte(s) is a
command byte. The command byte may be followed by parameter or
data bytes depending on the command. It is only valid to receive an
ESC (0x1B) as a command identifier.
The following table describes the supported Retail VFD commands.