6-6 Chapter 6: NCR 7454 4x20 Customer DisplayCommand DescriptionsStructure/Logic DescriptionThe Retail VFD firmware is comprised of operating system typeservices such as common subroutines, interrupt service routines, anddata processing routines. Normally, the firmware is always activerefreshing the Retail VFD to keep display data visible. The firmwarewill also act on interrupts due to host parallel communication. Thiscommunication is in the form of commands from the host to the RetailVFD that require the Retail VFD to modify the display appearance.The Retail VFD firmware uses a micro‐controller and interfaces toparallel links.Host/Retail VFD Command InterfaceThe Retail VFD accepts two types of data; display data and commanddata. If a byte received from the host is any character except ESC (0x1B)character, it is processed as a character and displayed on the RetailVFD. If an ESC character (0x1B) is received, the subsequent byte(s) isprocessed as a command. If the subsequent byte is an invalidcommand it will be ignored. If an ESC is received, but no subsequentbyte 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 acommand byte. The command byte may be followed by parameter ordata bytes depending on the command. It is only valid to receive anESC (0x1B) as a command identifier.The following table describes the supported Retail VFD commands.
