Specifications

52 Chapter 5 - Software Interface
Checksum byte = <AAh> + <Lead in byte> + <8 Data bytes>
where the addition is performed with 8-bit unsigned numbers and overflow is
ignored.
By default, the host is not required to send a properly calculated Checksum in
command packets. A dummy value, such as 0, is required to provide the correct
packet length.
If a higher confidence is needed in the serial communications, the host may use the
Parameter command, (see page 95), to enable Checksum verification by the
controller. With this function enabled, the controller checks each command packet
for a valid Checksum value before processing the command.
Key Byte
An optional format extends the standard serial packet by adding a Key byte. This
extended packet is used in specialized installations where more than one serial
controller is to be connected on a single serial communication link. In such an
installation, a unique Key value may be programmed into each controller with the
Key command, (see page 88), and stored in NVRAM.
A command intended for only one of the interconnected controllers is sent in an
extended packet. Although all controllers on the link receive the command, only
the one with the matching Key processes the command. If a standard packet is sent
along the link, all the interconnected controllers will process the command (it acts
as a global command).
Similarly, responses from each controller contain the programmed Key byte. This
permits the host to discriminate between touch data generated by the controllers.
As there is no standard way of allowing the controllers in this type of installation
to send data on the same serial data line, a custom wired OR configuration is
necessary for the hardware to function properly. The controllers must also have
automatic touch reporting disabled with the Mode command and be polled with a
Touch query issued to each controller. See Touch command, page 102. Other
hardware considerations must also be evaluated when attempting this type of
installation.
The structure of the extended serial communications packet is:
<Lead In byte><8 byte Command or Response><Key
byte><Checksum byte>