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The collection of the three bytes in our message (0x42, 0x41, 0x44) may be referred to as a
packet, since these bytes are always associated with each other.
Just as all words are not the same length (number of letters), packets may also vary in length.
One way to denote the size of the word is to put the number of letters you have in the word as
the first number in your packet. Your data packet containing the word (or command) BAD will
then become 0x03, 0x42, 0x41, 0x44.
Checksum (Check Character)
During transmission of the packet, an error could change the value of the data. Suppose in our
example, for instance, the packet 0x03, 0x42, 0x41, 0x44 is actually 0x03, 0x42, 0x30, 0x44.
How can you determine that the numbers you get are the same as the numbers sent? Using a
check-character is a method of assigning a value to the packet to check if any bytes have been
modified.
If all the data bytes are added together and this sum made into a byte, that byte could be called
a check character, or more commonly referred to as a checksum. For the VC4™ instrument, the
checksum is the negated sum of all the bytes in the packet. In our example, the packet is 0x03,
0x42, 0x41, 0x44, 0x36. For this packet, the sum modulo 0x100 of all the bytes added to the
checksum must equal zero (0). Any other result indicates there is an error with the data.
The slave's data contained within the packet is interpreted in a somewhat different manner
than our example. The data is composed of two sections, a command and one or more
parameters. The command indicates what type of information is being transmitted in the
packet. The parameters contain specific arguments or data values to be interpreted. Parameters
for most VC4™ instrument's packet require at least four bytes for the Date and Time stamps.
This information is important to provide a date and time reference for each communication
from the instrument. You should ensure that the date and time have been set accurately in
each VC4™ monitor.
Every packet sent by the VC4™ instrument also contains an address, a length, a command and
its associated parameters, and a checksum. The VC4™ instrument assumes that the master's
address is 0 (zero). You assign a unique address to each slave which communicates with the
master.
ACK/NAK Handshake
Each slave (VC4™) uses a handshake scheme between itself and the master. The simplest
response back from the instrument is called an ACK (an abbreviation for ACKnowledge). When
the slave receives a command packet from the equipment, it will send back an ACK response if
the command is received, but no additional data has been requested in the command.
If however, the checksum does not match, the slave will send a NAK (an abbreviation for
Negative AcKnowledge). A NAK indicates that a data packet has been received, but the
checksum did not match with the packet data. The master may send the request again. An
example of an ACK packet is 0x40, 0x00, 0x05, 0x20, 0x9B (40 + 0 + 5 + 20 + 9B = 0x100).
VC4™
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User Manual
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