-8-
8.4 ASCII COMMUNICATION - All communication to and from the annunciator is performed using
ASCII characters. This allows the information to be processed with the "string" functions
common to high level computer languages such as BASIC and C. For computers that support
standard serial port interfaces, no special machine language software drivers are required.
The use of the ASCII format also allows for the connection of these devices to an auto answer
modem for long distance operation without the need for a local supervisory computer. The
ASCII characters also make system debugging easy using standard terminal emulation
software.
8.5 COMMUNICATIONS PARAMETERS
Baud Rate:
9600
Data Bits:
8
Stop Bits:
1
Parity:
None
8.6 COMMAND STRUCTURE - The annunciator operates with a simple command/response
protocol to monitor all functions. A command must be transmitted to the unit by the master
(computer or PLC) before the slave can respond with useful data. A slave unit can never
initiate a communications sequence.
Communication to the annunciator is performed with two character ASCII command codes.
The general format used for the commands is illustrated below using the READ DATA
command as an example. The hexadecimal values for the characters are shown only as a
reference for those using low level (assembly language) decoding and will not appear on the
communications terminal screen. All of the characters used in the communications protocol
are standard ASCII characters and appear on the computer keyboard as shown with the
exception of the "not acknowledge" (NAK) which is the industry standard "control U".
header
start
node
space
command
end
AS CII
>
(
01
R
D
)
HEX
3Eh
28h
30h 31h
20h
52h 44h
29h
Command Header ">" (3Eh) - Each command must begin with the command header
sometimes referred to as a prompt character. The ASCII character used is the ">" which
means that a command message will be sent from the master to the slave.
Start of Text "(" (28h) - The command header must be followed by the start of text indicator.
Node Number "01 -99" (30h 31h) - The node number or address of the device being
contacted.
Space (20h) - Following the node number is an ASCII space character (not printable, value
20h) to act as a delimiter between the node number and the two character command word.
Command Word "RD" (52h, 44h) - The command words are standard two letter (upper case)
commands sent by the master for gathering specific information about the status of a slave.
The commands are listed under STANDARD COMMANDS below.
End of Text ")" (29h) - The end of text indicator says this is the end of the command.