Unitech PT600, Programming Reference Manual

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Chapter 2 MULTIPOINT Communication
2.1 MULTIPOINT Communication Protocol
MULTIPOINT communication protocol is supported by built-in communication
tool on PT600. It is the default setting for RS232 port. User can enable/disable
MULTIPOINT protocols by keypad setting or system call in the application.
If RS232 port is set to NONE communication protocol, that means MULTIPOINT
protocol is disabled and the application can treat RS232 port as a character I/O
device like COM ports on PC. Otherwise, the MULTIPOINT is active and PT600
will interpret RS232 input data as on-line commands from host computer and send
data to host computer in data packet according to MULTIPOINT protocol.
MULTIPOINT protocol is an asynchronous serial multi-drop protocol for
communication with the host computer. One host computer can be connected up
to 32 PT600 terminals, while each PT600 has a unique address. The address is an
ASCII character ( ‘A’ – ‘Y’ or ‘0’ – ‘6’) plus 80H.
In MULTIPOINT protocol, host computer always issues POLLING DATA
request or ESC commands first, then wait for the addressed PT600 to response.
The maximum packet size including protocol control characters is 256 bytes.
Symbols in Transmission Packet
STX = 02H
ETX = 03H
EOT = 04H
ACK = 06H
NAK = 15H
ESC = 1BH
CMD = command character
ADDR = address (‘A’ – ‘Y’ or ‘0’ – ‘6’) + 80H
CS1 CS2= 2 checksum characters
<data>/<parameters> = character string
Host Transmission Packet
Transmission Format
Poll STX ADDR
Host Data STX ESC ‘0’ <data> CS1 CS2 ADDR