Structure of Serial Frame

This service is used to have a PC and other devices read and write the information and data in a PLC, and to have them control a PLC (RUN, STOP, PAUSE). In the system composing of a Master and a Slave, if station numbers are assigned, multi-drop communication is available.

Request Frame (Master) :

The frame that an outside communication device requests to a computer link module

ENQ	Stn	Cmd	Length	Data	BCC	EOT
1 Byte	2 Bytes	1 Byte	2 Bytes	N Bytes	2 Bytes	1Byte

Response Frame (Slave) :

The frame that a computer link module responds to an outside communication device

STX	Stn	Cmd	Length	Data	BCC	ETX
1 Byte	2 Bytes	1 Byte	2 Bytes	N Bytes	2 Bytes	1Byte

1. The structure of a sending frame and the one of a receiving frame are same.
2. The same as the command codes received from a request frame (Master) are used for response frame. But, if there is an error in communication or process, Code E is responded.
3. Description for Codes

Code	Hex Value	Description
ENQ	0x05	Master Frame Header
EOT	0x04	Master Frame Tail
STX	0x02	Slave Header
ETX	0x03	Slave Tail
Stn	0x00 ~ 0x1F, 0xFF	PLC Station Number
Cmd		Command
Length		Length of Data Device (Length Bytes), Hexadecimal
Data		Data Device according to Command (Length Bytes)
BCC		Remainder value when dividing the binary-sum fromCmd to the end of data by 256

4. Commands The commands used for exclusive communication service are as follows.

Command	Code	ASCII	Function
Read Word Data	0x52	R	Reads Word Memory Device.
Write Word Data	0x57	W	Writes to Word Memory Device.
Read Bit Data	0x72	r	Reads Bit Memory Device.
Write Bit Data	0x77	w	Writes to Bit Memory Device.
Change PLC Mode	0x4D	M	Changes PLC Mode.
Register Monitoring Device	0x58	X	Registers Monitoring Device.
Read Monitoring Device	0x59	Y	Reads
Respond Error	0x45	E	Responds Error in PLC.