Emsa HS500E Modbus TCP Handshaking, O s t / H S 5 0 0 E M o d b u s T C P H a n d s h a k i n g

C H A P T E R 6 : M O D B U S T C P P R O T O C O L

6.2MODBUS TCP HANDSHAKING

Modbus TCP handshaking is governed by the changing of the “Overall Length” value within a data packet. The Overall Length value is typically the first 2-bytes of a command or response and indicates the total number of data words in the packet (including one word for the Overall Length value).

Overall Length values are stored in the first holding register, 40001, of Device ID 1 (for commands) and Device ID 33 (for responses). When the value at register 40001 (of Device ID 1) changes from 00, the HS500E will recognize that a command is waiting to be executed. The HS500E will then execute the command and return a response at Device ID 33.

6 . 2 . 1 H o s t / H S 5 0 0 E M o d b u s T C P H a n d s h a k i n g

One implication of this process is that when the Host issues a command, it must first write the entire command to the holding registers for Device ID 1, leaving the Overall Length value to be written last.

For example, for the Host to issue the 6-word command “Read Data,” it must first write the last 5 words of the command to Device ID 1, beginning at register 40002.

L A S T 5 W O R D S O F A R E A D D A T A C O M M A N D

Word	MSB	LSB	Description
02	AA	02	Command ID: Read Data
03	00	01	Node ID
04	03	E8	Timeout Value of 1 second (measured in
			ms)
05	00	20	Read Start Address: 0x20
06	00	04	Read 4 Bytes

After writing the last 5 words of the command, the Host will write the Overall Length value to register 40001 of Device ID 1.

F I R S T W O R D O F A R E A D D A T A C O M M A N D

Word MSB

LSB

Description

01

00

06

Overall Length (in words)

The moment the Overall Length value at register 40001 of Device ID 1 changes to a “non-zero” value, the HS500E will recognize the waiting data and will execute the command.

H S 5 0 0 E – O P E R A T O R ’ S M A N U A L	P / N : 1 7 - 1 3 0 5 R E V 0 2 ( 1 2 - 0 5 )
	P A G E 5 5 O F 8 2