3 operational description, Chapter 1 – introduction, Power leader™ ethernet gateway – GE GEH6505A User Manual

Page 7: Message format, Gateway/host interface

Advertising
background image

POWER LEADER™ Ethernet Gateway

Chapter 1 – Introduction

3

1-3 Operational Description

The Ethernet Gateway transparently passes message

between the host and devices attached to the

Gateway. Figure 5 illustrates the stripping or adding

of Ethernet headers to the Modbus messages. This

section describes the nature of these messages and

how the Gateway routes them. The following

information is not necessary for configuration and use of

the Ethernet Gateway, but is provided for users who may be

developing custom applications and need such

information.

Message Format

Messages sent from the software to RS485 devices

via the Ethernet Gateway have a 15-byte header

inserted in front of the message. The header tells

the Gateway where to send the message, how long

the message is, and if parity errors were

encountered. This header has the following format:

SS

DD

EE

NN

CC

SS

Sequence of ten AA hex bytes indicating the start
of a message

DD Destination device port number – the Gateway

RS485 port to which the message should be
routed (0 - 3)

EE

Error status byte (0 = no parity errors, 1 = parity
errors encountered)

NN

Number of bytes in the Modbus message

CC

A one byte checksum calculated by adding the
first 14 bytes in the header

The header is stripped off the message by the

Gateway and the remainder of the message is sent

without changes to the destination device (or

interpreted by the Gateway if a configuration

message).
Messages from the RS485 devices to the host are

processed by adding the 15-byte header onto the

start of the message. For messages from devices to

the host, the byte in the DD position contains the

RS485 port from which the message came.
Cyclic redundancy check (CRC) handling is done

by the host on the Ethernet and the RS485 device

on the Modbus. The Gateway does not check the

CRC when receiving messages from the host or

from RS485 devices.

Ethernet

header

information

Modbus

message

Host

P MC S

Modbus

messag e

Ethernet

header

information

Modbus

message

Modbus

messag e

Modbus messages fr om host to device - Ether net Gateway str ips off header

Modbus messages from device to host - Ether net Gateway adds header

Message tr aveli ng on Ethernet

Message tr avel ing on RS485

Ethernet

Gateway

Ethernet

Gateway

RS485
device

RS485
device

Figure 5. Ethernet headers on Modbus messages.

Gateway/Host Interface

The Gateway uses TCP/IP (Transmission Control

Protocol/Internet Protocol) to interface with the

host on the Ethernet.
The Gateway initially opens a socket and waits for a

host device to attempt to connect with the socket.
Once a connection is established, data messages

may be transmitted to the Gateway (and ultimately

the RS485 devices) and messages from RS485

devices passed to the host.

Advertising