Xylem Expert 7060_7070_Modbus User Manual

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Communication protocol Serie30

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MODBUS communication

4.1

MODBUS Communication Basics

Mode:

RTU Mode (ASCII is not supported)

Coding System:

8–bit binary, hexadecimal 0–9, A–F

Two hexadecimal characters(bytes) contained in each 8–bit field of the message

Bits per Byte:

1 start bit

8 data bits, least significant bit sent first

1 parity bit: none, even, odd (programmable) – default: none (Keller devices)

1 stop bit

Error Check Field:

2 Byte Cyclical Redundancy Check (CRC)

Baudrate:

programmable 9’600 or 115’200

Frame Layout:

> 3.5 char (byte)

start delimiter

8 bit

addr

8 bit

fkt code

n x 8bit

data

(max n = 252)

16 bit

CRC

LOW / HIGH

> 3.5 char (byte)

end delimiter

The entire message frame must be transmitted as a continuous stream of characters. If a silent interval of more than 1.5

character times occurs between two characters, the message frame is declared incomplete and should be discarded by the

receiver.

Bus Addressing:

Unicast addresses: each slave has an address between 1 and 247.

Broadcast addresses: address 0 is used for broadcast frames, the slave accepts the command but must not generate an

answer.

Special addresses: addresses 248 – 255 are reserved. The address 250 is used for non-bus (point-to-point) communication to

address a slave with unspecified or unknown address (not MODBUS standard).

Function Code:

The 8-bit value function code describes the operation that is performed on the slave. Implemented MODBUS functions are

described in the following chapter.

4.2

Using MODBUS with KELLER products

Both protocols are active. For MODBUS only a subset of the MODBUS functionality is implemented and the implemented

MODBUS function codes are not used in the KELLER-protocol. Please note, that the responses are different for both protocols.

Furthermore note that for MODBUS communication NO initialization is required to be performed before any other command.

Attention: The Error Check Field byte alignment differs.

Modbus

KELLER-Protokoll

CRC-16

L:H

H:L

MJK Devices

MJK Products

MJK

MJK