User`s manual

Western Reserve Controls 1782-JDM User’s Manual

PUB 24.0 PRELIMINARY

4 DeviceNet I/O Assembly Formats

The following tables define the format of the DeviceNet data sent to and received from the JDM. The exact

format of the output and input data that is sent from and received at your DeviceNet master is defined by the

Modbus command used. The following tables provide the general format and illustrate the differences

between the ASCII and RTU modes. Review the Modbus spec or your Modbus device’s user’s manual for

more details.

4.1 Output Data, Modbus Command

There are two types of output command assembly formats. The output command format depends upon

whether or not the command data length is less than or equal to 7 bytes or is 8 or more bytes.

If the command length is 8 bytes or more, then a “fragmented message” is created by the host and is sent

to the JDM. This will be the more common format for your Modbus command strings and the format that the

JDM expects to receive. See Table 4-1.

Table 4-1 I/O Assembly Object Instance 2 Data Format – Fragmented Command

Byte

(Offset value)

Data

0 Record number

1 Address

2 Command

3 Data www – LSB

4 Data www – MSB

5 Data xxx – LSB

6 Data xxx – MSB

…

N Last data – LSB

N + 1 Last data – MSB

If the Modbus command string data is 7 bytes or less, then use the command format shown in Table 4-2.

This will generate a “non-fragmented DeviceNet message”. The 3F (hex) leading byte value indicates to the

JDM that this message is be a short string that was not fragmented by the sender.