User`s manual

Western Reserve Controls 1782-JDM User’s Manual
PUB 24.0
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1.3 JDM Basic Operation
The JDM operates as the DeviceNet front-end gateway between the DeviceNet system and the Modbus
devices. It is a DeviceNet server device that can be assigned by the system implementer to one specific
master. The DeviceNet Master can receive and send data to and from the JDM via the methods described in
this section. It interprets messages received from the DeviceNet connection, and if its a Modbus message,
reformats and sends it to the Modbus device. Likewise, it accepts and interprets responses from the
Modbus device, which are reformatted and passed back to the DeviceNet system as required.
The JDM has one DeviceNet address. All Explicit DeviceNet messages to the JDM itself (to read / write its
internal data) are sent to this address. The Modbus devices are communicated to using poll messages sent
to the JDM and poll responses received from the JDM. See Section 2.4 and 2.5 for details.
The JDM Parameter Object allows you to define the specific operation of each JDM. These parameters
include set-up for the serial comm link and the fault and idle actions and data. See Section 3.6 for details.
Each Modbus device connected to the JDM has a Modbus address between 1 and 247. This address is
used by the host system and the JDM to identify which device on the Modbus network is being selected for
each message.
Table 1-1 Data Message Options
I/O Type Polled Cyclic Bit-Strobe Change-of-State Explicit Message
Modbus Device
Commands
√√
Modbus Device
Status
√√
JDM Parameters √√
The following sections describe how the data is accessed.
1.3.1 Polled I/O
The master can poll the run-time data used in the JDM. The communications is a two-part transaction: there
is a Poll Command from the host to the JDM and the JDM responds with a Poll Response using the
response of the addressed Modbus device.
How do I send a poll command to write data?
The poll command can set multiple registers of data per output command, depending upon the Modbus
command used. First, define the Modbus command string desired to send to the Modbus device. Enter this
data in binary format – the normal format for RTU mode. This format is also used if in Modbus ASCII mode –
the JDM will translate from binary to ASCII. Do not include the leading colon (for ASCII mode) or the CRC or
LRC. Determine the number of bytes to be written, with the data in binary format. Then set up your scanner
to write the defined number bytes of data to the JDM plus one (1).
Any Modbus command and register that is appropriate to the target Modbus device are valid. See the
Modbus device’s user’s manual for more details on which commands are supported.