User`s manual

Western Reserve Controls 1782-JDM User’s Manual
PUB 24.0
3
for each message. Therefore, each message to a Modbus device has 2 addresses associated with it – the
DeviceNet address of the JDM and the Modbus address of the specific device.
Table 1-1 Data Message Options
I/O Type Polled Cyclic Bit-Strobe Change-of-State Explicit Message
Modbus Device
Commands
√
√√
√
Modbus Device
Status
√
√√
√
Modbus Device
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 (with or without data) to the JDM and the JDM responds with a Poll
Response. When a poll command is received, input data are transmitted from the JDM to the host.
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.
How do I send a poll command to read data?
The poll command can read multiple Modbus Device status registers’ data per poll command, as
supported by the device, up to a limit of 100 bytes of data on the DeviceNet message. First, set up your
scanner to read the number of bytes of data expected from the JDM
plus two (2)
, depending upon the
command and number of register data desired. The command response from the Modbus device is
returned from the JDM to the DeviceNet master in response to a poll command.
1.3.2 Explicit Messages
As mentioned explicit messages are typically used to read and write configuration data, but they can be
used to access any available data in either the JDM or the Modbus device.