Western Reserve Controls, Inc. 1782-JDC User’s Manual
6
1.4 Basic Operation
The JDC operates as the DeviceNet front-end to the serial device(s). Connecting the JDC to a
single device, that device can then be assigned by the system implementer to one specific
master. The DeviceNet Master can receive and send data to and from the 1782-JDC via the
methods described in this section. It formats and sends the data to the device and likewise
accepts responses from the device, which are reformatted and passed back to the DeviceNet
system as required.
The JDC has one DeviceNet address. All DeviceNet messages to the JDC itself (to read /
write its internal data) are sent to this address. All DeviceNet messages to and from the serial
device are sent to the JDC DeviceNet assembly objects using poll commands.
The JDC Parameter Object allows you to define the specific operation of each JDC. These
parameters include all the set-up required for the serial comm. link.
The following chart defines the various messaging methods used for “typical” data types at
your serial device and a brief explanation follows.
Table 1-1 I/O Message Types
Typical Data Polled Cyclic Bit-Strobe Change-of-State Explicit Message
Commands
√
√√
√ √
√√
√
√
√√
√
Status
√
√√
√ √
√√
√
√
√√
√
Parameters
√
√√
√
1.4.1 Polled I/O
The DeviceNet Master uses the JDC’s predefined polled IO connection to send input and
output data to the JDC. The input data to the JDC has a one-byte record number, a one-byte
length indicator and up to 124 additional bytes of transmit data. When a poll is received and
the record has changed since the last poll was sent, the JDC sends the associated transmit
data out the serial port to the remote ASCII device. When the JDC receives serial data from a
device on the serial link, the poll response data to the Master contains a one byte record
number, then a one byte status number that reflects errors or events on the bus, a one byte
length indicator that is the length of the message in the response and up to 124 bytes of
received data. The record number is incremented in the poll response when new data is
received on the serial link. The status byte is zero if no errors occurred.
1.4.2 Explicit Messages
As mentioned, explicit messages are typically used to read and write configuration data. This
data allows the JDC to change its internal operating parameters such as baudrate and parity.
The JDC does not allow for direct communication to the serial ports using explicit messaging.
1.4.3 Cyclic Input Message
Cyclic I/O is the function by which a slave device sends its input data to the master at a
specific time period without the host explicitly requesting it. When the specified time interval
(defined by you) elapses, the user-specified input data are transmitted to the master. This
data is the same format as a poll response.
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