User`s manual

Western Reserve Controls 1782-JDM User’s Manual
PUB 24.0
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1.3 JDM Operational Philosophy
A Modbus network system consists of a single MB Master and one or more MB Slave Devices. The JDM,
as the proxy to the DeviceNet Master, becomes the ModBus master. All other devices on the Modbus link
must be defined as Slaves.
- The JDM is a Slave on the DeviceNet bus
- The JDM communicates on DN with the DN Master using the Polled I/O technique for transmitting and
receiving MB data.
- As just mentioned, the JDM acts as the Master on the Modbus line. It initiates all communications
(commands) to the MB devices, which are generated by the DeviceNet Master.
- All Modbus devices connected to it must be set up as slaves, which only communicate in response to the
MB commands issued by the DeviceNet Master (PC, PLC, controller, etc.) thru the JDM.
- Each MB device on the serial line must be set to a unique MB address, 0-255. The JDM, as MB link
master, does not get a MB address assigned to it.
- The DN master software program must form a valid MB command string in a data array, pre-pend a
Record Number and String Byte Length, and then send the string to the JDM. The string must contain:
- slave MB address which is being addressed for this command string
- MB command number
- slave starting register for the command
- and typically, depending upon the command number and command format, the number of registers
referenced
- and any data required (for outputs, for example)
In order to accomplish this, the user must understand how to generate an appropriate ModBus command.
The format of this string generally follows the format of a MB command in RTU mode (hexadecimal)
format. (See Table 5-1 in the Manual.)
- The Master then transmits this string to the JDM. The JDM will interrogate the Record Number, and, if
the Record number is different from the previous record, will format the MB command string and transmit
the new command out the serial port.
- The JDM does not interpret any data sent to it from the DN Master. It simply reformats the command
string for either ASCII or RTU mode MB data transfers (as defined in Parameter 1 of the JDM) to send to
the MB slaves. (For instance in RTU mode the JDM calculates and appends the 2-byte CRC to the end of
the string received from the Master.)
- The JDM then listens for the MB response. When received, the JDM checks the serial port status (parity,
overflow, etc.), checks and strips the CRC/LRC, reformats the data (per Table 5-2 in the Manual), pre-
pends a new RX record number, a status byte and a Length byte, and sends the resultant string to the DN
Master using a DeviceNet Poll Response.
- The DN Master’s software then has the task of deciphering the received string and taking appropriate
action.
1.4 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