Operating instructions

PTQ-MCM ♦ Quantum / Unity Platform Reference
Modbus Communication Module
Page 106 of 139 ProSoft Technology, Inc.
April 29, 2008
The following illustration shows the three basic steps performed during the pass-
thru operation.
Modbus
Master
PTQ- MCM Quantum
1
2
3
The module receives a modbus write command. For example:
1 Modbus Function: Preset Multiple Registers (function 16)
2 Count: 10 words
3 Destination Address: 500
4 The module verifies that the destination address is a valid address in the
processor. If not, it sends a response with an exception code (illegal data
address). If the destination address is valid the module will insert the data into
its internal backplane command queue to transfer to the Quantum. After the
request is placed in the queue the module sends the response to the master
device.
5 The time period for the data to transfer to the Quantum processor will
basically depend on the processor's scan rate. Depending on the processor
scan rate the module could receive a command while the queue is full. In this
situation the module will send a "Slave Device Busy" exception response
(exception code 6) to the modbus master and the data will not be copied to
the processor.
6 The module writes the data directly to the Quantum address defined by the
modbus command. For the example from item 1 the data would be written
starting at Quantum address 40501.