Operating instructions

Reference PTQ-MCM ♦ Quantum / Unity Platform
Modbus Communication Module
ProSoft Technology, Inc. Page 105 of 139
April 29, 2008
8.2.7 Pass-Thru Control Blocks
Overview
Caution: This feature allows a remote modbus master to directly update the processor memory.
The user should carefully read this section before considering this feature in order to avoid
transferring data to unexpected memory areas in the Quantum processor.
The pass-thru operation consists of the slave port receiving a write command
from the master and passing the data directly to the processor without updating
the PTQ-MCM internal database.
The module supports two pass-thru modes: block pass-thru and direct pass-thru.
The module also supports swap byte option for each mode.
During block pass-thru operation the module generates a specific block to the
processor when it receives a write command from the master. The module uses
different block IDs depending on the type of write command received (modbus
functions 5,6,15 and 16). The user should implement processor logic to handle
the pass-thru block once it is received. The block contains relevant information
such as data count, destination address, and data
During direct pass-thru operation the module passes the data received from a
write command to the processor. The module will use the destination address
from the modbus command to select the start memory address where the data
will be copied to.
Pass-Thru Configuration
Each port can be configured for pass-thru functionality.
Type: 0 #Type of port (0=Master, 1=Slave, 2=Block Pass-Thru,
#3=Block Pass-Thru, 4=Direct Pass-Thru, 5=Direct Pass-Thru Swap)
The following codes are supported for Pass-Thru:
Code Description Modbus Functions Supported
2 Block Pass-Thru 5, 6, 15, 16
3 Block Pass-Thru with Byte Swap 6, 16
4 Direct Pass-Thru 6, 16
5 Direct Pass-Thru with Byte Swap 6, 16
The maximum number of bits supported for modbus function 15 is 800 bits. The
maximum number of words supported for modbus function 16 is 125 words.
Direct Pass-Thru
When a PTQ-MCM port is configured for pass-thru operation it will work as a
modbus slave transferring the data received directly to the processor memory
(holding register memory). The configuration of backplane commands is not
required to transfer data from the module to the processor during direct pass-thru
operation. This feature does not require any processor program to receive the
data from the module.
The module uses the destination address received from the modbus command to
determine the memory location in the processor where the data will be written to.