Manual VIPA CPU 21x Chapter 9 Deployment CPU 21xSER-1
HB103E - Rev. 05/45 9-9
Data that the CPU has to provide for the Modbus master are stored in a
FIFO send buffer (first in first out) with a size of 2x256Byte. In opposite to
the other protocols the data remain in the send buffer until they are reques-
ted by the Modbus master via a read command (function code 01h, 03h).
If the interface receives data from the master (function code 05h, 06h, 10h)
these are stored in a FIFO receive buffer with a size of 2x256Byte and may
there be read by the CPU.
The embedding of the data into the Modbus protocol happens
automatically.
Please regard that the Modbus master may access the IN res. OUT buffer
by according presetting of the read function code. By means of a read
access to the IN buffer (function code 02h, 04h) the master may read data
that it has sent to the Modbus slave before. The data remain in the buffer
until they are overwritten by the Modbus master.
The following picture shows the communication principle. More information
is also to be found in the chapter "Modbus slave function codes" further
below.
Protocol FIFO buffer
256Byte
256Byte
256Byte
256Byte
IN
OUT
RS232C/RS485
InterfaceProgram
SER_RCV
SFC 218
CFG
RECEIVE
SEND
SER_SND
SFC 217
SER_CFG
SFC 216
CPU 21xSER-1
Read
01h, 02h,
03h, 04h
Write
05h, 06h,
10h
Modbus-Maste
05h, 06h, 10h
02h, 04h
01h, 03h
Principles for
Modbus Slave
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