User Frames; General Information; Structure Of The Reading And Writing Processes - JUMO mTRON T Manual

6.1

General information

The central processing unit allows the user to individually compile Modbus frames for his ap-
plication. This achieves a high level of flexibility and the user can thus reduce the data ex-
changed on the bus to the volume he requires.
This provides a significant advantage with regard to the transmission speed. Numerous vari-
ables can be compiled in a large data packet, thus improving the transmission speed, which,
in turn, means that the bus is subject to less protocol overhead and fewer changes between
the request and response.
The maximum size of a frame is defined to 254 bytes. This corresponds to 127 words for each
command. The activated reading or writing frames are cyclically processed in sequence. The
frame length specified in the setup program is always used. This length is automatically calcu-
lated using the number of entries (64 per frame) in the respective frame. All frames can be used
for the Modbus master or Modbus slave.
NOTE!
There is no locking function against multiple use of the same input variables within several
reading frames. The user must prevent this from happening to avoid a data collision.
6.2

Structure of the reading and writing processes

Block diagram of the inputs and outputs for the master function
Ext. variables
Cyclical value check
and alarm generation
(1)
Frame processing
Write frames:
Data are collected.
Read frames:
Data are stored in
single variables.
Frame
config.
Reading/writing Modbus (master)
(1) A cyclical function compiles the individual variables into frames.
(2) To do this, it accesses the frame arrangement configured in the setup program.
(3) The complete cyclically updated frames are available.
(4) Each interface configured as a master transmits (reading or writing) the frames configured for the
other end.
(5) The other end receives this frame and responds.
System I/O
With all
process values;
areas for 64
modules
Ext. variables;
limits checked
Ext. variables;
interface
cyclical
Additional
frames
Modbus start adr.
+ frame length
(2)
Modbus
tables
for slave
With attributes
of every variable:
Type,
address,
read only ...
(3)

6 User frames

Device Modbus
address slave
Transfer Serial and
in virtual TCP
area
(4)
Modbus
master
Serial and
TCP
SCADA
system
PC
PLC
(5)
External
slave
devices
67