Modbus-Tcp - JUMO LOGOSCREEN nt Interface Description

6.3 Modbus-TCP

Structure of a
Modbus-TCP
telegram
Identical for
request and
In this case, the Logoscreen nt is configured as a slave, and handles the
incoming queries as a server through port 502. The port can also be changed,
see Chapter 3.5 Configuration of the Ethernet interface, page 13.
Modbus-TCP is a standardized procedure for packing a Modbus telegram
inside a TCP frame (tunneling) and transmitting it via Ethernet.
The Modbus telegram (without CRC) is transmitted with an additional, 6 or 7
byte long MBAP header. The seventh byte corresponds to the first serial byte,
but has a different designation in this case.
MBAP header
2 bytes 2 bytes
trans- protocol ID
action ID
Must be 0
for Modbus
response
As a comparison: the normal Modbus telegram, see Chapter 4.4 Structure of
a Modbus telegram, page 17:
This protocol can be used, for instance, by the JUMO SVS 2000 visualization
program (or similar programs) to read from and write to the Logoscreen nt over
a company-wide Ethernet network. It is possible to access all the device
variables in the Modbus address tables (see Chapter 7 Address tables, page
43).
Only one Modbus master (client) at a time can access a Logoscreen
nt by means of the Modbus-TCP.
A connection that has been opened by a client will be closed after 30
seconds of inactivity!
A closed Modbus-TCP port (from the Logoscreen nt or the other end)
can only be opened again after 10 seconds.
6 Ethernet protocols
2 bytes 1 byte
length unit ID
Length of Corresponds
query or to device
response address. For
in TCP, must be
bytes 0xFF or 0
from (incl.)
unit ID broadcast)
Slave Function
address code
1 byte 1 byte
Modbus
telegram
other bytes
as below,
but without CRC
(0 =
Data field
x bytes
CRC16
2 bytes
39