J-Bus/ModBus Protocol
PR 5220 Instrument Manual
EN-118 Sartorius
5.2 ModBus-TCP/-UDP
Connection is via the network interface.
The telegram consists of six blocks:
Trans
Proto
Size
Addr
Func
<data>
Legend
Trans
16
Sequential transaction number. The number is reflected
by the instrument in such a way that the reply can be
assigned to the enquiry.
Proto
Reserved for future extension, here always 0.
Size
16
Number of subsequent bytes
Addr
8
Normally, the device address is not used with ModBus-
TCP/-UDP.
It is used if ModBus-TCP/-UDP is connected behind a
ModBus RTU gateway.
With ModBus-TCP/-UDP, PR 5220 ignores this parameter.
Func
8
Function code (see Chapter 5.3)
<data> Further data (see Chapter 5.3)
The telegrams can be exchanged using TCP or UDP via Port 502. Normally, this is a fixed setting in the
ModBus-TCP master.
With UDP, the typical response time is 4 ms and max. 8 ms. With high network traffic, failed telegram
transmissions must be expected. Suitable measures must be taken in the ModBus-TCP master to force a
repeated transmission in the event of telegram loss.
With TCP, the typical response time is approx. 10 ms. With high network traffic, transmission can be delayed
(max. 120 seconds in extremely disturbed networks, or with long transmission distances as encountered e.g.
with the Internet). Normally, however, no telegrams are lost.
Comparison
ModBus-TCP ModBus-UDP
Reliable transmission:
As long as the line is not interrupted, no telegram is
lost.
Unreliable transmission:
With high network traffic or transmission over long
distances, telegram loss must be expected.
Low speed:
With transmission problems, transmission can be
delayed considerably.
High speed:
The reply is transmitted quickly, or not at all.
Suitable for:
- Parameter transmission
- Result logging
- Non-time-critical process control
Suitable for:
- Transmission of dynamic values
- Visualization
- Time-critical process control (requiring timeout
handling)
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