VM600 networking manual MAVM600-NET/E 8 - 9
Edition 9 - February 2018
Description of Modbus TCP protocol
SETTING UP A MODBUS CONNECTION (CPUM SW VERSION 067 OR EARLIER)
8.4.5.2 System errors
All other VM600 Modbus server errors, such as conversion errors, sensor not connected or
data invalid are expressed using coils (status bits). Every card and channel has some status
bits and most important bits shall be mapped and made available through Modbus.
For example, the fact that vibration values were not correctly acquired or processed does not
cause Modbus error exceptions. The validity of the acquired and processed value should be
checked using the status bits.
There are two types of configuration error status, physical errors and logical errors:
• A physical error occurs if the master requires data from a card that is missing or is not
responding to the CPUM. For example, this error can be caused if a card is not
responding due to a hardware defect or reconfiguration.
• A logical error occurs if the requested data point on a card or output is not properly
configured.
For both types of configuration error, the response will be completely filled with zeros.
For physical configuration errors the Physical Configuration Error latch is set.
For logical configuration errors the Logical Configuration Error latch is set.
Every Modbus client has own error latches. The error latch will be cleared on the first
non-configuration status request after error status request.
8.5 Description of Modbus TCP protocol
The Modbus TCP protocol has exactly the same layout as the Modbus RTU protocol, with the
exception of the framing sequence check pattern (CRC) and the address interpretation.
NOTE: All requests are sent using TCP on registered port 502.
Requests are normally sent in half-duplex mode on a given connection. Effectively, there is
no benefit in sending additional requests on a single connection while a response is
outstanding.
The Modbus 'slave address' field is replaced by a single-byte 'unit identifier'. This may be
used to communicate using devices such as bridges and gateways which use a single
IP address to support multiple independent end units. The VM600 does not use this feature.
In the response, this field is always set to the received value, but it is not checked by the
system.
The request and response are prefixed by six bytes shown in Table 8-4.
Table 8-4: Modbus TCP prefix bytes
Bytes Function
0 Transaction identifier (usually 0) copied by server
1 Transaction identifier (usually 0) copied by server
2 Protocol identifier = 0
3 Protocol identifier = 0
4
Length field (upper byte) = 0
(since all messages are smaller than 256)
5 Length field (lower byte) = number of bytes following
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