Date: 2021-08-17
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Note:
Doubling a Modbus Slave Address will not result in a complete network short-circuit -
if the ID is requested by a Modbus master, both addressed slave devices will respond. This will cause the Modbus Master to
display misleading data accordingly or assumes a timeout due to the fact the data packets are not valid.
In this case, check the uniqueness of the slave address and, if necessary, assign a free Modbus address.
Baud Rate
The baud rate defines the data transmission speed for Modbus queries and answers. Please note that the polling speed through
the master must be identical to the answering speed configured at the slave to avoid communication lost issues.
Parity
When transmitting data in the form of a bit stream, the parity bit will ensure an error detection can be performed.
The value of the parity bit is calculated by the transmitter and communicated to the receiver accordingly. The receiver of the
data stream uses the same Modbus mathematical algorithm to verify valid data and find corrupt data packets. Thereby Sender
and receiver must therefore agree beforehand on how to perform the parity calculation:
The parity calculation can be interpreted as even or odd.
Example: even parity
If devices agree to calculate with even parity, the number of all "1 bits" will be counted within the data word. The task of the
parity bit is to set the result to an even number:
Therefore, if the number of bits to be checked within a data packet is even, the parity bit must be transferred as 0, otherwise the
total count will be not an even value. Differently configured devices would therefore declare an odd or an even value valid and
discard other data packets accordingly as invalid.
Der CS141 offers three options
:
n
No parity control
o
Odd parity control
e
Even parity control
By default, the CS141 is shipped with the value n for no parity check.
Stop Bit
A stop bit defines the end of a data word within a data stream and is used in asynchronous data transfer:
Usually, a corresponding start bit is sent before a data word transmission starts. Start/Stop bits allow a receiving device to
recognize the beginning as well as the end of a data word inside a data stream. In principle, it is also a high signal, but the level
of the signal differs from the subsequent data word.
At the end, depending on the configuration, one or two stop bits are set to determine the explicit end of this data word. As a
consequence, no valid data word can be present between stop bits and the next start bit:
The receiving device will recognize these data as discardable and ignores them accordingly.
If data transfer issues cause a synchronisation lost, the device will look for predefined bit chains for re-synchronization.
Note:
If more than one Modbus device is on the same ID, they will all start sending data to serve the master's query.
Amongst other things, the start and stop bits within the network will begin cross talking, causing problems with the assignment of
valid and invalid data packets.
The Stop Bit function defines whether the CS141 should send one or two corresponding stop bits
