How to configure Modbus

For this configuration step, navigate to the following menu:

Since Modbus is standardized, the basic configuration is intuitive to handle. To configure the Modbus agent, go to services and open Modbus.

TCP Port 502

The TCP port 502 is a static port setting within the Modbus standard that cannot be changed or customized without leaving standards - This port value is hardcoded inside the CS141 source code.

Slave Address

The Modbus slave is the ID that makes a Modbus device addressable. The Slave ID mentioned by a master's query will cause this device to answer. This ID may be customized but can only exist once inside a Modbus network.

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 determines the data transmission speed, i.e., how quickly Modbus requests can be received and processed. It is essential that the master and slave devices are configured with the same baud rate, as mismatched settings will prevent successful communication.

Parity

When transmitting data as a bitstream, the parity bit serves as a safety mechanism that allows for error detection. The value of the parity bit is calculated by the sender and transmitted to the receiver. The receiver can then use the same mathematical algorithm to check whether the data has been corrupted.

Parity calculation can be based on either even or odd parity. Therefore, both sender and receiver must agree in advance on whether to use even or odd parity.

Example: even parity

When using even parity, the number of all '1' bits in the data to be secured is counted. The role of the parity bit is to adjust this number, if necessary, so that the total becomes an even number.

If the number of bits to be checked is already even, the parity bit must be transmitted as 0; otherwise, the overall parity would be incorrect.

The 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 synchronization lost, the device will look for predefined bit chains for re-synchronization.

The Stop Bit function defines whether the CS141 should send one or two corresponding stop bits

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.

Apply/Cancel

This function saves the entered data and restarts the corresponding services on the fly. A complete reboot of the CS411 is not necessary:

• Apply:                     Save changes and restart the service as required
• Cancel:                   Withdraw settings and return to current state.

Note:

The standard UPS Modbus addresses can be found in the appendix of the CS141 manual.

The CS141 uses a valid Modbus reading range from 0-65535. Some Programs like "Modbus Poll" use the same range. Other Modbus polling clients may use 1 - 65536 - in this case, you need to correct this by adding 1 to the original value.