1
.
General information
This manual is about connecting the RS-485 / Modbus RTU connection to a Calculus gateway.
2
.
Cabling
1. Type
Calculus uses a 4 core cable type communication cable to connect a device and the gateway: UNITRONIC BUS LD 2X2X0.22 - Fieldbus cables Modbus
Always use a suitable cable for RS-485 / Modbus RTU communication with the following specifications:
- Use a cable with flexible conductors for manageability.
- A thickness of 24AWG or 0.22mm² of the conductors is sufficient.
- Always use appropriate conductor sleeves for the flexible conductors.
- Use twisted pair conductors for the differential data signal.
- Use a third conductor, or second twisted pair, for signal ground (GND).
- An impedance of 120 ohms.
- A separate jacket, or shielding, which you connect only on one side to the earth ground.
For short distances or interference-free environments, it is sufficient usually to:
- Use a cable with only 1 twisted pair for the differential signal.
- To then use the sheath for the signal ground (GND), this time connected at both ends of the cable.
Be careful with this, however. Identifying and solving malfunctions is harder than causing them!
2. Topology data bus
Always connect devices in a ring according to the "daisy chain" principle and limit the length of the separate conductors (stubs) from the bus to the slave devices to 1 meter. See also the following image for an illustration.
Do not mix other topology structures in your data bus.
3. Data bus limits
Depending on the speeds used and risks of interference, lengths of up to 1,200 meters are feasible with a maximum of 32 connected devices.
Caution! For long distances, end resistors of 120 ohms are required to avoid reflections.
3
.
Connecting RS485
1. Connect RS-485 to Calculus gateways
Make sure that connecting a data bus is always performed de-energized to avoid short circuits on the communication bus.
Gateways sometimes have an internal battery so to be sure to work de-energized, it is best to disconnect the 3-pin removable terminal block connector (green connector) from the gateway for connection of the data bus. After mounting, reinsert the green connector into the gateway.
Connection RS-485 on a green connector with the screws visible upward is the same for all Calculus gateways:
Left: Data + (brown)
Middle: Data - (white)
Right: GND (green)
See image below:
1.1 C04 gateway
The connector for RS-485 on the C04 is always located bottom on the right side of the device, at the location provided for the expansion slot. This connector is rotated relative to the other I/O connectors on the C04.
1.2 C07W gateway
With the C07W gateway, RS-485 / Modbus RTU is an option and consequently a connector brought out through an expansion card. As you take the gateway with the side with the RS-485 expansion card in front of you, the the connector in question is always on the far right, in the first rectangular recess for the connectors of the expansion cards.
Note: Is the RS-485 connector not in this location? Then you probably have an old RS-485 card. This has separate user instructions. If these are not familiar to you and you need help? Then create a ticket at my.calculus.group and we will will be happy to assist you further.
1.3 C06W gateway
On the C06W gateway, the RS-485 / Modbus RTU connector is located on the top left of the gateway.
2. RS-485 connecting to devices (Modbus Slaves)
Consistently connect the "Data +" and "Data -" correctly to all devices on the bus according to the "daisy chain" principle.
In RS-485 systems, there is no defined standard for the labeling. However, there is a clear standard about the differential nature of the signals, namely "+" and "-."
This means that on some devices A is "+" and on some is "-." This also applies to B.
If there is no +/- indication when connecting then we make the assumption that A is +; if this does not work we make the assumption that A is -.
See image below for an example of connections with differences between the labeling. In this example, the ASPAR and DSE have A as +, the Siemens PLC has B as +.
What is important is that the + of the gateway is connected to the + of the device otherwise it will lead to malfunctions.
To illustrate, a Modbus daisy chain on an energy meter:
And a device on the end of the data bus:
4
.
Slave device settings
To enable communication between a gateway and a Modbus slave (e.g., an energy meter), the settings on the Modbus slave must match the settings on the gateway.
The gateway settings can be accessed through the Calculus portal. Their operation is explained in the highlighted text on this page.
By default, Calculus uses the following gateway settings:
- Baud rate: 9600
- Data length: 8 bits
- Parity: none
- 1 stop bit
The slave address of each device on the same bus must be unique. By default, we start with Slave ID 1 and give the next device Slave ID 2 and so on.
If the hardware is not pre-configured by Calculus, you as the client must document the settings per device properly and pass this information on to Calculus. This way proper monitoring can be guaranteed.
For higher data rates due to time critical applications, or due to a large amount of registers or slave devices, this is usually discussed before installation and a plan of action is created.
