Cabling
Type
Calculus uses a 4-core cable type as a communication cable between a device and the gateway: Fieldbus cables Modbus UNITRONIC BUS LD 2X2X0.22
Always use a suitable cable for RS-485 / Modbus RTU communication with the following specifications:
A cable with flexible conductors for handling.
A thickness of 24AWG or 0.22mm² of the conductors is sufficient.
Always use suitable wire sleeves for the flexible conductors.
Use twisted pair conductors for the differential data signal.
Use a third conductor, or second twisted pair, for the signal ground (GND).
An impedance of 120 ohms.
A separate mantle, or shielding, which you only connect to the earth on one side (earth/ground).
For short distances or a trouble-free environment, it is usually sufficient to:
Cable to be used with only 1 twisted pair for the differential signal.
The sheath can then be used for the signal ground (GND), this time connected to both sides of the cable.
However, be careful with this. Identifying and solving malfunctions is more difficult than causing them!
Databus topology
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.
Don't mix other topology structures in your data bus.
Data bus limits
Depending on the speeds used and risks of interference, lengths of up to 1,200 meters are achievable with a maximum of 32 connected devices.
Attention!
At long distances, terminal resistances of 120 ohms are required to avoid reflections.
Connecting RS-485
Make sure that the connection of a data bus is always de-energized to avoid short circuits on the communication bus.
Gateways sometimes have an internal battery, so to be sure that you work without voltage, it is best to disconnect the 3-pole detachable terminal block connector (green connector) from the gateway to connect to the data bus.
After assembly, reinsert the green connector into the gateway.
Attention!
Connection RS-485 to a green connector, with the screws facing up, is the same for all Calculus gateways:
Left: Data+ (brown)
Middle: Data– (white)
Right: GND (green)
C04 gateway
The connector for RS-485 on the C04 is always located at the bottom on the right side of the device, in the place provided for the expansion slot.
This connector is rotated relative to the other I/O connectors on the C04.
C07W gateway
With the C07W gateway, RS-485 / Modbus RTU is an option and therefore a connector that is brought out via an expansion card.
If you take the gateway with the side with the RS-485 expansion card in front of you, the connection in question is always on the far right, in the first rectangular recess for connectors of the expansion cards.
Attention!
Is the RS-485 connector not in this place? Then you probably have an old RS-485 card. This one has separate instructions for use. If these are not known to you and do you need help? Then create a ticket on my.calculus.group and we will be happy to help you.
C06W gateway
With the C06W gateway, the RS-485 / Modbus RTU connector is located at the top left of the gateway.
2. Connecting RS-485 to Devices (Modbus Slaves)
Consistently connect the Data+ and Data- correctly to all devices on the bus according to the daisy chain principle.
Tip!
In RS-485 systems, there is no fixed standard for labeling. There is a clear standard about the differential nature of the signals, namely "+" and "-".
This means that for some devices A is "+" and for some "-".
This also applies to B.
If there is no +/- indication when connecting, we make the assumption that A is +; if this does not work, we make the assumption that A is - .
See the 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 the B as +.
It is important that the + of the gateway is connected to the + of the device, otherwise this will lead to malfunctions.
To illustrate a Modbus daisy chain on an energy meter:
And a device at the end of the data bus:
Slave device settings
To enable communication between a gateway and a Modbus slave (e.g. a power meter), the settings on the modbus slave must match the settings on the gateway. The gateway settings can be consulted via the Calculus portal. How this works is explained in this article: Reading modbus RS-485 in settings C07W.
Calculus uses the following gateway settings as standard:
Baud rate: 9600
Data length: 8 bits
Parity: none
Stop bits: 1
The slave address must be unique for each device on the same bus. By default, we start with Slave ID 1 and the next device we give Slave ID 2 and so on.
Attention!
If the hardware is not pre-configured by Calculus, you as a customer must properly document the settings per device and pass this information on to Calculus for correct monitoring.
For higher data rates due to time-critical applications, or due to a large number of registers or slave devices to be read, this is usually discussed before installation and a plan of action is drawn up.