Yes. All Didactum Ethernet-based measurement and control devices support this Modbus-RTU USB adapter.
RS-485 is a differential signaling protocol. This means that the same data is transmitted (remotely) on both wires. However, this occurs with exactly opposite polarity.
If one wire carries logic 1, the other wire carries logic 0. By transmitting these two opposite signals over a twisted pair of wires, high data rates (baud rates) can be achieved. Cable lengths of up to 1,200 meters are supported. Any induced interference (noise) is effectively canceled out by the two opposite polarity signals.
The maximum number of devices is 32. If you are monitoring additional IP-enabled sensors and I/O contacts with your SNMP-compatible monitoring system, Didactum engineers recommend connecting a maximum of 16 Modbus RS485 devices.
A signal ground is absolutely necessary because the data is transmitted electrically and, to put it simply, a return channel is required.
A shielded cable in itself does not constitute a signal ground. If the shielded cable is connected at only one point, the wiring is correct. There is no signal ground if no third conductor (at both ends of the cable) is connected. If the shield is counted as a conductor, then a total of four conductors are required.
If the device has a common ground via a nearby power supply, a 2-wire RS-485 connection may work. Without grounding, this only works over a very short distance. In this case, the RS-485 transmitter-receiver chip forms its own "virtual" ground. This is created by currents within the chip's semiconductors. However, only a very low DC voltage is permitted. The DC voltage specification that the RS-485 transmitter-receiver chip tolerates is given in the respective chip manufacturer's datasheets. In our opinion, such a solution is not very practical.
Unfortunately, we cannot go into the individual specifications of the many cable manufacturers here. However, please take a look at the datasheets of the respective cables. Here you will find information on data transmission rates and possible cable lengths. Please note that most communication failures and interference are due to a poor signal ground.
To limit ground loop current, the EIA-485 specification recommends using 100-ohm resistors in series between each signal ground connection. This provides a signal ground sufficient to compensate for potential differences.
In such a case, use optically isolated RS-485 repeaters. These repeaters have two data lines on each side. Each side has its own signal-to-ground connection, which is necessary for reliable data communication. Internally, the RS-485 repeater requires two isolated power supplies, one of which is typically generated internally using a high-isolation transformer. Data is transmitted via optocouplers. The isolation ratings of these repeaters are usually in the range of 500 to 2000 volts.
It is important that both data lines are twisted pair. Sufficient noise reduction for RS485 cannot be achieved without twisted pair cable. Since a single twisted pair with a third single conductor is often not readily available, two twisted pairs should be obtained. The use of shielded cables is frequently recommended. The shielding protects the signals from interference, crosstalk, and other electrical devices. The wire gauge is specified in gauge. Typically, this is around 22 gauge. It is important to keep both the resistance and capacitance as low as possible. The longer the cabling, the more important it is to use a high-quality cable. Ask about cables specifically designed for data networks.
It is essential to ensure that the RS485 devices are not wired randomly. Maintain a very organized cable chain with consistent spacing between the RS485 devices.
Although RS485 devices may function without termination over short cable runs, a 120-ohm terminating resistor must be installed at each end of the cable. This termination is necessary to prevent reflections on the line. A missing termination can lead to communication errors. By protocol, Modbus RTU and BACnet MS/TP devices are required not to establish communication if signals are not sufficiently detected.
Yes. The expansion board has an integrated line bias voltage. This is always switched on.
Both the manufacturers of many RS485 chips and the RS-485 specification (EIA-485) refer to the two data lines as A and B. Even if your data connections are labeled A and B, it's possible that they are incorrectly labeled. This depends on whether the designer considered the chip specification or the protocol specification. To avoid this confusion, many Modbus RS485 devices are labeled with "+" and "-". If you are unsure about the polarity of the data lines and assume everything else is correct, simply try swapping the data lines. This will quickly reveal whether your Modbus RS485 devices are communicating successfully. Reversing the data lines will not damage the Modbus RS485 devices.
The "MS" in BACnet MS/TP stands for "Master/Slave." In MS/TP, all devices alternate between being masters and slaves. The only requirement is that all devices on the network must have unique MAC addresses (also called station IDs) and unique device IDs. Modbus RTU also works on the master/slave principle. However, there can only be one master within a Modbus RTU network. All other Modbus RS485 devices must be slaves! Furthermore, each slave must have a unique address. This prevents multiple slaves from responding simultaneously.