#RS-485
RS-485 interface in fire alarm systems.
Communication interfaces in fire alarm systems
Modern fire alarm systems are usually built on a modular principle. The system consists of several independent units connected to each other using some interfaces.
In the simplest case, this is a control panel (CP) and addressable expanders to which security or fire detectors are connected.
The control panel is installed at the security post, addressable expanders are mounted directly at the security facility in order to minimize the length of wires to the detectors.
Expanders are distributed throughout the security facility, detectors or actuators are connected to them, all that remains is to connect the expanders to the control panel.
There are a huge number of ways to do this.
These include dedicated wired communication lines, a radio channel, signal transmission over the power supply network, etc.
In this article, we will consider perhaps the most common communication interface – RS-485.
RS-485 Specification
The RS-485 interface (another name is EIA/TIA-485) is one of the most common standards for the physical layer of communication. The physical layer is a communication channel and a method for transmitting a signal (layer 1 of the OSI open systems interconnection model).
A network built on the RS-485 interface is made up of transceivers connected using a twisted copper pair – two twisted wires.
The RS-485 interface is based on the principle of differential (balanced) data transmission. Its essence lies in the transmission of one signal via two wires, with the original signal going via one wire (wire A), and its inverse copy via the other (wire B).
In other words, if one wire has «1», the other has «0» and vice versa. Thus, there is always a potential difference between the two wires of a twisted pair: at «1» it is positive, at «0» it is negative. It is this potential difference that transmits the signal.
This transmission method provides high immunity to common-mode interference.
Common-mode interference is interference that affects both wires of the line equally.
For example, an electromagnetic wave passing through a section of a communication line induces potential in both wires.
If the signal is transmitted by potential in one wire relative to the common, as in RS-232, then the interference on this wire can distort the signal relative to the common wire («ground»), which absorbs interference well. But with differential transmission, distortion does not occur.
The standard RS-485 interface has up to 32 transceivers on the bus. However, there are implementation options with a larger number of nodes (up to 256).
The length of the communication line is determined by the standard specification of 1200 m.
The line topology is linear, with matching resistances at both ends of the line.
Data transfer rates can reach 10 Mbps depending on the cable geometry and materials used in the insulation, the twisted pair will have a corresponding characteristic impedance, which is usually determined by its manufacturer.
The RS-485 specification recommends that this characteristic impedance be 120 ohms.
Features of using RS-485 in fire alarm systems
As can be seen from the specification data, RS-485 already in the base is quite suitable for the communication interface between the fire alarm system units.
Let's note all the important and subtle points.
Number of devices on the bus: if 32 may be too few, then in the design of receivers with increased input resistance their number can reach 256 — quite sufficient in almost all cases.
You just need to remember to specify the maximum number of devices in the system, so that there is a reserve for development. By the way, the control panel can also have several RS-485 ports.
Using a twisted pair as a communication cable should not cause any problems.
There are many offers on the market, it is better to choose a cable where the twisted pair is combined with power wires in a common outer shell.
The maximum length of 1200 m seems sufficient, in many cases this is true.
But sometimes you have to deal with long distances. Here you also need to take into account that the permissible topology of the communication line is linear.
That is, you need to sequentially go around all the devices in the system, combining them into a line and not forgetting to include matching resistances at both ends.
By the way, this is one of the few disadvantages of the interface, because with a linear topology, the total length of the line can be significantly longer than with a connection, for example, in a «star» or «tree».
Limitations on both length and topology are removed by using special devices — signal repeaters.
About the data transfer rate: 10 Mbps is hardly needed for fire alarm tasks. For transmitting alarms and any other types of notifications in the system, tens of kilobits per second are quite sufficient.
The optimal range of speeds is considered to be from 9.6 to 64 Kbps.
The thing is that if the speed is not very high, the requirements for other interface characteristics are significantly reduced.
For example, you can not strictly adhere to the linearity of the topology or exceed the maximum permitted length of the communication line. This, of course, is not very correct, but sometimes you want it that way!
Based on the author's experience, at a speed of 9.6 Kbps, you can allow yourself the following: at a length of up to a kilometer, you can completely abandon the linear topology, i.e. lay the cable as God puts it into your soul.
If the linearity is observed, it is possible to significantly increase the maximum length of the communication line without using additional repeaters (2–3 km is quite acceptable with a high-quality cable).
But this is all empirical and requires testing for stability in each specific case.
The RS-485 specification defines only the physical layer and says nothing about the organization of data transmission. However, most fire alarm systems use protocols built on the Master-Slave principle.
This principle determines the order of access to the bus.
Since only one device can transmit at a time, an arbitration system is required to eliminate conflicts when several transmitters simultaneously access the bus.
So, the Master-Slave method is the easiest way to do this. The master role is usually played by the control panel. It issues data to the bus addressed to a specific slave node (Slave). This data is a kind of request.
All devices receive this request, but only the one to whom the request was intended responds.
There is one important feature here: as a rule, the control panel sequentially polls all expanders, so the more addressable devices on the bus, the longer the polling period, the longer the information will be transmitted from the expander to the control panel. This should be taken into account when designing the system.
Using Repeaters
As mentioned above, the limitations of line length and topology can be overcome using repeaters.
A repeater is a device that has two transceivers and cuts into the gap in the line. When receiving data from one side, the repeater repeats it on the other side, restoring the signal level.
Thus, one repeater doubles the maximum length of the communication line. In total, up to 10 repeaters can be connected.
As a rule, repeaters have galvanic isolation between their sides, which allows you to isolate individual sections of the line from each other. In addition, the repeater allows you to make branches from the main communication line, which can be very useful when laying cable.
Using media converters
A big advantage of RS-485 is that there is a huge number of serially produced converters for organizing data transmission via various physical media. It is often inconvenient or impossible to use a traditional twisted pair.
In this case, you can move to another dimension — choose something more suitable for data transmission.
For example, a radio channel, if it is not possible to use a wired communication line. Or Ethernet, if there is already a ready-made network or you need to connect remote objects via the Internet.
In the case when it is necessary to organize data transmission over a dedicated line over a significant distance, it makes sense to pay attention to fiber optics.
Using single-mode fiber, you can organize a channel up to 20 km.
With a high level of electromagnetic interference, it also makes sense to use optics — an optical data transmission system is absolutely not sensitive to external interference and interference.