Overview of the common interfaces used in Domotics
a) Power Line Carrier system (PLC)
A chip solution for the transmission of metering values is offer by the PLC system. It consists in an assembly of electronic blocks that function like this: the information received from the counters (pulses or digital telegram) is counted or processed in order to obtain the accounting values.
These values, with all the additional information related to the integration of PLC in network (link layer, network layer, session layer, transport layer) will be integrated in telegram that will be use to modulate the power line voltage at a medium frequency.
The signal will be amplified and using and isolator coupling will be transmitted on the power network. In a similar way, from the power line, the PLC will receive and demodulate the signals transmitted by other systems.
Converting and filtering, the signal will be processed and interpreted by processor and will be transmitted to the executions elements. With the sensitive signal detection and sophisticated digital filtering technique, this PLC communication is highly immune to electrical noise and interference.
Figure 1 Modulator and demodulator system used for an AMR system that uses the power line that transmission support for data transfer
b) RS232 Interface
The RS2323 interface is use especially for the transfer between field instruments named Data Terminal Equipment (DTE) – and Data Communication Equipment (DCE) or MODEM assuring the transfer of information using a voltage modulation of signal and could be use for peer-to-peer connection. The logical levels are:
- “0” logic implies to drive the voltage between +3V to +15V for data transmission line, and –3 to –15V for “handshaking” signals.
- “1” logic implies to drive the voltage between –3V to –15V for data transmission line, and +3 to +15V for “handshaking” signals.
- The following signals are used for control the data flow:
Table 1 RS232 Signal description and pins for 9 and 25 pins connector
Description |
Signal |
9 pin DTE |
25 pin DTE |
Source DTE or DEC |
Carrier Detect |
CD |
1 |
8 |
from MODEM |
Receive Data |
RD |
2 |
3 |
from MODEM |
Transmit Data |
TD |
3 |
2 |
from DTE |
Data Terminal Ready |
DTR |
4 |
20 |
from DTE |
Signal Ground |
SG |
5 |
7 |
from MODEM |
Data Set Ready |
DSR |
6 |
6 |
from MODEM |
Request to Send |
RTS |
7 |
4 |
from DTE |
Clear to Send |
CTS |
8 |
5 |
from MODEM |
Ring Indicator |
RI |
9 |
22 |
from MODEM |
In figure 2 we present the connectors that could be use with this interface:
EIA 574 |
EIA 561 |
|
|
c) RS485 Interface
The RS485 interface is use to connect devices that work together on the same bus. The RS485 standard is a balanced differential system; the voltage produced by the driver appears across a pair of signal lines that transmit only one signal. A balanced line driver will produce a voltage from 2 to 6 volts across its A and B output terminals and will have a signal ground (C) connection.
Although proper connection to the signal ground is important, a balanced line receiver in determining the logic state of the data line doesn’t use it. The standard include for the corresponding drivers to use a control line that can enable or disable the line. Thus means that is possible to establish a control function for the drivers, in reception this driver will be disconnected from the line, and in transmission the lines will be place in TSL (high impedance mode).
Figure 3 RS485 Driver and the form of signal on bus
At reception a balanced differential line receiver senses the voltage state of the transmission line across two signal input lines, A and B. The polarity between the lines A and B will determine the logical value received. Thus, if the voltage received on input is at last 200mV, the logic state will be considered at 1 logic. The maximum voltage level taking into account will be 6V. For the o logic level, the polarity will be –and the limits of voltage will be comprised between -200mV and –6V.
On RS485 bus we can connect function of driver until 128 interface circuits (for details please read the data sheet form producers). Optical interface is used for the communication of data measured and of tariff established for different consumption periods of the day. Use a pulse-modulated signal that carries the information between the PDA or other mobile data concentrator and the energy counter. The specification of interface is established in the standard IEC 62056-21 Ed.1.0 (2002).
Ethernet interface is the most widely used local area network (LAN) technology and is used for approximately 85 percent of the world. The IEEE 802.3 specified the standard that was developed initially by Xerox Corporation. In parallel with the standard bus was developed the protocol TCP/IP, protocol base on the transmission of data packets.
Ethernet devices compete for access to the network using a protocol called Carrier Sense Multiple Access with Collision Detection (CSMA/CD), operating at Layer 2 (Data Link) of the OSI (Open Systems Interconnect) model. An Ethernet LAN may use coaxial cable, special grades of twisted pair wiring, or fiber optic cable. "Bus" and "Star" wiring configurations are supported.
Bluetooth interface is a standard for a small, cheap radio chip to be plugged into computers, printers, mobile phones, etc., that use for data transmission on a ISM band at 2.4GHz, and was conceive to cable-replacement technology. In Europe, a band of 83.5 MHz width is available; in this band, 79 RF channels spaced 1 MHz apart are defined.
Ericsson Company proposed the interface in 1999 and the Bluetooth chip was designed to replace cables by taking the information normally carried by the cable, and transmitting it at a special frequency to a receiver Bluetooth chip. Bluetooth wireless technology makes it possible to transmit signals over short distances between telephones, computers and other devices and thereby simplify communication and synchronization between devices. It is a global standard that:
- Eliminates wires and cables between both stationary and mobile devices
facilitates both data and voice communication - Offers the possibility of ad hoc networks and delivers the ultimate synchronicity between all your personal devices.
Bluetooth wireless technology provides a universal bridge to existing data networks, a peripheral interface, and a mechanism to form small private ad hoc groupings of connected devices away from fixed network infrastructures Bluetooth radio uses a fast acknowledgement and frequency-hopping scheme to make the link robust, even in noisy radio environments
Description of two/four wires networks
a) RS-485, RS-422 Interface
When a network needs to transfer small blocks of information over long distances, RS-485 is often the interface of choice. The network nodes can be PCs, microcontrollers, or any devices capable of asynchronous serial communications.
The RS-485 standard is flexible enough to provide a choice of drivers, receivers, and other components depending on the cable length, data rate, number of nodes, and the need to conserve power.
The interface popularly known as RS-485 is an electrical specification for multipoint systems that use balanced lines. RS-485 is similar to RS-422, but RS-422 allows just one driver with multiple receivers whereas RS-485 supports multiple drivers and receivers. The RS 422 drivers use one twisted pair of wires per signaland the drivers generate a differential signal.
b) RS-485 Features
An RS-485 network can have up to 32 unit loads, with one unit load equivalent to an input impedance of 12k. By using high-impedance receivers, you can have as many as 256 nodes. An RS-485 link can extend as far as 4000¢ and can transfer data at up to 10 Mbps, but not both at the same time. At 90 kbps, the maximum cable length is 4000¢, at 1 Mbps it drops to 400¢, and at 10 Mbps it drops to 50¢.
For more nodes or long distances, you can use repeaters that regenerate the signals and begin a new RS-485 line. Most RS-485 links use the familiar asynchronous protocols supported by the UARTs in PCs and other computers. A transmitted word consists of a start it followed by data bits, an optional parity bit, and a stop bit.
RS-485 is designed to be wired in a daisy-chain or bus topology. Any stubs be as short as possible. Most links use twisted pairs because of their ability to cancel magnetically and electromagnetically coupled noise.
c) RS-485 and RS-422 Physical Topologies
RS-485 and RS-422 are in wide use as an interface for telecommunications, industrial, medical, security and networking applications. The reasons for their popularity are low cost, flexibility and very desirable feature set.
The published TIA/EIA 485 and RS-422 standards define only the electrical characteristics of the drivers and receivers. They did not standardize such things such as cables and connectors, pin outs, bus arbitration, signaling protocols, or physical wiring topology. Many different implementations have come into use and they are often incompatible with each other.
1. Point to Point RS-422 or RS-485
This is the simplest configuration, just one driver and one receiver. If termination is used, it is only required at the receiver. Most RS-422 cables used to connect telecom or data-com equipment are point-to-point links. Generally twisted pair cable is preferred due to its noise canceling ability. Cables may include a single channel over one pair of wires or there may be multiple channels routed through a multi-pin connector and bundled cable.
Figure 4 Point to point connection
2. Bi-directional, Half Duplex (2 wire) RS-485
This is the classic RS-485 topology. It takes advantage of RS-485 capability to support multiple drivers on a bus. RS-422 devices should not be used in this configuration. This topology enables bi-directional communication from many nodes over long distances at low to medium data rates, all on a single pair of wires. It can implement a very functional, very flexible and very economical data network.
Because signals travel in both directions, this bus should be terminated at both ends to prevent reflections. Stubs used to connect each node to the common bus should be kept as short as possible. Only one node can drive the bus at a time, making this a half-duplex communication channel.
Only one driver should be active at any one time. Therefore the transceivers used on a half-duplex bus should support a Driver Enable (DE) function. Each node should be configured to listen for its node address and to act upon data or commands matching its address.
Figure 5 “Two Wire ” Half-Duplex Network
Usually a two wire interface with multiple peripherals uses a 'polled-response' half-duplex software protocol where each device has a unique device code. Generally, this requires externally powering adapter, but for short cable runs to non-terminated equipment operating at medium baud rates (like 9600 or less) an external power supply may not be needed.
The designer should use twisted pair wire with a impedance of 100 to 120 ohms. Low-cost CAT-5 UTP (Unshielded Twisted Pair) wire works fine. The user must externally wire the cable with pin 3 connected to pin 7 (TX+ to RC+) and pin 8 to pin 2 (TX- to RC-) on the RS-485 connector side of the adapter. The 2 conductor cable which connects to the outside world is then wired to pin 7 (+DATA) and to pin 2 (-DATA) of the RS-485 compatible peripheral. The photo below shows a typical transmission and reception over an RS-485 2-wire interface
Figure 6 Typical 2-wire RS-485 interface
3. Bi-directional, Full Duplex (4 wire)
The 4-wire topology simplifies bus arbitration in multi-node RS-485. A single Master node is the only driver allowed on the topmost wire pair. All other nodes listen to all data traffic that passes on the “party-line” type multi-drop bus.
Nodes may transmit on the lower pair of wires when addressed by the master node or by using a token-passing bus arbitration scheme. The master node may drive its bus while any of the slave-nodes are driving the lower pair, making this 4-wire network a full-duplex communication channel. All communication occurs from master to slave or slave to master, so any peer-to-peer communications must be routed through the master node.
Figure 7 “ Four Wire ” Full-Duplex Network
A 4-wire topology can simplify the problem of bus arbitration. The master node is the only driver allowed on the topmost pair. All other nodes transmit on the lower pair only when given permission. If bus contention does occur on the lower pair the master node can command all slave-nodes to stop driving.
A 4-wire RS-485 interface uses 2-wires to transmit to all connected peripheral receive data lines, and 2-wires connected from all peripheral transmit pairs back to the server's receive wire pair. In such an arrangement, the server can be transmitting and receiving at the same time (full duplex) and no 'polled-response' protocol or unique device code is necessary resulting in an overall improvement in communication speed.
RS-485 and RS-422 networks are sometimes said to be complex to set up. One reason for this is that there is no single bus topology defined in the basic standards documents. However this flexibility makes it possible to implement a range of different topologies that can fill many needs for low cost, high speed, reliable data communication.