Electronic communications is all about interlinking circuits (processors or other integrated circuits) to create a symbiotic system. For SCAAD and Telemetry systems that to swap information, they must share a common standard communication protocol. Communication protocols have been designed to achieve data exchange.
Serial Data Communications
The most common serial communication protocols are RS232, RS485, RS422, USB, and Ethernet. But because USB and Ethernet require powerful interfaces with complex protocols, many efficient devices utilized RS232, RS485, and RS422 which is what this note is all about. For more information, read Application AN236(SerialComm)
Serial Protocol Comparison Chart
|Cable||Single ended||Single ended multi-drop||Multi-drop|
|Number of Devices||1 transmitter
|Communication Mode||Full duplex||Full duplex,
|Maximum Distance||50 feet at 19.2 kbps||4000 feet at 100 kbps||4000 feet at 100 kbps|
|Max Data Rate (50 feet)||1 mbps||10 mbps||10 mbps|
The Advantages of RS485 and RS422 vs RS232
RS485 and RS422 use differentiation. Two wires are required for each signal. The figure below shows a single RS485 / RS422 signal being transmitted. To transmit a logic 1, line B is high and line A is low. To transmit a logic 0, line B is low and line A is high. The advantage of this arrangement is that signals can be transmitted faster and over greater distances than is possible with a single wire.
The main differences between RS422 and RS485 is the types of communications allowed.
RS422 allows only one-way (simplex) communications between one driver, and as many as ten receiving devices. To control devices and need no feedback from them, RS-422 multi-drop network will work well. The output data driver from the master is always enabled. Full Duplex communication.
RS422 (One Master and up to 10 Slaves)
1 Driver up to 10 Receivers
RS485 was designed to address the multi-drop limitation of RS422, allowing up to 32 devices to communicate. In RS485 each driver can be switched off allowing multiple units to send data. The Output data drivers from the masters are disabled unless data is coming out of a master.
RS485 (Many Masters and Many Slaves)
Up to 32 Driver/Receiver Pairs
RS-422 and RS-485 transceivers are sometimes confused with each other. One type is assumed to be a full-duplex version of the other. RS-485 meets most all the RS-422 specifications, RS-485 drivers can be used in RS-422 applications. RS-422 drivers cannot be used with RS-485. 4-Wire RS-485 networks are the same as RS-422 Multi-Drop Networking. An RS-422 device is used as the master and an RS485 device is used as a slave in a 4-Wire RS-485 bus.
RS232 serial ports consists of just two communication wires:
A. one for sending data and another for receiving. As such, serial devices should have two serial pins:
B. the receiver, RX, and the transmitter, TX.
Communication data is based upon the voltage on the wires, relative to the ground level. A ground connection must be made between devices that communicate with RS-232. This is an RS232 wiring diagram:
A digital 1 voltage is: -3V to -25V
A digital 0 voltage is: +3V to +25V
RS232 data is sent serially, each bit is sent one after the next because there is only one data line in each direction. This mode of data transmission also requires that the receiver knows when the actual data bits are arriving so that it can synchronize itself to the incoming data. To achieve this a logic 0 is sent as a synchronization start bit.
RS422 (Single Driver Communications)
RS422 is designed to be tolerant of noise and forgiving of long cable runs. It is typically used between one transmitter receiver pair to one other transmitter receiver pair. Each output can drive up to 10 receivers. It achieves this by using a differential current drive output which has high immunity to noise. The noise immunity enables RS422 systems to operate over very long connections, much better than RS232, US, and Ethernet.
Each signal uses two wires to pass the data. The differential voltage on the A and B wires represent the digital value. If B>A the value is 1. If A>B then value is 0.
|Input Signal||A B||Output Signal|
A multi-drop wiring has many desirable advantages, RS422 devices cannot be used to construct a truly multi-point network. A true multi-point network consists of multiple drivers and receivers connected on a single bus, where any node can transmit or receive data.
A multi-drop wiring has many desirable advantages.
RS422 devices cannot be used to construct a truly multi-point network.
A true multi-point network consists of multiple drivers and receivers connected on a single bus where any node can transmit or receive data.
RS422 networks are often used in a half-duplex mode similar to RS485. The RS-422 products that can do multi-drop communications, are actually RS-485 4-wire systems.
RS485(Multiple Driver Communications)
RS485 is similar to RS422 upon which it is based. The main difference is that up to 32 transmitter receiver pairs may be present on the RS485 lines at one time. RS-485 is generally a 2-wire half duplex system. AN RS485 4-wire full duplex system is very similar to RS-422.
In RS485 half-duplex, only one signal pair (A, B) is used. When instruments are described as having an RS485 interface this tells you nothing for sure about the signals being transmitted. Usually though only the Transmit Data (TX) and Receive Data (RX) of a normal serial port are converted to RS485 or RS422. The other signals of the serial port are not used. Three arrangements are commonplace: Write only, 4-wire (full duplex) and 2-wire (half duplex). The “2-wire” RS-485 connection is shown below.
RS485 half-duplex can use just two wires to communicate with up to 32 device, one at a time.
Each device has the ability to turn off its output drivers, so only the one device that is linking to the host outputs a signal on its TX lines.
Wireless Data Communications
Serial data can be converted to RF signals in wireless data communication modems, and then the data can be sent over un-wired areas.
To extend range, Go Wireless for remote access over 10s to 100+ square miles, us an RV-M7 or RV-M21 data radio modem. They have RS232, RS485, and RS422 options.
Wireless Connection to SCADA Slave (Long Range. Many miles)
A single master device initiates all transactions on the network. Slave devices respond to the master’s queries, by returning data or performing an requested action. Slaves are often connected using RS485 or RS422 serial connections.
To Reduce Cost and increase flexibility, use a Wireless Remote Terminal Unit (RTU) with build in Slave Devices. Raveon’s RV-M21 radio modem can control lights, pumps, and read switches and many other things. M21 has digital IOs, switched DC, open collector switches, and analog inputs.
For Short Range SCADA systems: Use wireless technologies such as Bluetooth, Wi-Fi, or Zigbee. They work reliably over 1-10 meters.
Wide Area SCADA and Telemetry systems rely on low-power technologies operating on licensed radio bands and license free bands.
In the unlicensed bands, popular technologies are: LoRaWAN and Sigfox. Raveon provides many long-range wireless solutions.
For use on private licensed bands, long-range radio modems cover hundreds of square miles.
For Global Coverage: Systems utilize cellular technologies, such as the 4G based LTE-M and NB-IoT technologies and 5G in the future.
Cellular networks work well, but may not have coverage in all areas you need, and they do charge monthly fees. If you setup your own wireless SCADA or Telemetry system, you have not monthly fees, and it will keep working when Cellular networks fail or are over-busy.
Wireless Communication Range
The range of a communication system is determined by:
- Antenna gains and antenna heights.
- Transmit power
- Receive sensitivity
- Frequency of the system
- The energy per bit (data rate)
Raveon’s data radio modems have many different range limitations because every radio and every system has a variety of parameters that determine range. To see the advantages of LoRa technology long-range RF, see AN205(LoRa_Range) application note, and for general RF coverage information see this App Note: AN206(RadioCoverage)
Antenna gains and antenna heights can make hug improvements to the range of a communication system. Antenna performance is some of the most economical ways to improve a system. But Antenna gains are sometimes limited by regulatory rules or practical considerations.
Every RF engineer knows the equation for RF communication that shows the higher the frequency of the system, the more the loss.
L = C + 20 * log(D) + 20 * log(F)
C is 36.6 if D is measured in miles. L is the free space path loss. You can see that the loss goes up as the distance (D) goes up and as the frequency (F) goes up. If you use a frequency like 900MHz instead of 150MHz, the path-loss is 15dB more. This common formula is often cited as proof that the atmosphere attenuates high frequencies more than lower.