Category Archives: Guides

What is an LDO and How Does it Work?

When you need a voltage regulator for your circuit and do not have much of a voltage head room, the trick is to use an LDO or a low-dropout regulator. Normal regulators need voltage headroom of roughly around 3V to allow good regulation, but LDOs can do with a lot less – of the order of a few 100 millivolts. However, there are other considerations as well.

To regulate and control an output voltage, it is necessary to source it from a higher input voltage supply. For normal regulators, the voltage headroom or the difference between the output regulated voltage and the minimum input unregulated voltage must be more than 3V. For example, if you need a regulated voltage of 5V, it must be sourced from a minimum input voltage of 8V. That ensures the regulated output voltage never dips below 5V. With circuits getting more complex and noise sensitive, new designs must deal with higher currents and lower voltages. Hence, headroom voltages of 3V or more may not be available in all cases, and it is necessary to use LDOs.

Although manufacturers offer datasheet specifications for basic parameters of regulators, they cannot list all parameters for every possible circuit conditions. Therefore, to use the LDO in the best possible manner, designers must necessarily understand the key performance parameters of the LDO and their impact on given loads. A close analysis of the surrounding circuit conditions helps to determine the suitability of a specific LDO.

In applications, LDOs primarily isolate a sensitive load from a noisy power source. The pass transistor or the MOSFET regulating and maintaining the output voltage accurately is always on and dissipates continuous power. This is different from switching regulators, which work as on-off switches. That makes LDOs less efficient and designers must handle the thermal issues related. System power requirements primarily drive the use of LDOs as voltage regulators. Since they are linear devices, they are also used for noise reduction and for fixing problems related to EMI and PCB routing.

As the power dissipation of an LDO is primarily governed by the current through it, LDOs are an obvious choice for very low current loads, bringing with their use simplicity, cost economics, and ease of use. For load currents of more than 500mA, designers must consider other parameters also, such as the dropout voltage, load regulation, and transient performance.

LDOs comprise three basic functional elements – a pass element, a reference voltage, and an error amplifier. Under normal operation, the pass element behaves as a voltage controlled current source. A compensated control signal from the error amplifier drives the pass element. The error amplifier senses the output voltage and compares it with the reference voltage. LDO regulator designs use four different kinds of pass elements – PNP transistor based regulators, NPN transistor based regulators, P-channel MOSFET-based regulators and N-channel MOSFET-based regulators.

While using a specific LDO in their circuits, designers need to consider the performance of the LDO with respect to its dropout voltage, load regulation, line regulation, and the power supply rejection ratio or PSRR.

What are Polymer and Hybrid Capacitors?

The growing complexity of active electronic components and their applications has resulted in the use of different types of passive components, especially capacitors. The advances in conductive polymers now offer a universe of capacitors for embedded systems applications and others.

Some advanced capacitors use conductive polymers for their electrolyte. Others such as hybrid capacitors use the conductive polymers in conjunction with a liquid electrolyte. Both these polymer-based capacitors offer improved characteristics over conventional ceramic and electrolytic capacitors, namely, life cycle, safety, longevity, reliability, stability, ESR or Equivalent Series Resistance and voltage rating. These special hybrid and polymer capacitors show distinct performance advantages in terms of ideal voltages, environmental conditions, and frequency characteristics.

Polymer Capacitors

Layered Polymer Aluminum Capacitors: These use conductive polymer as the electrolyte with an aluminum cathode. They operate within a voltage range of 2-25 VDC and manufacturers make them in capacities of 2.2-500 µF. Packaged in molded resin as low profile SMDs, they offer very low ESR.

Wound Polymer Aluminum Capacitors: Although they use conductive polymers and aluminum, they are constructed with a wound foil structure. They operate over a wider voltage range of 2.5-100 VDC and their capacities range from 3.3-2700 µF. With low ESR values, the capacitors are packaged as SMD, although layered capacitors are more compact in comparison.

Polymer Tantalum Capacitors: They use a tantalum cathode and conductive polymers as electrolyte. They are available in capacitance values of 2.7-680 µF and their operating voltage ranges from 1.8-35 VDC. Among the most compact capacitors on the market, polymer tantalum capacitors are available in SMD packages.

Hybrid Polymer Aluminum Capacitors: These use a combination of conductive and liquid polymers as electrolyte along with an aluminum electrode. The polymer offers low ESR as well as high conductivity. The liquid electrolyte offers higher capacitance ratings as it has a larger surface area, while being able to withstand high voltages. These capacitors come in a capacitance range of 10-330 µF with voltage range of 25-80 VDC. Although compared to other types, the ESR value for hybrid capacitors are on the higher side, the values are far lower than what conventional capacitors offer.

Advantages of Polymer Capacitors

Although different in material and construction, the four types of capacitors share common desirable electrical properties.

Superior Frequency Characteristics: As polymer capacitors have very low ESR, the impedance at their resonance point is also very low, resulting in reduced ripple in power circuits by nearly five times when compared to that produced by conventional tantalum capacitors.

Capacitance Stability: Ceramic capacitors tend to drift in response to DC bias and temperature. Polymer capacitors are devoid of such problems. This stability is of importance in automotive and industrial applications, where the operating temperatures vary broadly. Even under common operating conditions such as high temperatures and high frequencies, where ceramic capacitors show an effective capacitance loss of over 90%, polymer capacitors remain stable.

Enhanced Safety: Conventional capacitors can short circuit and fail, and these are causes for safety issues. Mechanical stresses or electrical overload can create discontinuities or defects in the oxide films that forms the dielectric leading to safety failures. The self-healing capability of the polymer capacitors eliminates such failure modes.

PIXY: Versatile CAM for Your Raspberry Pi

If you are looking for a small, fast, low-cost, easy-to-use, and readily available vision system for your Raspberry Pi or RBPi, then the Pixy can be a great choice. Pixy or CMUCam5 is somewhat more than a normal camera that you may have used so far for your single board computer. It comes with several features not available on most camera systems.

First, Pixy is versatile – use it for all kinds of projects. Along with the hardware, you will receive all kinds of information – PCB layout, bill of materials, schematics, and other hardware documentation. All software/firmware is GNU-licensed and open-source. The configuration utility provided with Pixy runs on all platforms – Windows, MacOS, and Linux. RBPi can communicate with Pixy over one of several interfaces – analog/digital output, USB, UART, I2C, or SPI. The Pixy comes with all libraries for RBPi, BeagleBone, and Arduino and supports programs written in Python and C/C++. The cable provided with Pixy can connect directly to Arduino, and it also works with BeagleBone and RBPi.

On the performance side, Pixy can learn to detect and recognize objects that you have taught it and outputs what it detects 50 times per second. With a Pixy, an RBPi and a servo control board, you can reconstruct Wall-E, the waste-collecting robot.

Pixy resulted from a partnership of the Carnegie Mellon Robotics Institute with Charmed Labs. First started as a Kickstarter campaign, Pixy is now the most popular vision system since it first started selling in March 2014. You can gage the versatility of Pixy from the activities it can do in association with an RBPi – pick up objects, chase a ball, locate a charging station, and more – doing all this with a single vision sensor.

Although there are other vision systems that can sense or detect practically anything, almost all of them have two drawbacks. One, they output huge amounts of data, a few megabytes per second. Two, enormous computing power is necessary to process this data, leaving the attached SBC with little else to cater to other tasks.

Pixy gets around these barriers as it pairs a powerful and dedicated processor along with its image sensor. The processor does all the processing of the data captured by the image sensor, and sends only the relevant information to the attached SBC. For example, yellow ball detected at x=50, y=110. Therefore, the RBPi can easily talk to Pixy and still have enough computing power left over for other activities. That also means you can have multiple Pixy cams hooked up to your RBPi. For instance, you can make a robot with a 360-degree sensing capability with four Pixys.

Although Pixy began with interfacing capabilities with the Arduino controller, it has matured sufficiently to be able to communicate with other controllers as well. The Pixy comes with all sorts of software libraries and a Python API for connecting to Linux-based controllers, such as an RBPi.

On-board Pixy is a color-based filtering algorithm that helps in detecting colored objects. The popular color-based filtering method makes Pixy singularly fast, efficient, and relatively robust. Pixy examines each RGB pixel from the image sensor and computes the saturation and hue to use as its primary filtering parameters.

Fanless Mini-PC Consumes only 5W

Industrial control applications, digital signage and thin client users require low-power computer systems. The F200 mini PC from Giada Technology is an ultra-compact unit measuring only 4.6 in x 2 in and a thickness of only 1.2 in. Other desktop PCs use up more than 100 W of power, but the F200 takes up only 5 W at full load. At this level of power consumption, there is no need for cooling, and consequently, the mini PC is a fanless unit.

The fanless F200 mini PC takes up the minimum real estate on your desktop. With a VESA mount, you can do a clean installation of this mini PC on the back of a monitor or display, where it fits easily. An Intel Celeron N2807 processor with dual cores powers the mini PC, and it can operate at up to 2.16 GHz. With 8 GB of DDR3 DRAM and 16 GB of eMMC flash directly soldered on its motherboard, the fanless F200 gives out very little heat. The sturdy build resists shocks and vibrations. You can operate the F200 with Android, Linux, Windows 7 or Windows 8.1. If you want to add a solid-state disk of your choice, F200 has an mSATA II slot as well.

Unattended operations on the F200 are easy because of its built-in capabilities. You can schedule power on and off, and program it for auto power-on after a power failure. Therefore, F200 is eminently suitable for simple digital signage and other industrial installations. For those looking for pushing signage content over a network, the F200 offers a SIM card slot for Wi-Fi, BT or 3G module connectivity.

The F200 ultra compact mini PC can act like a virtual desktop. As industrial environments can be typically harsh, Giada Technologies has made F200 durable, noise proof and dust resistant. Its Intel Celeron Processor N2807 operates with two cores, 2 threads at 1.58-2.16 GHz, reaching a TDP of 4.3 W.

Display interfaces on the F200 consist of Intel HD Graphics, with Microsoft DirectX 11 on a single HDMI port. With an optional VGA output, the display resolution can be 3200 x 2000 at 60 Hz for DP, or 3840 x 2160 at 24 Hz for HDMI.

F200 offers three expansion slots. The first is for a SIM card capable of connecting 3G enabled Wi-Fi modules. There are two Mini-PCI Express slots where you can connect full-length mSATAII SSDs, full-length PCIe, USB Wi-Fi & BT or a 3G enabled module.

The F200 is rather rich in IO interfaces. A single port offers Microphone and audio in/out. A Realtek Gigabit Ethernet Controller connects to a single RJ45 port on the back panel. There are two USB2.0 ports and a single USB3.0 port, one COM port, and one SIM card slot. 12/19V DC in is through a Jack on the back panel. Optional ports include IEEE 802.11 ac/b/g/n, a Bluetooth module and an IR module.

Several built-in features provide system management on the F200. For example, there is JAHC support, a watchdog timer, auto power on, wake on LAN and RTC wake up to control the mini PC. The operating temperature range is from 0-40°C.

AT21CS01 from Atmel: This EEPROM Does Not Require External Power Source

AT21CS01 from Atmel is a two pin serial EEPROM. Astonishingly, it does not have a Vcc or power supply pin characteristic of any IC and does not require an external power source to work. This amazing memory IC operates with only a data pin and a ground pin. The memory in the IC is organized as 128×8 bits, that is, a total of 1-kbits.

The single-wire device, AT21CS01, operates with only an SI/O and GND pin. The SI/O signal functions as a combination of data and power line. That means, apart from moving data in and out of the IC, the SI/O pin also provides power to the device. During high time of the protocol sequence, the IC’s parasitic power scheme provides the IC with power.

Each AT21CS01 is factory programmed to include a unique serial number of 64-bits. The SI/O line can be accessed directly from outside the application, because the device complies with the IEC 61000-4-2 ESD tolerance. This memory IC comes in 4-ball WLCSP, 8-lead SOIC and 3-lead SOT23 packages. Market availability is slated for the fourth quarter of 2015.

Possible applications for AT21CS01 include ink and toner print cartridge identification, storing data for analog sensor calibration and management of after-market consumables. There are several advantages in using AT21CS01.

Manufacturers claim AT21CS01 consumes 33% lower power in its active mode when compared to devices offered by the competition. For instance, at 25°C, the typical write current for an AT21CS01/11 is 200 µA, the typical read current measures about 80 µA, and a typical standby current of 700 nA. Each memory location can endure 1 million write cycles.

With such features, the AT21CS01 is eminently suitable as identification markers for cables, batteries, consumables, wearables and IoT. To support different voltage requirements, the AT21CS01 comes in two variants. AT25C501 is suitable for applications operating in the range of 1.7 to 3.6V. However, when operating with Li-Ion or polymer batteries, applications require higher voltage ranges, such as 2.7 to 4.5V, for which, the AT21C511 is suitable.

With its ultra-low active and standby currents, the AT25C501 beats the competition by consuming at least one third less power. The single-wire interface follows the I2C communication protocol. This IEC 61000-4-2 Level 4 ESD compliant device can withstand discharges of +8KV in contact and +15KV in air.

The innovative memory is organized into for zones of 256-bits each, with a security register additional to the 1 Kb memory space. Each EEPROM has a 64-bit serial number programmed at the factory and includes 16-bytes extra for user programmability. That means the user can improve on the uniqueness of the serial number on each device.

The advantages of using AT25C501 are many. The designer needs only one pin from the ASIC/MPU/ASSP/MCU. Because of its smaller footprint, layout is simple and the consumed PCB area reduces. That makes it easy to integrate identification capabilities in cables and or consumables. Its lower energy consumption is a boon for instruments working on batteries. The high-speed mode of AT25C501 even in low power applications results in high performance.

An Energenie Pi-Mote controller Board for Your Raspberry Pi

Those looking for a low-cost automation and home control solution can use the Pi-Mote controller board from Energenie. The Pi-Mote controller board is an add-on for your single board computer, the Raspberry Pi or more simply, RBPi. With this combination, you can control electrical appliances connected to special radio controlled electrical sockets.

Working at 433.92 MHz, the Pi-Mote controller board for radio-controlled sockets is easy to install and command. The product offers a safe and simple way to let your RBPi control mains powered devices and appliances. The Pi-Mote controller board from Energenie is compatible with all RBPi models such as the A, A+, B, B+ and B2.

The Pi-Mote controller has a range of up to 30 meters and puts out an output power of 3V, 27mA at +12 dBm. The output is encoded at four data bits, offering a 20-bit address pre-set with OTP. The user can select the output modulation with software from OOK or FSK.

The product actually comes in two parts, the RF board and the electrical socket. The RF board attaches to the RBPi for controlling several 13A, 3-pin electrical sockets. Although the original Energenie sockets are meant for use in the UK, plug adapter sockets are available, which make these almost universal. You can also get kits with a 4-way extension lead and other compatible sockets from Energenie. All can be controlled from the Pi-Mote controller board.

A small Python program allows the add-on RF transmitter board to control up to 4 radio controlled sockets simultaneously by toggling the socket on and off individually. The add-on board attaches to the GPIO pins of the RBPi. In its basic form, each board transmits a frame of information to the sockets. The frame is made up of a 20-bit address and a 4-bit control data. Additionally, the frame uses the On-Off Keying or OOK technique, a basic form of Amplitude Shift Keying or ASK. The source addresses are pre-programmed and the user cannot change them.

When using the Pi-Mote controller, you are required to insert the radio-controlled socket into the mains wall socket and switch it on. The socket then enters a learning mode, which is indicated by the slowly flashing LED in front of the socket housing. You can force a socket to enter the learning mode at any time by pressing the green button on its housing form, holding it for five seconds and releasing it.

Once it is in the learning mode, send the socket a signal from the program running on the RBPi. The LED on the socket housing gives a brief flash and stops glowing. This indicates the socket has accepted and memorized its address. You can then program the rest of the three sockets in turn; otherwise, they will react to the same address. When using more than one socket, insert each into separate mains wall outlets, maintaining a physical separation of at least 2 meters so they do not interfere with each other. The sockets must not be put into a single extension lead.

Comparing Wireless Standards 802.11ad & 802.11ah

Wireless LAN standards were first set up for serving the needs of laptops and PCs in homes and offices. These were IEEE 802.11a and b, and these later served to allow connectivity in different places such as in shopping malls, Internet cafes, hotels and airports. The main functionality of the standards was providing a wireless link to a wired broadband connection for email and Web browsing.

Initially, speed of the broadband being a limited factor, a relatively slow wireless connection was enough. Therefore, 802.11a offered up to 54Mb/s at 5GHz and 802.11b up to 11Mb/s at 2.4GHz, with both frequencies being in the unlicensed spectrum bands. To reduce interference from other equipment, both standards were heavily encoded using forms of spread-spectrum transmission. In 2003, a new standard 802.11g used the 2.4GHz band maintaining the maximum data rate of 54MB/s.

However, by this time, people started realizing the need for higher throughput, especially with increased data sharing amongst connected devices in the home or small office. By 2009, a new standard, 802.11n came up, which improved the single channel data rate to over 100Mb/s. The new standard also introduced spatial streaming or MIMO, multiple inputs, multiple outputs. The new modems had up to four separate transmit and receive antennas, carrying independent data that was aggregated in the modulation/demodulation process.

However, new WLAN usage models were continually raising the demand on throughput, such as projection to TV or projectors, streaming from camcorders to displays, video streaming around the house, airplane docking, public safety mesh and more. Catering to these VHT or very high throughput demands made it necessary to generate two new standards 802.11ac (an extension of 802.11n) and 802.11ad.

Standard 802.11ac runs in the 5GHz band, providing a minimum of 500Mb/s on a single link and 1Gb/s overall throughput. On the other hand, 802.11ad provides up to 6.7Gb/s using a spectrum of about 2GHz at 60GHz, but at short range. Operation at high frequencies limits the transmission range and obstacle penetrating capacity of the signals.

With the proliferation of local sensor networks working on low power, billions of IoT or Internet of Things and M2M or machine-to-machine device connections, a new standard is now deemed necessary. This new standard is the 802.11ah, working in the license-exempt 1GHz band and its final version is expected in 2016.

Standard 802.11ah is a down-clocked version of the 802.11ac standard. While adding some enhancements in the MAC and PHY layers, the new standard offers advantages such as power savings, multiple station support, better coverage and mobile reception.

For the standard 802.11ah, three main use-case categories are under consideration. These are Wi-Fi extended range networks, backhaul networks for sensors and meter data and sensor networks. The standard 802.11ah extends the transmission range with 1 and 2MHz mandatory modes, allows ultra-low power consumption, thereby offering multi-year battery life for large scale sensor networks and is optimized for long sleep times while handling small packet sizes.

Therefore, with 802.11ah, you can have several devices such as light sensors, temperature sensors and smart meters set up throughout the home, enabling your home devices and appliances to be considered smart.

Why is WAM Better than QAM?

Almost all wired and wireless applications today use the QAM or Quadrature Amplitude Modulation. These include the Fiber infrastructure, Wireless Backhaul, DSL modems, Cable modems, Cable TV, Satellite TV, Wi-Fi, Cellular and numerous other communication systems.

QAM systems use two AM or Amplitude-Modulated signals combined into a single channel – increasing the effective data rate while using the same amount of bandwidth. A QAM signal has two carriers, each with the same frequency, but differing in phase by 90 degrees. The term quadrature arises from the difference of one quarter of a cycle. If you call one of the amplitude-modulated signals the in-phase signal, the other becomes the quadrature signal.

Typically, the quadrature signal, multiplied with a sine wave, is subtracted from the in-phase signal multiplied with a cosine wave. The resulting signal is then amplified and transmitted over the air, wires or cables.

At the destination, a reversal of operations takes place by multiplying the in-phase output by a cosine wave and the quadrature output by a sine wave. Filtering them individually leaves only the lower frequencies. As these operations are entirely reversible, they ensure the preservation and exact recovery of the originally transmitted data.

Proliferation of communications devices and systems is putting considerable stress on bandwidth availability. For the past 40 years, QAM has completely dominated the advanced communication systems. Consequently, there are over seven billion connected devices using QAM technology.

Lately, a new Wave Amplitude Modulation or WAM technology is challenging this dominance of QAM. MagnaCom, who has patented and trademarked the WAM technology, claims the use of WAM can enhance nearly all wired and wireless applications. Additionally, WAM being backward compatible to legacy QAM systems, its use will not require any changes to the RF, radio or the antenna.

WAM technology uses a purely digital modulation scheme and is scalable. While using the same analog and RF circuits that QAM does, WAM needs no redesign and consumes only about one square millimeter space in modern semiconductor design. As the WAM technology is scalable, designers can now implement a smaller and lower cost solution.

Speaking technically, WAM represents a multi-dimensional signal construction technique working in the Euclidean domain. For the first time, designers can break the orthogonal signal construction. This provides an optimal handling of nonlinear distortion, increasing the system capacity. Overall, there is significant improvement over the legacy QAM systems.

The benefits of using WAM include a system gain advantage of over 10dB, while increasing the distance covered by over four times at half the power. This results in double the spectrum savings, offering better noise tolerance, major increase in speed and easier design at lower costs. Additionally, there is no need to replace any of the existing QAM equipment, as WAM is entirely backward compatible with QAM.

Compared to QAM, the new technology modulates information differently resulting in major system benefits. The use of spectral compression allows WAM to improve spectral efficiency by enabling an increase of the signaling rate. This allows reduction of complexity to a lower order. The use of nonlinear signal shaping by WAM offers inherent diversity of time and frequency domains, resulting in a lower cost and lower power design of the transmitter.

Get VGA from your Raspberry Pi

Those of you who use the single board computer, the Raspberry Pi or RBPi, know that it has two video outputs. It offers high definition video via the HDMI port and a composite video via the RCA port. For viewing the output of the RBPi on a VGA monitor, one must use an HDMI to VGA adapter or similar. However, there is a simpler and cheaper method now available – the Gert VGA 666.

The Gert VGA 666 is a breakout/add-on board, useful only for the RBPi Model B+. The board does not work on other RBPi Models such as A and B, as it requires the additional GPIO pins that are only available on the Model B+. Gert van Loo has designed this Gert VGA 666 board and has released it as an open source hardware design. Incidentally, Gert van Loo was associated with the initial design of the original RBPi and is one of the architects of the BCM2835 chip that forms the heart of the RBPi.

The Gert VGA 666 is a useful and neat solution for attaching a VGA monitor/screen to your RBPi. Additionally, this works out much cheaper than buying a converter or adapter for converting HDMI to VGA. A parallel interface from the GPIO pins drives the hardware natively for the VGA connection, using the same CPU load as the HDMI connection does. Users have the added advantage of setting up a dual screen, one for HDMI and the other for VGA. This is possible as the RBPi can drive both interfaces at the same time. With no CPU load, you can expect a VGA video display with resolution of 1080p60 or 640×480.

You can buy this adapter in the form of a kit, comprising the PCB for Gert VGA 666, a 40-pin header connector for the GPIO, a 15-pin female VGA connector, 20 through-hole resistors and two Pi supply stickers. When assembled and fitted on the RBPi, the board uses up nearly all the GPIO pins on the Model B+. Therefore, it will not be possible to use any other add-on boards at the same time when using the VGA adapter.

The decision to offer the adapter as a kit stems from the requirement of meeting EMC compatibility regulations. A fully assembled board would be required to meet most EMC regulations. However, these regulations do not cover the kit, as it is a homemade electronic product.

After soldering the board, plug it into the RBPi and power up the combination. However, the adapter does not work directly and you will need an intermediate solution for video output. You can use either an HDMI or a DVI-D monitor. If that is not available, use a composite monitor or TV via the RCA port. However, using the composite video means you will need to program the NOOBS on the RBPi.

After booting, you must install the necessary drivers for the Gert VGA 666 adapter. This requires an Internet connection, preferably via an Ethernet connection. If you simply plug in the Ethernet cable, Raspbian will automatically start to use it.

RS-485 – The Wired Communication Standard

TIA/EIA-485 is a popular wired communication standard published by the TIA/EIA or the Telecommunications Industry Association/Electronics Industries Association. This standard is also known as the RS-485 and uses differential signaling enables the standard to transmit data over long distances for factory automation and in noisy industrial environments.

This is because differential signaling allows rejection of common mode noise, while the twisted pair cable ensures the most received interference comes as common mode. When used over long distances, the standards improve the chances for ground potential differences, while the wide CMR or common mode range of the standard ensures that the network operates satisfactorily, even when there are large common mode voltages present.

In practice, both the transmitter and the receiver have non-inverting and inverting pins. Bidirectional communication over a single cable can use half-duplex devices, where the corresponding Receiver and Transmitter terminals connect to the same IC pins. Networks can also use two cables for bidirectional communication, and employing full-duplex devices, only, the Receiver and Transmitter terminals now must connect to separate pins.

The number of transceiver models available in the market is huge, and that makes it a challenge picking out the best and most cost-effective device for a specific application. That requires considering the common design considerations, examining the electrostatic discharge (ESD) protection and comparing the Human Body Model. Other important points to be considered are the over voltage protection (OVP) and data skew in case of high-speed transmissions.

Requirements of RS-485

Although the published standard for the RS-485 is over 14 pages long, the most important requirements are:

The differential output voltage generated must be over ±1.5V, while the receiver must be capable of detecting signals with a minimum of ±200mV. This combination makes sure the devices can tolerate attenuation from long cables and there is a robust noise margin available.

As the standard allows multiple drivers on the bus, each transmitter must have an enable pin giving it tri-state output capability. This ensures true bidirectional transmission over a single cable.

Transmitters must have high output current capability to drive long cables and cables with double termination, especially for high-speed bidirectional transmission.

The CMR should be at least -7V to +12V. This allows using RS-485 over networks of 1220 m or 4000 feet. Long distances can involve ground potential differences and a high CMR helps to tolerate them in noisy environments. Additionally, devices with different supply voltages can also communicate on the same bus because of a large CMR.

The receiver input resistance must be over 12KΩ. According to the standard, there can be 32 devices on a bus.

The Basic RS-485 Transceiver

People often use less expensive RS-485 transceivers for simple, short, low-node-count networks. This works because short networks do not involve much CMV or common mode voltage and OVP, and they can work with the CMR specified by the standard.

When there are less than 32 modes in the network, fractional unit load devices are not necessary. Moreover, when cables are not frequently connected and disconnected, ESD protection is also not necessary. However, most basic devices now include the ±8 to ±15 KV Human Body Model for ESD protection.