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HID Signo Readers Announced.

Posted by Zack Hamm on March 2, 2020

HID announced an entirely new reader line today, called Signo. What’s immediately noticeable is they are more sleek and stylish than the iClass R or RP models, but looking further, we found that there are some distinct differences that might just make switching to this new reader platform sensible.

For starters, the keypad reader model looks more functional, and the mullion keypad reader is a definite necessity. The keypads are capacitive touch style which should make them more reliable in harsh environments.

Dimensions for the readers is almost identical, with the Signo readers being a little slimmer, but probably not by very much. See Feature Comparison Matrix.

What’s missing though, like in the RP series, is a long range parking lot reader like the R90. This is a needed technology that should be added in the future (are you listening, HID?).

The Signo series seems to lump all the reader technologies in together, making the product selection a little less confusing than previous iClass reader selections. This is most welcomed. Supported technologies are 125Khz proximity, iClass, SEOS, Mifare, plus mobile credentials via Bluetooth and NFC, plus Apple’s Enhanced Contactless Polling technology for apple wallet credentials.

Other features are better support for crypto keys (no more base encryption key in the wild, for now), automatic tuning/detuning for optimized read range, and OSDP support out of the box. Reader tamper is now a dry contact relay (THANK YOU). But the biggest thing installers are going to enjoy is that the Signo readers support remote management. No more configuration cards to go around to every reader just to turn of the 125Khz prox read feature set. This should have been done LONG AGO. Firmware updates, configuration, and reader management can be done via mobile device or over OSDP (assuming your PACS supports it).

From our take, these readers appear to have been developed largely for the Campus environment (the Apple ECP is a dead giveaway), but certainly have the feature sets that would make them desirable in the commercial, government, and industrial markets as well. We don’t have any evaluation copies yet, but will definitely be looking at these for new projects where they fit and offer additional security, style, and convenience.

Feature Comparison Matrix

Reader	RP40	Signo 40
Dimensions	3.3″ x 4.8″ x 1.0″	3.15″ x 4.78″ x 0.77″
Read Range (typ)	iCLASS: 2.4″ 125Khz Prox: 2.8″ to 4.3″	iCLASS: 1.6″ to 4″ 125Khz Prox: 2.4″ to 4″
Power	85ma @ 16VDC	75ma @ 12VDC
Comm	Wiegand & (optional) OSDP	Wiegand & OSDP
Reader Tamper	Open Collector Output	Dry Contact Relay
Configuration	Programming Cards	Mobile Device or OSDP
Weatherproof	If optional gasket installed	Yes
Certifications	UL294, EAL5+	UL294, EAL6+
Price	~$200.00	~$200.00

Tags: Access Control

Posted in: Access Control, Company News, Security Technology

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Why You Should Be Skeptical of Security Apps in the Cloud

Posted by Zack Hamm on January 30, 2020

The “Cloud” for most people has become an integral part of our daily lives. It’s everywhere, yet many people couldn’t tell you what the cloud really is if you asked them. It’s just there.

In a nutshell, the cloud could be defined as “an internet server you don’t own, yet hosts your data”. All this means is you’re using Software As a Service (SAAS), and someone else has developed the application, hosts it on their server, and you use it, or maybe even pay for it via subscription (like Ring or Canary).

Yet all too often, when we really closely inspect these services or applications we tend to find flaws, or sometimes downright intentional abuse, that leads to exposing more of our personal data than we would like. In the case of many apps for home or personal use, we can choose to accept some loss of privacy in exchange for the free or bundled software and storage. However, in the business world this is rarely tolerated, and in some cases it’s against the law.

For a fantastic example of this abuse, the Electronic Frontier Foundation has published an excellent article about Ring and its application that exposes multiple abuses of privacy. The biggest takeaway from the article for us was that Ring appears to be using pinned certificates in a way that prevented data security experts from analyzing encrypted traffic generated from the application itself. Now imagine an app from a state sponsored corporate entity like Huawei sending images, stored files, GPS data, voice recordings, or video from your phone or tablet to some server in another country. See how big the risk is?

Click the link below to read the full article, but this summary puts it best:

“Ring claims to prioritize the security and privacy of its customers, yet time and again we’ve seen these claims not only fall short, but harm the customers and community members who engage with Ring’s surveillance system.”

https://www.eff.org/deeplinks/2020/01/ring-doorbell-app-packed-third-party-trackers

Tags: Cloud, privacy, Ring, security

Posted in: Company News

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Quantum Resistant Encrypted Communications

Posted by Zack Hamm on October 25, 2019

If quantum computing can reduce previous calculation times of decades down to seconds, then what does that spell for current military, government, and corporate technologies that rely on encryption algorithms? The current idea is that encryption isn’t unbreakable, it’s just not practical because by the time the message is encrypted, it’s no longer relevant and most likely the encryption key (or even mechanism) has changed. With Quantum computing, this all goes out the window, as now brute force decryption could theoretically be done in real time. This terrifies everyone, and rightfully so.

What about Blockchain? Under the bitcoin blockchain algorithm concept, the encryption difficulty is adjusted after every 2016 blocks are mined. This is typically an incremental increase or decrease as more or fewer hardware resources are thrown at mining bitcoin. Thus, if you added a super quantum computer (or cluster of quantum computers), the difficulty would proportionally (and drastically) increase with the number of blocks that were mined (just in a much shorter time frame).

On October 23, 2019, Bitcoin’s price fell from around $8200 per bitcoin to under $7400 in a matter of minutes, because Google announced that Sycamore, its quantum computing platform, had achieved “quantum supremacy” and passed an impossible test. The premise is that it completed a test calculation in 200 seconds that would take the world’s most powerful supercomputer 10,000 years to finish. While the claim is somewhat questionable, there is no doubt that quantum computing has taken a foothold and will only advance from here.

Google Sycamore Quantum Computer

The problem here is that someone trying to break the encryption isn’t likely to be actively participating in the blockchain. They’re just going to be trying to decrypt packets or streams, or whatever they are intercepting and trying to decrypt. So unless the encrypted blockchain communications platform changed the encryption difficulty to a level that it couldn’t actively mine itself (thus defeating the point of encryption to begin with), the communications would always be deciphered by the quantum computer. Thus… all encrypted communications would have to be done with quantum computers. It’s another round of keeping up with the bad guys, and technology has to step up to meet the challenge.

The downside is that this is financially well outside the possibility for most businesses and private individuals, meaning we are back to the equivalent of clear text passwords and messages as well as unencrypted financial transactions. This would be disastrous for the financial and credit industries, as well as creating havoc in terms of privacy act enforcement, worldwide (think GDPR).

How this plays out is anybody’s guess right now and the technology isn’t quite there, but in the mean time, all one can do is hope that Google really means it when they say “Don’t Be Evil”.

Posted in: Company News

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NSA Warns: Update Windows Or Else

Posted by Zack Hamm on June 8, 2019

For the first time I’ve ever seen, the National Security Agency has made a public announcement about a private company’s product, warning that all Windows users that are still using older versions of Microsoft Windows XP, Vista, Windows Server 2003/2008, or Windows 7 should upgrade or face serious remote exploit risks. Their advisory can be found here: https://www.nsa.gov/News-Features/News-Stories/Article-View/Article/1865726/nsa-cybersecurity-advisory-patch-remote-desktop-services-on-legacy-versions-of/

Honestly though, if you’re still using Windows XP or Windows 2000 and it’s connected to the internet, you kinda deserve everything you get. I get it, there are still some ancient programs out there that never got upgraded and you just “can’t live without it”. And I can even understand if you’re still using Windows 7 (by the way, patch that too), but really, it’s been 18 years since Windows XP was released… stop clinging and move on.

Further information from the Microsoft CVE-2019-0708 security advisory:

Block TCP Port 3389 at your firewalls, especially any perimeter firewalls exposed to the internet. This port is used by the Remote Desktop Protocol (RDP) and will block attempts to establish a connection.
Enable Network Level Authentication. With NLA enabled, attackers would first have to authenticate to RDS in order to successfully exploit the vulnerability. NLA is available on the Windows® 7, Windows Server® 2008 and Windows Server® 2008 R2 operating systems.
Disable remote Desktop Services if they are not required. Disabling unused and unneeded services helps reduce exposure to security vulnerabilities overall and is a best practice even without the BlueKeep threat.
Note that Windows® 10 systems are already protected from this vulnerability, as it only affects the older versions of Windows® listed above.

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2019 Q3 and Q4 Class Schedule

Posted by Zack Hamm on May 22, 2019

The following North Carolina SP-FA/LV Exam Prep classes are set for the remainder of the 2019 calendar:

August 14, 2019 8:00A – 5:00P Greensboro ADI, 4500 Green Point Dr #103, Greensboro, NC 27410
October 16, 2019 8:00A – 5:00P Raleigh ADI, 2741 Noblin Rd # 101, Raleigh, NC 27604
December 11, 2019 8:00A – 5:00P Greensboro ADI, 4500 Green Point Dr #103, Greensboro, NC 27410

For additional information or registrations, contact National Training Center at (702) 648-8899 or sales@nationaltrainingcenter.net. Or register on-line at http://www.nationaltrainingcenter.net/index.xml.
Full NTC class schedule link: http://www.nationaltrainingcenter.net/instructor-led-training.xml

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Simple Home Security Tips

Posted by Zack Hamm on April 14, 2019

For the most part and except for the rare high net worth executive or celebrity, our firm does not actively engage in home security consulting for residential properties. We typically just aren’t cost effective, and there are plenty of other very good sources of information out there that can provide good service for your needs. Still, I find that I get asked this advice from time to time, and even though we don’t typically provide this service, here are some ideas that can be helpful in protecting your loved ones and your home:

Have a Security Mindset. A simple rule I learned from my father as a kid, “Leave your place the way you want to find it when you return”. This applies to home security very easily.
Use Lighting. Leave the lights on if you’re coming home after dark (or have automatic lights that turn on a schedule or at dusk).
Smart Lights. For techies, get light switches or sockets that can be controlled by Google Home or Amazon Alexa. Then set schedules or use voice commands to turn lights on/off. This helps make the home appear to be occupied.
Outdoor Lights. Keep a light on the porch or in the yard. Lights are a great deterrent for criminal behavior. Motion detection lights are useful too, although they tend to false quite a bit and may come on more than necessary.
Be Neat. Leave the house neat and orderly (it’s hard to tell if someone has ransacked your house if it’s already a mess…). There’s also a little bit of the “broken window mindset” here, that people won’t respect your home as a sovereign domain if it’s unkempt and in disrepair. Plus, don’t leave items in your yard that may help burglars or vandals break into or damage your home (ladders, tools, bricks/lumber, gas cans, etc).
Lock Doors. Lock the doors before you leave. If keys are a hassle, install a PIN pad for your deadbolt on your main or side entrance door. They are easily installed and inexpensive. Oh, and make sure you have a deadbolt lock on all doors. Install one if not.
Use door barricades if needed. Metal exterior doors are best. For use when you are at home, using simple devices that function as a night latch are very helpful in supplementing deadbolts (you did get a deadbolt, right?). Be sure to install the night latch at least one foot higher or lower than the deadbolt, to add more strength and resist kick ins. By the way, forget the chains and use at least 3″ screws to fasten to the door frame and studs behind. Here’s a suggestion https://www.amazon.com/dp/B00D2K367Y/
Use Your Alarm. Arm the alarm system if you have one. Many people have alarm systems, but never actually use them. Use it! (A great feature I’ve added to my house is a “go away light”, that is a little red light that turns on if the alarm has been tripped and can be seen as I’m driving up to the house. If it’s on, nobody goes inside, we call the police.)
Get an Alarm. If you don’t have an alarm system, get one. Most any commercially available alarm system is sufficient for home use. Large companies like ADT and CPI market themselves as inexpensive, but tend to have high monitoring fees. Otherwise they’re all about the same. PRO TIP: If you get an alarm system, get smoke detectors connected to it and pay for the monitoring fee. The fire department will automatically be called whether you’re home or away.
Get a dog. This age old burglar deterrent really does work pretty well. And no, it doesn’t really matter what kind of dog, although a Chihuahua might not be the most intimidating.
Lock Windows. Use your window locks, even on the second floor (criminals have ladders too). If your windows are the older double-hung wooden type, an easy trick to secure them is to drill a ¼” hole in the far left or right side side of both sashes, and then insert a 10 gauge nail through both sashes. Even if they managed to unlock from the outside, the window can’t be raised or lowered.
Cut Back Shrubs. Keep shrubs cut back and trees limbed up so you can see your house windows and doors from the street. Shrubs near the house should be trimmed neatly and cut back. Hedges should be trimmed so they don’t offer an advantage to a stalker or potential burglar.
Fences. If your property would benefit from the use of a fence, they provide a natural barrier and boundary to your property. Most houses don’t have them, but for some homes they might be a good fit.
Cell Charger by Your Bed. Most people don’t have a home telephone anymore, so make sure to have your cell phone charger at your bed at night. If you need the phone in a hurry because of a break in, you don’t want it in another room.
Night Lights. Most grown adults don’t want or need night lights, but they are very helpful for night time navigation in events where you’ll likely have the advantage of night vision and know the layout of your home.
Flashlight. Having an alternate source of light is critical in emergency situations. There should be one flashlight per person in the household. The nightstand is an obvious place to keep it.
Have a plan. Have a plan what to do in case of an emergency, a fire, or home invasion. If you have children, discuss the plan with them too. Keep it simple so everyone will remember it.
Owning a Gun. Owning a weapon like a gun is a personal choice, and for some may not be right, or even legal. If you do have a lethal weapon, make sure it is stored and locked up securely, and that you can get to it quickly and safely if needed. Otherwise, you’re better off not having it or not using it in the case of a home invasion (you don’t want it used against you or your family). Training and regular practice with the weapon is also strongly recommended.
Non-Lethal Weapons. If a firearm is not for you, other non-lethal options such as Pepper Spray, Blunt Weapons, or Tasers may be used in the case of home invasion, but still also carry the responsibility of choosing to use them appropriately and have proper training. Don’t use pellet guns, air-soft guns, or the like as a means of self-defense. In the event the intruder also has a gun, he’s more likely to use it if he believes you also have a gun.
Handcuffs. A final consideration is what you’re going to do with the intruder if you’ve managed to stop them. How will you hold them for police? Having a set or two of handcuffs in a drawer is a safer and easier alternative to tying them or trying to lock them in a room.

These are but a few ideas for home safety and security that can be easily adopted by most people. If you need more detailed information or want a thorough security plan developed, contact a professional security consultant to help you determine your needs and the best plan to make sure you are prepared and protected.

Tags: Alarm Systems, CPTED, Home Invasion, home security, Security Lighting

Posted in: CPTED, Fire and Life Safety, Security Technology

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The use of shielded category Ethernet cable for IP Video

Posted by Zack Hamm on August 17, 2018

It comes up from time to time from customers and vendors when and where to use shielded category twisted pair cable for Ethernet. Most vendors hate it. It’s hard to terminate, doesn’t flex well and nobody ever seems to agree on how it should be grounded. For the most part, it wasn’t much of a problem for CAT3 or even CAT5 cable. But with higher and higher bandwidth (and thus frequency) demands on the cable, using CAT6A cable in certain environments for network applications becomes important. And while there’s a decent argument for why you may not need CAT6A cable for IP video (see this article for more information), many of our clients are using it as a corporate standard, regardless of the application.

Although CAT6 cables have improved the cable twist to handle gigabit Ethernet and reject noise, this by itself is not enough for environments that have high electromagnetic interference (EMI). What is EMI? Think of EMI as gremlins that are trying to attack the signal of your network cable. EMI is generated as electromagnetic waves in the radio frequency (RF) spectrum, and can come from many sources. The most logical is an RF transmitter, like a radio station tower or even HAM radio antenna; but other sources can be harder to spot, such as a nearby computer, high voltage power lines, a leaky transformer, or fluorescent light fixture that’s going bad. Running network cables in your ceiling or plenum space could potentially put these cables near those kinds of sources, and thus introducing the gremlins to degrade or even interrupt communications over the Ethernet network cable. Since Ethernet is a collision based network strategy, this usually looks like a slow connection, as the network repeatedly keeps re-transmitting packets that were found to be in error. Result, poor network performance and potentially bad video.

Most people are familiar with UTP cable (Unshielded Twisted Pair), versus STP (Shielded Twisted Pair), and UTP is commonly used in CAT5e cabling that is predominant for gigabit Ethernet networking in most commercial and residential applications. STP cables have an additional metallic braid that forms a sort of shield (google “Faraday shield” for how it works) around the conductors, and reduces the amount of interference that can be injected into the cable. Still, both types of conductors (STP and UTP) have one thing in common, the twisted pair, that by itself reduces interference by its inherent design.

Photo courtesy of Axis Communications.

The drawback of STP cables is that they increase the total cost of the installation. STP cables are more expensive due to the shielding (and usually are higher quality), which is an additional material that goes into every foot of the cable. The shielding also makes the cable heavier and stiffer. Thus, it is more difficult to handle during installation (pulling cable over long distances through a conduit is hard enough with flexible cable).

While most installations can be done effectively using UTP cable, we recommend using STP cable for high EMI environments like manufacturing, laboratory, or research facilities where other high energy or RF generating devices may be in use. Also, if you’re forced to run category cable in a cable tray that’s shared with power conductors (low or medium voltage), use STP cable even though the cable tray is separated and may have it’s own shielding for the power conductors. It is also highly recommended to use an STP cable where the camera is installed outdoors or where the network cable is routed outdoors.

Oh, and what to do with that drain wire? Our suggestion is to use some of the pre-fabricated shielded keystone jacks like this one.

Tags: IP Cameras, Security Technology

Posted in: IP Video

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Bitcoin Hacked! Hackers steal $70M dollars! And Other Sensational Journalism…

Posted by Zack Hamm on December 7, 2017

Bitcoin… Another story. The mainstream media, in their usual quest for drama and ratings, is in a fever pitch about the December 5 hack of Nicehash.com that resulted in the theft over over 4736 bitcoins (~$77M dollars as of this writing). See here for what is claimed to be the blockchain identifier for the transfer: https://blockchain.info/address/1EnJHhq8Jq8vDuZA5ahVh6H4t6jh1mB4rq

The claim that seems the most ridiculous is that “Bitcoin is NOT safe, and is hackable!”. This is nonsense, and it is like saying that the US dollar isn’t safe because your neighborhood Bank of America was robbed. The fact is, nicehash.com didn’t have adequate security measures in place to prevent the hack (even with the most annoying Captcha I’ve ever used), and it probably has ruined the company.

Nicehash is was a very popular and easy to use mining service where people (including myself) can mine for Bitcoin (and other cryptocurrencies) using their PC or specialized hardware built for mining. Nicehash pays miners a “fee” for mining cryptocurrencies, and pays them on a round basis. The nice thing was they paid in bitcoin, no matter what coin you were actually mining. Naturally, they had to have a pretty large amount bitcoin to be able to make these payments, and they advertised it regularly on the web.

Nicehash also had a policy of not making payments to external wallets (meaning, under the control of the individual miner, and not on nicehash.com) unless they had a mining balance of .01 bitcoin or more. That’s about $170.00 and many miners had just slightly less than that balance that was stolen from the community wallet that nicehash.com paid miners from. And because Bitcoin transfers are generally not traceable to an individual, the money is gone. In short, everybody loses.

The trouble sets in when someone, somehow, found a way to get into their Bitcoin wallet and transfer the coins out to themselves. The FBI is almost certainly involved, as well Interpol, Europol, and possibly some other European or Slovenian police agencies.

So why still invest in Bitcoin? Because Bitcoin is based on a blockchain technology that is very reliable and secure. The concept of bitcoin and it’s blockchain is not hackable in itself. Rather, nicehash.com was hacked and lost their bitcoin. There are different types of blockchain strategies, and some are more efficient, quick, secure, and anonymous than others. Bitcoin was the first cryptocurrency to use this concept, and while you can “see” what address funds are transferred to and from, you cannot see “where” physically (geographically) or any other identifying information for who or where the funds were sent to or from.

So what is blockchain? Google is your friend here, as there are tons of videos and wikis about blockchain and how the different types all work. But in a (very simplistic) nutshell, blockchain is the concept that all transactions in an ecosystem are using a distributed cryptographic ledger, and most importantly, the SAME ledger. This means that if Zack, Sally, Mike and Kim are all in a trading club and are sending money to each other, they each have a copy of the ledger, and when Mike sends Zack funds, it is recorded on all four ledgers and the ledgers all have to agree (using a cryptographic algorithm) on the transaction date/time, amount, and transferees bitcoin address. If they don’t agree, the transaction is invalid and the transaction is rejected, thereby preventing someone from just inserting a million dollar credit to their own ledger. As you can also imagine, for something like Bitcoin that’s been logging and recording all these transactions around since 2009, that ledger can be quite large… about 2 gigabytes large… and still growing.

The cool thing is this technology can be applied to other types of transactions, such as deed transfers, contracts, information exchanges, or gaming, to name a few. Because the transaction is secure, encrypted, and shared, it is virtually “hackproof”. What isn’t “hackproof” is anything stored online, like Nicehash’s wallet, or any other online wallet that you yourself don’t have the private keys for and can transfer to cold storage. Online wallets are very convenient. Coindesk.com is very popular and has exploded in recent weeks due to the popularity and price spike for Bitcoin, but it’s generally not considered a good idea to keep large sums of Bitcoin stored there unless you have an immediate need for it. Keep it in an offline wallet and use cold storage.

So in short, Bitcoin is just like any other fiat cash currency, the bearer holds the value, and if you don’t take steps to protect it, someone else can (and probably will) steal it.

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Door Handedness

Posted by Zack Hamm on November 17, 2017

One of the questions that comes up all the time in access control design layouts is door handedness. It’s not a hard concept to understand once you see it graphically, but it’s sometimes tough to remember in the field if you’re not accustomed to working with door hardware on a daily basis.

While we would typically prefer security doors to swing into the secured space (easier to barricade if needed in an emergency and the hinges are typically on the secured side), usually the handedness of a door isn’t left up to security and is based more upon building code and/or the function of the space.

The following graphic explains it better than I’ve seen it anywhere, and shows you the door swing based upon being on the Outside (or “unsecured” side where the card reader or key would be).

Courtesy of Specialtydoors.com

Also, note from the table below that a Left Hand door isn’t the same as a Right Hand Reverse door, as the lock hardware has to change in order to be able to latch properly.

Left Hand: Door swings inward to the Left, uses LH Hinge, LH Strike, LH Lock.
Right Hand: Door swings inward to the Right, uses RH Hinge, RH Strike, and RH Lock.
Left Hand Reverse: Door swings outward to left, uses RH Hinge, RH Strike, and LH Lock.
Right Hand Reverse: Door swings outward to Right, uses LH Hinge, LH Strike, and RH Lock.

So next time someone says that a door is a “Right Hand Reverse” door, you’ll know that they really mean the door swings out to the right towards you if you’re standing on the outside.

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Alarm Circuit Supervision – Why You Should Care

Posted by Zack Hamm on September 12, 2017

EOL resistors installed at panel instead of terminating device.

The wrong place for EOL resistors.

Nearly every project we work on, we recommend that alarm initiating devices, especially door contacts, are to be configured using end of line (EOL) resistors for 4-state supervisory circuits. The actual resistance value and configuration can vary by system manufacturer, but typically it involves wiring a 1K Ohm resistor in series and another in parallel with the switch, at the terminating device itself (not in the panel or junction box above the door). This ensures that we have circuit supervision from the alarm panel (or card reader panel) all the way down to the device termination, so we know if the device is in a normal state (1), an alarm state (2), shorted state (3), or cut state (4). This is known as 4-state supervision, because it distinguishes between 4 possible scenarios for the supervised device.

For an example of why you need device supervision, I recently had a high profile client contract me to do a security survey of their research building. One of the basement doors had a door contact on it that was not supervised and had been cut and shorted about 30 feet away from the door. The card access system monitoring the door didn’t use 4-state monitoring and the door appeared to be “closed” all the time, even though it was commonly known that maintenance staff actively used the door for that area. It had been in that state for several years before it was identified and later repaired.

This simple addition offers greater security to the system, yet often gets omitted by vendors in the installation because it requires extra time and expense, and even causes confusion with some installers (really). Worse, we sometimes end up with installations like the picture above that adds the EOL resistors to the panel with Dolphin connectors. This type of installation does not offer any real security, and potentially introduces the opportunity for spurious connections inside the panel. Thankfully, vendors like GRI manufacture magnetic contacts that come pre-assembled with the resistor array included. They include 1K, 2K, 3.3K, 5.6K, 10K, and 33K resistors in a variety of contact packages, and also sell resistor packs for retrofit installations.

Years ago, one of the best explanations I ever read about alarm circuit supervision was from an Andover Controls card access panel installation guide. I had learned already about 4-state supervision and why you should do it, but the following illustration shows it more clearly than anywhere else I ever saw it. I ran across it again the other day and decided I would put it in an article here on the site. The illustration below shows how the first two iterations of EOL resistors do not offer any significant line supervision, and could easily be defeated. The third configuration offers 4 unique resistance values that correspond with the 4 possible state conditions.

Credit to Schneider Electric / Andover Controls for the illustration.

Posted in: Security Technology, Training

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