Applications engineering of security systems has gained impressive digital tools which include transmission, processing, and protecting the data. To make the most effective use of these new tools, additional knowledge is required in the areas of operating systems, digital video, IT topologies, Internet Protocol, and network security. Although the experienced security dealer integrator probably has earned his wings using some of this technology, there are always additional insights to gain.
OTJ (On the Job) training is when you try to learn a particular device or software application during the course of its first installation. Although this will always be a way you acquire new skills, it is not the best course of learning. Training is available from a variety of sources including from vendors, trade associations, technical schools and publications such as Security Dealer and its wealth of on-line resources. Plus check out this month's, “The Training Center” (page 79).
Video is an area of particular interest, since the issues associated with IP video are generating so much buzz in the industry and also seem to be the object of the most confusion and misconceptions. Contributing to the confusion is the endless list of transmission acronyms you need to be versed on.
This one's tricky. TTL can stand for “transistor-transistor logic,” a common type of digital circuit in which the output is derived from two transistors. The term is commonly used to describe any system based on digital circuitry. TTL also can be “time to live,” a field in the Internet Protocol (IP) that specifies how many more hops a packet can travel before its discarded or returned.
TCP/IP stands for Transmission Control Protocol/Internet Protocol, the suite of communications protocols used to connect hosts on the Internet. TCP/IP uses several protocols, the two main ones being TCP and IP.
TCP/IP is used by the Internet, making it the de facto standard for transmitting data over networks. Network operating systems that have their own protocols, such as Netware, also support TCP/IP.
IR (infrared) telemetry is a line of sight technology used for data transfer between devices, usually over limited distances.
RF (Radio Frequency) refers to a wide range of wireless technologies such as Wi-Fi, cellular, and products which use proprietary formats over regulated frequency segments. These devices include supervised wireless alarms and encrypted voice systems.
Wi-Fi (Wireless Fidelity) was originally a brand licensed by the Wi-Fi Alliance to describe the embedded technology of wireless local area networks (WLAN) based on the IEEE 802.11 standard. As of 2007, common use of the term Wi-Fi has broadened to describe the generic wireless interface of mobile computing devices, such as laptops in LANs. Common uses for Wi-Fi include Internet and VoIP phone access, gaming, and network connectivity for consumer electronics such as televisions, DVD players, and digital cameras.
A broadband alternative, BPL (broadband over power lines) technology is a proven method for linking small offices, branches and telecommuters. BPL combines power and data on the same wire. It is another option where cable and DSL are scarce or unavailable, or where existing broadband customer service and pricing are poor. The only premises equipment required for BPL is a special modem plugged into an electrical receptacle. Industry forecasts are that by 2012, 33 percent of new broadband customers will access their services using BPL and 13 percent of current broadband users will switch over to BPL.
RS-232 is a standard for serial binary data interconnection between a DTE (Data Terminal Equipment) and a DCE (Data Circuit-terminating Equipment). It is commonly used in computer serial ports. Because the voltage levels are higher than logic levels used by integrated circuits, special intervening circuits are required to translate logic levels, and to protect circuitry internal to the device from short circuits or transients that may appear on the RS-232 interface.
Maximum cable lengths: Cable length is a limiting factor with RS-232. The maximum cable length is 50 feet, or the cable length equal to a capacitance of 2500 pF. This means that using a cable with low capacitance allows you to span longer distances without going beyond the limitations of the standard. If for example UTP CAT-5 cable is used with a typical capacitance of 17 pF/ft, the maximum allowed cable length is 147 feet. The cable length mentioned in the standard allows maximum communication speed to occur. If speed is reduced by a factor of 2 or 4, the maximum length increases dramatically. Although the RS-232 standard was originally developed for 20 kbps by halving the maximum communication speed, the allowed cable length increases by a factor of ten.
EIA-422 (formerly RS-422), now TIA-422, is a technical standard which specifies the “electrical characteristics of the balanced voltage digital interface circuit.” It provides for data transmission, using balanced or differential signaling, with unidirectional/non-reversible, terminated or non-terminated transmission lines, point to point, or multi-drop. In contrast to RS-485 (which is multi-point instead of multi-drop), EIA-422 does not allow multiple drivers but only multiple receivers.
Several key advantages offered by this standard include the differential receiver, a differential driver and data rates as high as 10 Megabaud at 12 meters (40 ft). The specification itself does not set an upper limit on data rate, but rather shows how signal rate degrades with cable length. EIA-422 only specifies the electrical signaling characteristics of a single, balanced signal. Protocols and pin assignments are defined in other specifications. A common use of EIA-422 is for RS-232 extenders.
While the RS-232 standard defines a bi-directional interface between exactly two communicators, the RS-423 standard defines a uni-directional interface between one transmitter and many receivers. RS-423 allows for distances up to 4000 feet but limits data rates to only 100 kb/sec for a maximum of ten receivers. The voltage levels are +3.6 to +6 volts to represent a binary 0 and -3.6 to -6 volts to represent a binary 1. The voltage levels are defined relative to an earth ground potential assumed to be zero volts. Consequently a difference in ground voltage levels will result in the Common Mode Voltage problem that will confuse the data values.
RS-423 is very similar to RS-422. RS-423 is a serial interface between one DTE and one DCE, unlike RS-422. A DB25 connector is sometimes used in place of the typical DB37, although the DB37 is still used. Like RS-422, all signals use both the A and B lines of a pair, but the B lines in RS-423 are all tied to the Ground (GND). RS-423 is also a single ended signal rather than the balanced signal of RS-422. Most RS-423 signals are synchronous.
EIA-485 (formerly RS-485 or RS485) is an OSI Model physical layer electrical specification of a two-wire, half-duplex, multipoint serial connection. The standard specifies a differential form of signaling. The difference between the wires' voltages is what conveys the data. One polarity of voltage indicates a logic 1 level; the reverse polarity indicates logic 0. The difference of potential must be at least 0.2 volts for valid operation, but any applied voltages between +12 V and -7 V will allow correct operation of the receiver.
EIA-485 only specifies electrical characteristics of the driver and the receiver. It does not specify or recommend any data protocol. EIA-485 enables the configuration of inexpensive local networks and multidrop communications links. It offers high data transmission speeds (35 Mbit/s up to 10 m and 100 kbit/s at 1200 m). Since it uses a differential balanced line over twisted pair (like EIA-422), it can span relatively large distances (up to 4000 feet or just over 1200 meters).
In contrast to EIA-422, which has a single driver circuit which cannot be switched off, EIA-485 drivers need to be put in transmit mode explicitly by asserting a signal to the driver. This allows EIA-485 to implement linear topologies using only two lines.
The recommended arrangement of the wires is as a connected series of point-to-point nodes, a line or bus, not a star, ring, or multiple-connected network. Ideally, the two ends of the cable will have a termination resistor connected across the two wires and two powered resistors to bias the lines apart when the lines are not being driven. Without termination resistors, reflections of fast driver edges can cause multiple data edges that can cause data corruption. Termination resistors also reduce electrical noise sensitivity due to the lower impedance but bias resistors are required. The value of each termination resistor should be equal to the cable impedance.