Home' RTCA Documents for Review : DO-230H FRAC Contents 268
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kilometers), and can be interfaced with optically isolated links. Long before the RS-232 standard,
current loops were used to send digital data in serial form for teleprinters. The original IBM PC
serial port card had provisions for a 20 mA current loop. Current loop interfaces usually use
voltages much higher than those found on an RS-232 interface, and cannot be interconnected with
voltage-type inputs without some form of level translator circuit.
Wiegand Interface: The Wiegand interface is a de facto wiring standard that arose from the
popularity of Wiegand effect card readers in the 1980s. It is commonly used to connect a card swipe
mechanism to the rest of an electronic access control system. The Wiegand interface uses three
wires, one of which is a common ground and two of which are data transmission wires. A high
voltage level of +5VDC is used to accommodate for long cable runs (most reader manufacturers
publish a maximum of 500 feet) from door readers to the associated access control panel typically
located in a secure closet.
The original protocol used on a Wiegand interface had one parity bit, 8 bits of facility code, 16 bits of ID
code, and a trailing parity bit for a total of 26 bits. Many access control system manufacturers adopted
Wiegand technology but were unhappy with the limitations of only 8 bits site codes (0-255) and 16 bits for
card numbers (0-65535) so they designed their own formats with varying complexity of field numbers and
lengths and parity checking.
An advantage of the Wiegand signaling format is that it allows very long cable runs, far longer than other
interface standards of its day allowed.
Complying with Standards
Compliance with standards can only be achieved if they are read and understood. While it may be
impractical to have a copy of every standard on site, a copy the BICSI Telecommunications Manual should
be kept on site and referenced during the execution of the work.
Supervision on site must be directed toward ensuring that non-compliances with standards are avoided,
including the following:
Cable bending radius under limits specified by standard or manufacturer for cables used (e.g. 25
mm for Cat 5e, 10 times cable diameter for fiber)
Cable bending radius not observed at route around right angle edge (e.g. horizontal to vertical
transition at the top of a partition wall, no LB’s unless rated for telecommunications, etc.)
Cable sheath subject to excessive pressure and deformation due to over tightening of cable ties,
excessive cable bundles or use of under-sized staples
Minimum safety clearances from electrical power wiring not met or insulating barriers not provided
Telecommunications outlets too close to power outlets
Metal enclosures, frames, racks or ducts not effectively earthed (either no connected at all, or paint
in the way of connection on metalwork concerned).
Record books for distributors not provided, not completed, or not completed in full
Underground cable not used on underground routes
Orange HD or grey flex conduit used instead of white telecommunications conduit on underground
Telecommunications pits, manholes or chambers shared with power
Pits not provided with drainage
Pits in roadways, aircraft areas or car parks not provided with metal covers with proper load rating
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