PVD Product Data / Telemetry Considerations

Data signals come from the control room to the NV-704J-PVD or NV-716J-PVD. Within a group, the data signal is split-out to support data for up to four cameras. Since the distances between the NV-704J-PVD (or NV-716J-PVD) and the cameras are short (under 400 ft, 120m), the transmission-line effect (termination reflections) may be negated. For more details, see Transmission-Line Effects, below. P/T/Z telemetry cameras come with a terminator (100 or 120 ohm resistor) that can be switched on or not. It is important to have one and only one terminator on in this group, preferably at the furthest camera.

If the installation supports multiple groups, either from multiple NV-704J-PVDs or from different wire pairs to an NV-716J-PVD, it is recommended that a "Code-Converter" (available from the camera manufacturer) be employed at the control room. This device acts as a "fan-out repeater box" allowing telemetry signals to be individually buffered. This reduces the loading due to multiple cameras, terminators, and transmission-lines. It also provides insurance against having a wiring fault take down the entire control system.

Note: Different camera manufacturers employ different communications protocols. RS-422 is a one-way path from the control room out to the cameras. RS-485 is a bidirectional protocol that allows commands and responses from the cameras. Camera responses are not supported by RS-422. If required, wire pairs (outside the "PVD" wiring scheme) will be needed for the support of these camera responses.

Transmission-line Effects

Data distribution is accomplished by paralleling the data paths to up to four cameras. Known as star-wiring, this fan-out method works very well for camera implementations of RS-485 or outbound RS-422. This is because data is typically traveling at the glacial speed of 4800 baud (confirm the camera's protocol). Signals travel through twisted-pair wire at 575 feet (175 m) per microsecond. From transmission theory:

Propogation speed ÷ Frequency = Wavelength

575 feet/microsecond ÷ 4800 bits / second = 120,000 ft

175 m / microsecond ÷ 4800 bits / second = 37,000 m

Transmission-line effects (reflections, ringing, etc.) start at 1/4 wavelength, so divide by 4 to get 30,000 ft (9,000m). Then there are harmonics of the square-edged data pulses. If we take the fifth harmonic (very conservative because edges round off at long distances), we need to divide this number by five 30,000 ft ÷ 5 = 6,000 ft (1830m). This is longer than our maximum cable length, which is typically spec'd at 3,000 ft (1Km). This distance is much longer than any star-wiring-based stub because the power distribution limits us to 400 ft (120m).

This calculation shows that at these speeds, the system acts more like a lumped-element circuit than a transmission line. Indeed, oscilloscope measurements of the data path show clean square pulses under worst-case conditions.