Modern high-speed RS-485 data links are pushing the envelope to the absolute breaking point with regards to data rate versus cable length. Applications include position encoders, traffic monitoring, seismic networks, and GPS transceivers.
Data rates from as low as 8 Mbits/s up to 20 Mbits/s are driven across distances of 200 to 1000 feet (70 to 300 m). The designs of such data links are rather complex because additional efforts must be implemented to compensate for increased propagation delays of cable and transceivers, as well as frequency-dependent cable losses.
Figure 1 shows the conventional cable length versus data rate characteristic. At about 4000 feet, the voltage divider action between the cable resistance of a 24 AWG twisted-pair cable and the termination resistance matching the characteristic line impedance reduces the signal strength by half (6-dB attenuation). This drop serves as a reference for the maximum cable length, which is why many long distance networks mainly transmit at 10 to 20 kbits/s, rarely at 100 kbits/s.
1. Long-distance, high-speed data links suffer from frequency-dependent cable losses, predominantly occurring at high frequencies. These losses contribute to inter-symbol interference and increased jitter.
At higher speeds the R-L-C line characteristics kick in, limiting the maximum cable distance with increasing data rates for conventional transceivers. At even higher rates the transceiver characteristics solely determine the signal quality. The range between 1 and 10 Mbits/s is where many modern applications aim for longer cable runs.
Using conventional high-speed transceivers still produces enormous jitter content due to the transmission medium’s propagation delay, insertion loss, high-frequency rolloff, data pattern-dependent inter-symbol interference (ISI), and delay skews in multi-pair, screened, and unshielded twisted-pair cable (UTP).
Considering a data link consisting of a signal source (transmitter), the transmission medium (cable and connectors), and the signal sink (receiver), each section has its own propagation delay and delay skew that contributes to the total prop-delay and skew of the link.
Synchronous interfaces consist of two data links, one for the synchronizing clock and one for the actual data channel (Fig. 2). To ensure proper data communication, the signal timing between both channels must be accurate to prevent bit errors at the receiving end. Thus, the overall propagation delay is less important than the delay skew between the two channels.
2. Transceiver and cable delay-skew of multi-pair UTP require prop-delay compensation when operating across long distances. For long cable runs, system manufacturers often recommend cost-intensive, high-quality cables.
The terms of cable propagation delay and delay skew are easily understood. Propagation delay exists for all types of transmission media. This delay is the time that passes between when a signal enters a cable channel and then exits at the other end. The delay value for twisted-pair cabling is a function of the signal velocity through the cable, the cable length, and the signal frequency.
The velocity varies with the dielectric materials used in the cable and is a percentage of the speed of light. Most category 5 (CAT-5) cables have velocities of 65% to 70%, whereas dedicated RS-485 cables, such as Belden 3105A (single-pair) and 3109A (multi-pair) cables, have velocities of up to 78%.
Lower velocities contribute to higher delays for a given cable length, just as an increase in cable length causes a proportional increase in prop-delay. Like most other transmission parameters, prop-delays are frequency-dependent.
Multiple signal pairs in the same cable sheath commonly have different prop-delays. Known as delay skew, this difference is affected by the differences in twist rates from pair to pair to minimize crosstalk between signal pairs.
To ensure proper signal transmission, local-area network (LAN) standards specify prop-delay and delay skew in nanoseconds per 100-m cable lengths. For CAT-5 and CAT-5e cable, the maximum nominal propagation delay and delay skew are 550 ns/100m and 45 ns/100m, respectively. Even for dedicated EIA-485 cables with much shorter prop-delays, the delay skew can still reach 15 ns/100 m.
Depending on the signal data rate, the cable length and the transceiver timing performance, the skew between clock and data channel must be eliminated. Typically, this is accomplished through propagation delay compensation at the receiving end. Here, FPGAs include hardware-coded algorithms that measure the time difference between the first arriving signal (either clock or data) and the later arriving one. For this purpose, the implemented logic often uses a sampling clock that is eight times the transmission clock, which in turn is twice the actual data rate.
When considering sending a 5-Mbit/s data stream across a data link, the transmission clock is 10 Mbits/s or 5 MHz, while the oversampling clock in the FPGA is 40 MHz. To keep the delay compensation limited to one transmission clock cycle and, thus, relatively simple, the combination of signaling rate and transceiver and cable performance is important. Hence, at significantly higher data rates the use of high-quality cable with minimum skew is often advised.
Note that a discussion delay skew between transceiver components, also known as part-to-part skew, is mainly important for short data links as there is a chance to avoid delay compensation entirely. For long cable runs, however, the cable’s delay skew dominates as it is several times longer than transceiver part-to-part skew.
source: http://electronicdesign.com/communications/move-high-speed-data-across-long-distances
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