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MAX3292CSD Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX3292CSD Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 16 page Detailed Description The MAX3291/MAX3292 high-speed RS-485/RS-422 transceivers feature driver preemphasis circuitry, which extends the distance and increases the data rate of reliable communication by reducing intersymbol inter- ference (ISI) caused by long cables. The MAX3291 is programmable for data rates of 5Mbps to 10Mbps, while the MAX3292 is programmable for data rates up to 10Mbps by using a single external resistor. The MAX3291/MAX3292 are full-duplex devices that operate from a single +5V supply and offer a low-cur- rent shutdown mode that reduces supply current to 100nA. They feature driver output short-circuit current limiting and a fail-safe receiver input that guarantees a logic-high output if the input is open circuit. A 1/4-unit- load receiver input impedance allows up to 128 trans- ceivers on the bus. Inter-symbol interference (ISI) causes significant prob- lems for UARTs if the total RS-485/RS-422 signal jitter becomes 10% or more of the baud period. ISI is caused by the effect of the cable’s RC time constant on different bit patterns. If a series of ones is transmitted, followed by a zero, the transmission-line voltage rises to a high value at the end of the string of ones (signal 1 in Figure 12). As the signal moves towards the zero state, it takes longer to reach the zero-crossing, because its starting voltage is farther from the zero crossing. On the other hand, if the data pattern has a string of zeros followed by a one and then another zero, the one-to-zero transition starts from a voltage that is much closer to the zero-crossing (VA - VB = 0) and it takes much less time for the signal to reach the zero- crossing (signal 2 in Figure 12). In other words, the propagation delay depends upon the previous bit pat- tern. This is inter-symbol interference (ISI). Preemphasis reduces ISI by increasing the signal amplitude at every transition edge for about one baud period, counteracting the effects of the cable (see the section Setting the Preemphasis Interval). Figure 13 shows a typical preemphasis waveform optimized for data rates between 5Mbps and 10Mbps. When DI changes from a logic low to a logic high, the differential output switches to a strong high. At the end of the pre- emphasis interval, the strong high returns to a normal high level. Both levels meet RS-485/RS-422 specifica- tions, and the strong levels are typically 1.9 times larger than the normal levels. If DI switches back to a logic low before the end of the preemphasis interval, the dif- ferential output switches directly from the strong high to the strong low. Similarly, this explanation applies when DI transitions from high to low. Applications Information Data Rate vs. Cable Length In general, preemphasis allows either double the dis- tance for a fixed data rate or double the data rate for a fixed existing cable distance over existing RS-485 transceivers that do not feature preemphasis. Figure 14 shows that the MAX3291/MAX3292 transmits approxi- mately twice as far at the same data rate or twice as fast at the same cable length as a conventional RS-485 transceiver without preemphasis for 10% jitter. RS-485/RS-422 Transceivers with Preemphasis for High-Speed, Long-Distance Communication 10 ______________________________________________________________________________________ SIGNAL 2 SIGNAL 1 VA - VB BAUD PERIOD tISI Figure 12. Inter-Symbol Interference among Two Data Patterns: Signal 1 = 11111110, Signal 2 = 00000010 100ns 001 0 1 0 1 1 1 0 VY - VZ DI 2.5V/div 5V/div Figure 13. Typical Preemphasis Waveform with a 100ns Preemphasis Interval |
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