Driver types SN751728 and SN751748 and receivers SN75173A and SN75175A were specifically designed for RS-422-A and RS-485 applications. These parts were designed to be direct plug-in replacements for popular RS-422-A circuits (see Figures 9-85. -86. -87 and -88).
Converting from an RS-422-A system is easy if the AM26LS31 or M0487 drivers and AM26LS32 or MC3486 receivers were previously being used. Conversion from a dedicated station-to-station line to a party line does not require rewiring of printed circuit boards, only the substitution of RS-485 circuits in the boards.
SN75172B series driver inputs arc TTL compatible as previously mentioned. Figure 9-89 shows some additional features.
Using a pnp input transistor provides a relatively high input impedance and allows the input to be both TTL and low-level CMOS compatible. The maximum input high-level current of 20 µA and maximum low-level current of – 360 µA arc well within the capability of low-power Schottky or low-power CMOS gates. Input diodes will clamp any negative voltages at or below – 1.5 V.
The basic driver output circuit configuration in Figure 9-90 is a totem-pole with both source and sink current limiting, High-speed Schottky output pull-up and pull-down transistors provide up to 60 mA of output drive current.
The pull-up output transistor receives its current from the VCC rail through a resistor that is connected across the base-emitter junction of a current sensing transistor. Excessive high-level output current will turn on the current sensing transistor providing shut down drive to an output predrivc transistor (not shown).
The pull-down transistor has a different type of current sensing, Its base is connected to the base of a current-sensing transistor. As the output sink transistor current reaches operating limits its VBE rises to a level high enough to turn on the current-sense transistor. The current-sense transistor draws (by bypassing) base drive current from the output thus limiting the output sink current.
The differential outputs will typically provide 3,5 V VOH and 1.1 V VOL levels for a 2.6 V differential drive (VOD). If operating into a typical 120Ω line-to-line termination. Figure 9-91, the resulting drive current is about 22 mA. If termination is to ground, resistor values arc half of the line impedance. Figure 9-92. Output high-level currents would typically be: VOH/RL/2 or 3.5 V/62Ω = 56 mA.
Driver Speed Characteristics
The SN75172B and SN75174B drivers exhibit a differential output transition time of 75 ns maximum. This particular parameter is key to the maximum speed at which the driver may operate in accordance with EIA RS-485 specifications.
EIA RS-485 defines the device transition time, “tr” relative to one unit interval, “tb”. Sec Figure 9-93. If transition time for the device is known then the maximum operating data rate for RS-485 conditions can be calculated as follows:
For example, the SN75172B or 174B drivers have a specified maximum tr of 75 ns. At the worst-case level the maximum data rate would be:
A typical tr value for the SN75172B and SN75174B is 50 ns and would result in a max fb of 0.3/50 x 10^9 or 6 Mbps.
The SN75173A or SN75175A receiver data input circuit is shown in Figure 9-94.
Series input resistance and active impedances provide a typical input resistance of 20 kΩ. An input common-mode voltage range of 12 V to – 12 V and 50 mV of input threshold hysteresis provide excellent noise immunity. Differential input threshold voltage is ± 200 mV or less allowing the reception of signals that have been attenuated over long line lengths.
The receiver output has typical TTL output characteristics. The equivalent output circuit of the SN75173A/175A is shown in Figure 9-95.
In addition, the output has a high impedance when disabled or powered down. Output leakage current, when disabled, is less than 20 µA. High level output current is limited to 85 mA maximum when the output is shorted to ground.
The SN75173A and SN75175A are built with low-power Schottky technology resulting in propagation delay times of only about 20 ns (sec Figure 9-96).