The DS485 is a low power transceiver for RS-485 and RS-422 communications. The device contains a driver and a receiver. Driver slew rates up to 2.5 Mbps allow operation (see Applications Information section). The transceiver draws 200 μA of supply current when the unloaded or fully loaded drive is disabled and operates from a single +5V supply. The driver is short-circuit current limited and is protected against excessive power dissipation by thermal shutdown of the circuit, placing the driver output in tri-state? (high-impedance state) during fault conditions. The driver guarantees a minimum 1.5V differential output voltage with maximum load (VOD3) over the entire common-mode range. The receiver has a fail-safe feature that guarantees a logic high output if the input is open circuited. The DS485 is a surface mount and DIP package characterized for operation over the industrial and commercial temperature range.

truth table

logic diagram

Application Information

The DS485 is a low power transceiver designed for multidrop applications using RS-485. On the basis that the DS485 can transmit data up to 2.5 Mbps, the ratio of the drive's transition time is 10% of the unit interval (bit time). The maximum data rate may be further limited by the interconnected media. The DS485 provides a standard unit load on the RS-485 bus across a common-mode range of -7V to +12V. This enables up to 32 transceivers (standard unit load) to be connected to the bus. Multiple transceivers can be connected to the bus in which they support reduced unit load (see DS36C278). The DS485 also guarantees that the driver's output voltage differential ages to the worst-case load mode standard terminal load and referenced to a maximum total of 32 unit load mode voltage extremes. With a minimum 1.5V swing into this load, a 1.3V differential noise margin is always supported for transceivers with heavy standard common-mode rejection range. Due to the multipoint nature of the bus, contention between drivers may arise. This will not cause damage to the drivers as they have short circuit protection and thermal shutdown protection as well. Thermal shutdown senses dead temperature perature and puts the driver output into tri-state, which can raise the junction temperature to +150°C if a fault condition occurs resulting in excessive power dissipation. A typical multi-application is shown in Figure-URE below. Note that termination is usually required, but only a certain address is cated on both ends of the cable (rather than at each node). The monly pull-up and pull-down of COM may require the same resistor at the ends of the bus to provide a failsafe bias. All drivers are tri-stated when these resistors are biased to the line. See State Application Note 847 for a complete discussion of differentiated bus failsafe biasing.