EIA-485 (RS-485) may seem a little retro. But the standard is still the simplest way to drive digital data over long distances (i.e., 4000 feet) with multidrop capability. Recognizing this, Maxim Integrated Products' has come up with a pair of chips that simplify RS-485 implementations. The MAX13487E and MAX13488E half-duplex RS-485 transceivers achieve this simplification with a feature Maxim calls AutoDirection.
This feature automatically enables the driver when transmitting data, eliminating the driver-enable control signal. The AutoDirection circuitry consists of a state machine and an additional receive comparator that determines whether the device is trying to drive the bus or another node on the network is driving the bus (see the figure).
The internal state machine in Maxim's transceivers has two inputs: A-B, determined by a differential comparator, and driver input (DI). A is the non-inverting receiver input/driver output, and B is the inverting receiver input/driver output. DI is an internal state. When it is low, the non-inverting output is forced low, the inverting output is forced high, and vice versa. The state machine also has two outputs: DRIVER_ENABLE and its complement, RECEIVER_ENABLE.
When DI is low, the state machine drives the bus low. When DI is high, it drives the bus for a short time and then disables the driver. This allows external pullup/pulldown resistors to hold the bus in the high state (A-B > 200 mV). During each low-to-high transition of DI, the driver remains enabled until (A-B) exceeds the driver-disable threshold (VDT). The driver is disabled at this point, letting the pullup/pulldown resistors hold the A and B lines in the correct state.
The MAX13487E uses reduced slew-rate drivers that minimize electromagnetic interference and reduce reflections caused by improperly terminated cables, allowing error-free transmission up to 500 kbits/s. The MAX13488E driver slew rate isn't limited, making throughput of up to 16 Mbits/s possible.
Maxim Integrated Products
www.maxim-ic.com