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 ().
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
