MAX13487E/MAX13488E
Detailed Description
The MAX13487E/MAX13488E half-duplex, high-speed
transceivers for RS-485/RS-422 communication contain
one driver and one receiver. The MAX13487E/
MAX13488E feature a hot-swap capability allowing line
insertion without erroneous data transfer (see the
Hot-
Swap Capability
section). The MAX13487E features
reduced slew-rate drivers that minimize EMI and
reduce reflections caused by improperly terminated
cables, allowing error-free transmission up to 500kbps.
The MAX13488E driver slew rate is not limited, making
data throughput of up to 16Mbps possible.
AutoDirection Circuitry
Internal circuitry in the MAX13487E/MAX13488E, in
conjunction with an external pullup resistor on A and
pulldown resistor on B (see
Typical Application Circuit
),
act to automatically disable or enable the driver and
receiver to keep the bus in the correct state. This
AutoDirection circuitry consists of a state machine and
an additional receive comparator that determines
whether this device is trying to drive the bus, or another
node on the network is driving the bus.
The internal state machine has two inputs:
•DI
• The current state of A-B (determined by a dedicated
differential comparator)
The state machine also has two outputs:
• DRIVER_ENABLE—Internal signal that enables and
disables the driver
• RECEIVER_ENABLE—Internal signal that is the
inverse of the DRIVER_ENABLE signal, but it can be
overridden by an external pin
When DI is low, the device always drives the bus low.
When DI is high, the device drives the bus for a short
time, then disables the driver and allows the external
pullup/pulldown resistors to hold the bus in the high
state (A-B > 200mV). During each low-to-high transition
of DI, the driver stays enabled until (A-B) > V
DT
, and
then disables the driver, letting the pullup/pulldown
resistors hold the A and B lines in the correct state.
Pullup and Pulldown Resistors
The pullup and pulldown resistors on the A and B lines
are required for proper operation of the device
although their exact value is not critical. They function
to hold the bus in the high state (A-B > 200mV) follow-
ing a low-to-high transition. Sizing of these resistors is
determined in the same way as when using any other
RS-485 driver and depends on how the line is terminat-
ed and how many nodes are on the bus. The most
important factor when sizing these resistors is to guar-
antee that the idle voltage on the bus (A-B) is greater
than 200mV in order to remain compatible with stan-
dard RS-485 receiver thresholds.
Idle State
When not transmitting data, the MAX13487E/
MAX13488E require the DI input be driven high to
remain in the idle state. A conventional RS-485 trans-
ceiver has DE and RE inputs that are used to enable
and disable the driver and receiver. However, the
MAX13487E/MAX13488E does not have a DE input,
and instead uses an internal state machine to enable
and disable the drivers. DI must be driven high in order
to go to the idle state.
Hot-Swap Capability
Hot-Swap Inputs
When circuit boards are inserted into a hot or powered
back plane, differential disturbances to the data bus
can lead to data errors. Upon initial circuit-board inser-
tion, the data communication processor undergoes its
own power-up sequence. During this period, the
processor’s logic-output drivers are high impedance
and are unable to drive the DI and RE inputs of these
devices to a defined logic level. Leakage currents up to
±10µA from the high-impedance state of the proces-
sor’s logic drivers could cause standard CMOS enable
inputs of a transceiver to drift to an incorrect logic level.
Additionally, parasitic circuit-board capacitance could
cause coupling of V
CC
or GND to the enable inputs.
Without the hot-swap capability, these factors could
improperly enable the transceiver’s driver.
To overcome both these problems, two different pullup
switches (strong and weak) are turned on during the
power-up. When V
CC
rises, an internal power-up signal
enables a strong pullup circuit. It holds DI and RE high
with 1mA for 15µs. Once the timeout is expired, this
strong pullup is switched off. A weak pullup (100µA)
remains active to overcome leakage on the pin. This
second weak pullup disappears as soon as the micro-
controller forces a low state on these pins. Therefore, in
normal operation (after the first activation), these pins
can be considered as high-impedance pins (CMOS
inputs) without any pullup circuitry.
The AutoDirection state machine is initialized, forcing the
driver disabled. The receiver is enabled in AutoDirection
mode.
Hot-Swap Input Circuitry
The enable inputs feature hot-swap capability. At the
input there are two pMOS devices, M1 and M2 (Figure 9).
When V
CC
ramps from zero, an internal 15µs timer turns
Half-Duplex RS-485-/RS-422-Compatible
Transceiver with AutoDirection Control
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