MAX3080 Maxim, MAX3080 Datasheet - Page 15

• Manufacturer Part Number: MAX3080
• Description: Fail-Safe / High-Speed 10Mbps / Slew-Rate-Limited RS-485/RS-422 Transceivers
• Manufacturer: Maxim

Figure 5. Driver DC Test Load
The MAX3080 family guarantees a logic-high receiver
output when the receiver inputs are shorted or open, or
when they are connected to a terminated transmission
line with all drivers disabled. This is done by setting the
receiver threshold between -50mV and -200mV. If the
differential receiver input voltage (A-B) is greater than
or equal to -50mV, RO is logic high. If A-B is less than
or equal to -200mV, RO is logic low. In the case of a
terminated bus with all transmitters disabled, the
receiver’s differential input voltage is pulled to 0V by
the termination. With the receiver thresholds of the
MAX3080 family, this results in a logic high with a 50mV
minimum noise margin. Unlike previous fail-safe
devices, the -50mV to -200mV threshold complies with
the ±200mV EIA/TIA-485 standard.
The MAX3089 has several programmable operating
modes. Transmitter rise and fall times are programma-
ble between 2500ns, 750ns, and 25ns, resulting in
maximum data rates of 115kbps, 500kbps, and
10Mbps, respectively. To select the desired data rate,
drive SRL to one of three possible states by using a
three-state driver, by connecting it to V
by leaving it unconnected. For 115kbps operation, set
the three-state device in high-impedance mode or
leave SRL unconnected. For 500kbps operation, drive
SRL high or connect it to V
drive SRL low or connect it to GND. SRL can be
changed during operation without interrupting data
communications.
Occasionally, twisted-pair lines are connected back-
ward from normal orientation. The MAX3089 has two
pins that invert the phase of the driver and the receiver
to correct for this problem. For normal operation, drive
Slew-Rate-Limited RS-485/RS-422 Transceivers
Y
Z
______________________________________________________________________________________
V
OD
MAX3089 Programming
CC
R
R
. For 10Mbps operation,
V
OC
Fail-Safe, High-Speed (10Mbps),
CC
or GND, or
Fail-Safe
TXP and RXP low, connect them to ground, or leave
them unconnected (internal pull-down). To invert the
driver phase, drive TXP high or connect it to V
invert the receiver phase, drive RXP high or connect it
to V
when RXP is high.
The MAX3089 can operate in full- or half-duplex mode.
Drive the H/F pin low, leave it unconnected (internal
pull-down), or connect it to GND for full-duplex opera-
tion, and drive it high for half-duplex operation. In full-
duplex mode, the pin configuration of the driver and
receiver is the same as that of a MAX3080 (Figure 4). In
half-duplex mode, the receiver inputs are switched to
the driver outputs, connecting outputs Y and Z to inputs
A and B, respectively. In half-duplex mode, the internal
full-duplex receiver input resistors are still connected to
pins 11 and 12.
The standard RS-485 receiver input impedance is 12k
(one-unit load), and the standard driver can drive up to
32 unit loads. The MAX3080 family of transceivers have
a 1/8-unit-load receiver input impedance (96k ), allow-
ing up to 256 transceivers to be connected in parallel
on one communication line. Any combination of these
devices and/or other RS-485 transceivers with a total of
32 unit loads or less can be connected to the line.
The MAX3080–MAX3085, and MAX3089 with SRL = V
or unconnected, are slew-rate limited, minimizing EMI
and reducing reflections caused by improperly termi-
nated cables. Figure 14 shows the driver output wave-
form and its Fourier analysis of a 20kHz signal
transmitted by a MAX3086/MAX3087/MAX3088, and
MAX3089 with SRL = GND. High-frequency harmonic
Figure 6. Receiver Enable/Disable Timing Test Load
CC
RECEIVER
OUTPUT
. Note that the receiver threshold is positive
15pF
C
TEST POINT
RL
Applications Information
Reduced EMI and Reflections
256 Transceivers on the Bus
1k
S1
S2
1k
V
CC
CC
. To
CC
15