MAX3170
receiver threshold between -25mV and -200mV in the
V.11 and V.35 modes. If the differential receiver input
voltage (B - A) is ≥-25mV, R_OUT is logic HIGH. If (B -
A) is ≤ -200mV, R_OUT is logic LOW. In the case of a
terminated bus with all transmitters disabled, the
receiver’s differential input voltage is pulled to zero by
the termination. With the receiver thresholds of the
MAX3170, this results in a logic HIGH with a 25mV min-
imum noise margin.
Applications Information
Capacitor Selection
The capacitors used for the charge pumps, as well as
the supply bypassing, should have a low equivalent
series resistance (ESR) and low temperature coeffi-
cient. Multilayer ceramic capacitors with an X7R dielec-
tric offer the best combination of performance, size,
and cost. The flying capacitors (C1, C2) should have a
value of 1µF, while the reservoir capacitors (C3, C4)
and bypass capacitor (C5) should have a minimum
value of 3.3µF (Figure 6). To reduce the ripple present
on the transmitter outputs, capacitors C3, C4, and C5
can be increased. The values of C1 and C2 should not
be increased.
Cable Termination
The MAX3172/MAX3174 software-selectable resistor
network is designed to be used with the MAX3170. The
MAX3172/MAX3174 multiprotocol termination network
provides V.11- and V.35-compliant termination, while
V.28 receiver termination is internal to the MAX3170.
These cable termination networks provide compatibility
with V.11, V.28, and V.35 protocols. Using the
MAX3172/MAX3174 termination network provides the
advantage of not having to build expensive termination
networks out of resistors and relays, manually changing
termination modules, or building termination networks
into custom cables.
Cable-Selectable Mode
A cable-selectable multiprotocol interface is shown in
Figure 7. The mode control lines M0, M1, and DCE/DTE
are wired to the DB-25 connector. To select the serial
interface mode, the appropriate combination of M0,
M1, M2, and DCE/DTE are grounded within the cable
wiring. The control lines that are not grounded are
pulled high by the internal pull-ups on the MAX3170.
The serial interface protocol of the MAX3170 (and
MAX3171/MAX3173 and MAX3172/MAX3174) is select-
ed based on the cable that is connected to the DB-25
interface.
V.11 (RS-422) Interface
As shown in Figure 8, the V.11 protocol is a fully bal-
anced differential interface. The V.11 driver generates a
minimum of ±2V between nodes A and B when a 100Ω
(min) resistance is presented at the load. The V.11
receiver is sensitive to ±200mV differential signals at
the receiver inputs, A′ and B′. The V.11 receiver rejects
common-mode signals developed across the cable
(referenced from C to C′) of up to ±7V, allowing for
error-free reception in noisy environments. The receiver
inputs must comply with the impedance curve shown in
Figure 9.
For high-speed data transmission, the V.11 specifica-
tion recommends terminating the cable at the receiver
with a 100Ω minimum resistor. This resistor, although
not required, prevents reflections from corrupting trans-
mitted data. In Figure 10, the MAX3172 or MAX3174 is
used to terminate the V.11 receiver. Internal to the
MAX3172/MAX3174, S1 is closed and S2 is open to
present a 100Ω minimum differential resistance. The
MAX3170’s internal V.28 termination is disabled by
opening S3.
V.35 Interface
Figure 11 shows a fully balanced differential standard
V.35 interface. The generator and the load must both
present a 100Ω ±10Ω differential impedance and a
150Ω ±15Ω common-mode impedance as shown by
the resistive T-networks in Figure 11. The V.35 driver
generates a current output (±11mA typ) that develops
an output voltage of ±550mV across the generator and
load termination networks. The V.35 receiver is sensi-
tive to ±200mV differential signals at the receiver inputs
A′ and B′. The V.35 receiver rejects common-mode sig-
nals developed across the cable (referenced from C to
C′) of up to ±4V, allowing for error-free reception in
noisy environments.
+3.3V, Multiprotocol, 3 Tx/3 Rx, Software-
Selectable Clock/Data Transceiver
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