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MAX3171CAI

+3.3V Multiprotocol 3Tx/3Rx Software-Selectable Control Transceivers

Maxim Integrated 

+3.3V Multiprotocol 3Tx/3Rx

Software-Selectable Control Transceiver

No-Cable Mode

The MAX3171/MAX3173 enter no-cable mode when the

mode select pins are left unconnected or tied HIGH

(M0 = M1 = M2 = 1). In this mode, the multiprotocol dri-

vers and receivers are disabled and the supply current

is less than 8mA. The receiver outputs enter a high-

impedance state in no-cable mode, which allows these

output lines to be shared with other receivers (the

receiver outputs have an internal pullup resistor to pull

the outputs HIGH if not driven). Also, in no-cable mode,

the transmitter outputs enter a high-impedance state,

so these output lines can be shared with other devices.

Dual Charge-Pump Voltage Converter

The MAX3171/MAX3173 internal power supply consists

of a regulated dual charge pump that provides positive

and negative output voltages from a +3.3V supply. The

charge pump operates in discontinuous mode: If the out-

put voltage is less than the regulated voltage, the charge

pump is enabled; if the output voltage exceeds the regu-

lated voltage, the charge pump is disabled. Each charge

pump requires a flying capacitor (C1, C2) and a reser-

voir capacitor (C3, C4) to generate the V+ and V- sup-

plies. See Figure 5 for charge-pump connections.

The charge pump is designed to supply V+ and V-

power to the MAX3172/MAX3174 in addition to the

MAX3171/MAX3173 internal transceivers. Connect the

MAX3172/MAX3174 V+ and V- terminals to the

MAX3171/MAX3173 V+ and V- terminals, respectively.

Fail-Safe

The MAX3171/MAX3173 guarantee a logic HIGH

receiver output when the receiver inputs are shorted or

open, or when they are connected to a terminated

transmission line with drivers disabled. The V.11 receiv-

er threshold is set between -25mV and -200mV to guar-

antee fail-safe operation. If the differential receiver input

voltage (B - A) is ≥ -25mV, R_OUT is logic HIGH. In the

case of a terminated bus with all transmitters disabled,

the receiver’s differential input voltage is pulled to 0 by

the termination. With the MAX3171/MAX3173 receiver

thresholds, this results in R_OUT logic HIGH with a

25mV (min) noise margin.

The V.10 receiver threshold is set between +25mV and

+300mV. If the V.10 receiver input voltage is ≤ +25mV,

ROUT is logic HIGH. The V.28 receiver threshold is set

between 0.8V and 2.0V. If the receiver input voltage is

≤ 0.8V, ROUT is logic HIGH. In the case of a terminated

bus with transmitters disabled, the V.10/V.28 receiver’s

input voltage is pulled to ground by the termination.

With the MAX3172/MAX3174 receiver thresholds, this

results in R_OUT logic HIGH.

Applications Information

Capacitor Selection

The capacitors used for the charge pumps, as well as

the supply bypassing, should have a low-ESR and low-

temperature coefficient. Multilayer ceramic capacitors

with an X7R dielectric offer the best combination of per-

formance, 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 5). To reduce

the ripple present on the transmitter outputs, capacitors

C3, C4, and C5 can be increased. Do not increase the

value of C1 and C2.

Local Loopback Control Signal

For applications that require the use of local loopback

(LL) signal routing, an extra transceiver is available for

use on the MAX3172/MAX3174 multiprotocol termina-

tion network device.

Cable-Selectable Mode

Figure 6 shows a cable-selectable mulitprotocol inter-

face. The mode control lines (M0, M1, M2, and

DCE/DTE) are wired to the DB-25 connector. To select

the serial interface mode, the appropriate combinations

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 pullups on the MAX3170.

The serial interface protocol of the MAX3171/MAX3173

(MAX3170 and MAX3172/MAX3174) is now selected

based on the cable connected to the DB-25 interface.

V.11 (RS-422) Interface

As shown in Figure 7, the V.11 protocol is a fully bal-

anced differential interface. The V.11 driver generates

±2V (min) between nodes A and B when 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 input must comply

with the impedance curve of Figure 8 and reject com-

mon-mode signals up to ±7V developed across the

cable (referenced from C to C’ in Figure 7).

The MAX3171/MAX3173 V.11 mode receiver has a dif-

ferential threshold between -200mV and -25mV to

ensure that the receiver has proper fail-safe operation

(see Fail-Safe). To aid in rejecting system noise, the

MAX3171/MAX3173 V.11 receiver has a 15mV (typ)

hysteresis. Switch S3 in Figure 9 is open in V.11 mode

to disable the V.28 5kΩ termination at the inverting

receiver input. Because the control signals are slow

(64kbps), 100Ω termination resistance is generally not

required for the MAX3171/MAX3173.

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