Programmable DTE/DCE,
+5V RS-232 Transceiver
MAX214
Table 2. Control Pin Configurations
CONTROL INPUTS
SHUTDOWN
0
0
0
0
1
1
1
1
HI-Z
0
0
1
1
0
0
1
1
DTE/DCE
0
1
0
1
0
1
0
1
TRA, TRB, TRC
Transmit Mode
Receive Mode/5kΩ
Transmit Mode
Receive Mode/HI-Z
Disabled/HI-Z
Slow Receive/HI-Z
Disabled/HI-Z
Slow Receive/HI-Z
RS-232 PINS
RTA, RTB, RTC, RDTC
Receive Mode/5kΩ
Transmit Mode
Receive Mode/HI-Z
Transmit Mode
Slow Receive/HI-Z
Disabled/HI-Z
Slow Receive/HI-Z
Disabled/HI-Z
RRE
Receive Mode/5kΩ
Receive Mode/5kΩ
Receive Mode/HI-Z
Receive Mode/HI-Z
Slow Receive/HI-Z
Slow Receive/HI-Z
Slow Receive/HI-Z
Slow Receive/HI-Z
which improves noise margins. The polarity of each
receiver’s input threshold is shown in Tables 1a and 1b.
In normal operating mode, receiver inputs are internally
connected to ground with 5kΩ resistors. So uncon-
nected receivers with positive input thresholds have
high outputs, and those with negative input thresholds
have low outputs.
When shut down, all receivers have positive thresholds.
This allows the receiver inputs to respond to
TTL-/CMOS-logic levels, as well as RS-232 levels. The
guaranteed 0.8V input threshold ensures that receivers
shorted to ground will have a logic 1 output. Also, the
300kΩ input resistance to ground ensures that a receiv-
er with its input left open will also have a logic 1 output.
The receiver’s 0.5V of hysteresis provides clean output
transitions, even with slow rise-time and fall-time sig-
nals with moderate amounts of noise and ringing. The
receivers have no hysteresis in shutdown mode.
Shutdown Control
In shutdown mode, the charge pumps are turned off,
V+ is pulled down to V
CC
, V- is pulled to ground, and
the transmitter outputs are disabled. This reduces sup-
ply current typically to 4µA. The time required to exit
shutdown is about 250µs, as shown in Figure 3.
Receivers
Receiver outputs never go into a high-impedance state;
they are always active, even in shutdown mode (see
Table 2). These awake-in-shutdown receivers are use-
ful for monitoring external activity (for example, on RI),
while maintaining minimal power consumption.
Receivers in shutdown mode are slower (20kbps) than
when not shut down (116kbps), and lack the hysteresis
present in normal operation.
Drivers
The driver outputs are high impedance in shutdown
mode, even when back-driven with voltages up
to ±15V.
HI-Z Control
The receiver inputs are terminated with 5kΩ resistors, to
comply with the requirements of EIA/TIA-232E.
However, these internal resistors can be disconnected
by taking the HI-Z control pin to a logic high. This
makes all of the MAX214’s receiver inputs high imped-
ance, and facilitates the transmission of RS-232 data
from a single transmitter to multiple receivers. In this
case, all but one of the receiving ICs should be put into
the high input-impedance state.
__________Applications Information
Capacitor Selection
The type of capacitor (C1 to C4) used is not critical for
proper operation. The MAX214 requires 1µF capaci-
tors, although in all cases capacitors of up to 10µF
can be used without harm. Ceramic dielectrics are
suggested for the 1µF capacitors.
8
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