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MAX232 Datasheet

Multichannel RS-232 Drivers/Receivers

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RS-232 Drivers/Receivers
The receiver inputs withstand an input overvoltage up
to ±25V and provide input terminating resistors with
nominal 5kΩ values. The receivers implement Type 1
interpretation of the fault conditions of V.28 and
The receiver input hysteresis is typically 0.5V with a
guaranteed minimum of 0.2V. This produces clear out-
put transitions with slow-moving input signals, even
with moderate amounts of noise and ringing. The
receiver propagation delay is typically 600ns and is
independent of input swing direction.
Low-Power Receive Mode
The low-power receive mode feature of the MAX223,
MAX242, and MAX245–MAX249 puts the IC into shut-
down mode but still allows it to receive information. This
is important for applications where systems are periodi-
cally awakened to look for activity. Using low-power
receive mode, the system can still receive a signal that
will activate it on command and prepare it for communi-
cation at faster data rates. This operation conserves
system power.
Negative Threshold—MAX243
The MAX243 is pin compatible with the MAX232A, differ-
ing only in that RS-232 cable fault protection is removed
on one of the two receiver inputs. This means that control
lines such as CTS and RTS can either be driven or left
unconnected without interrupting communication.
Different cables are not needed to interface with different
pieces of equipment.
The input threshold of the receiver without cable fault
protection is -0.8V rather than +1.4V. Its output goes
positive only if the input is connected to a control line
that is actively driven negative. If not driven, it defaults
to the 0 or “OK to send” state. Normally‚ the MAX243’s
other receiver (+1.4V threshold) is used for the data line
(TD or RD)‚ while the negative threshold receiver is con-
nected to the control line (DTR‚ DTS‚ CTS‚ RTS, etc.).
Other members of the RS-232 family implement the
optional cable fault protection as specified by EIA/TIA-
232E specifications. This means a receiver output goes
high whenever its input is driven negative‚ left uncon-
nected‚ or shorted to ground. The high output tells the
serial communications IC to stop sending data. To
avoid this‚ the control lines must either be driven or
connected with jumpers to an appropriate positive volt-
age level.
On the MAX222‚ MAX235‚ MAX236‚ MAX240‚ and
MAX241‚ all receivers are disabled during shutdown.
On the MAX223 and MAX242‚ two receivers continue to
operate in a reduced power mode when the chip is in
shutdown. Under these conditions‚ the propagation
delay increases to about 2.5µs for a high-to-low input
transition. When in shutdown, the receiver acts as a
CMOS inverter with no hysteresis. The MAX223 and
MAX242 also have a receiver output enable input (EN
for the MAX242 and EN for the MAX223) that allows
receiver output control independent of SHDN (SHDN
for MAX241). With all other devices‚ SHDN (SHDN for
MAX241) also disables the receiver outputs.
The MAX225 provides five transmitters and five
receivers‚ while the MAX245 provides ten receivers and
eight transmitters. Both devices have separate receiver
and transmitter-enable controls. The charge pumps
turn off and the devices shut down when a logic high is
applied to the ENT input. In this state, the supply cur-
rent drops to less than 25µA and the receivers continue
to operate in a low-power receive mode. Driver outputs
enter a high-impedance state (three-state mode). On
the MAX225‚ all five receivers are controlled by the
ENR input. On the MAX245‚ eight of the receiver out-
puts are controlled by the ENR input‚ while the remain-
ing two receivers (RA5 and RB5) are always active.
RA1–RA4 and RB1–RB4 are put in a three-state mode
when ENR is a logic high.
Receiver and Transmitter Enable
Control Inputs
The MAX225 and MAX245–MAX249 feature transmitter
and receiver enable controls.
The receivers have three modes of operation: full-speed
receive (normal active)‚ three-state (disabled)‚ and low-
power receive (enabled receivers continue to function
at lower data rates). The receiver enable inputs control
the full-speed receive and three-state modes. The
transmitters have two modes of operation: full-speed
transmit (normal active) and three-state (disabled). The
transmitter enable inputs also control the shutdown
mode. The device enters shutdown mode when all
transmitters are disabled. Enabled receivers function in
the low-power receive mode when in shutdown.
Maxim Integrated | 15

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