Datasheet

The receiver inputs withstand an input overvoltage up to
±25V and provide input terminating resistors with nominal
5kΩ values. The receivers implement Type 1 interpreta-
tion of the fault conditions of V.28 and EIA/TIA-232E.
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 periodically
awakened to look for activity. Using low-power receive
mode, the system can still receive a signal that will acti-
vate it on command and prepare it for communication 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 con-
trol 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 connected 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 voltage level.
Shutdown—MAX222–MAX242
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 receiv-
ers‚ 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 current 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 outputs are controlled by
the ENR input‚ while the remaining 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 transmit-
ters have two modes of operation: full-speed transmit
(normal active) and three-state (disabled). The transmit-
ter 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.
MAX220–MAX249 +5V-Powered, Multichannel
RS-232 Drivers/Receivers
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