+5V-Powered, Multichannel RS-232
Drivers/Receivers
nominal 5kΩ values. The receivers implement Type ±
Shutdown—MAX222–MAX242
On the MAX222F MAX235F MAX236F MAX240F and
MAX24±F all receivers are disabled during shutdown.
On the MAX223 and MAX242F two receivers continue to
operate in a reduced power mode when the chip is in
shutdown. Under these conditionsF 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 MAX24±). With all other devicesF SHDN (SHDN for
MAX24±) also disables the receiver outputs.
interpretation 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 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.
The MAX225 provides five transmitters and five
receiversF 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 MAX225F all five receivers are controlled by the
ENR input. On the MAX245F eight of the receiver out-
puts are controlled by the ENR inputF while the remain-
ing two receivers (RA5 and RB5) are always active.
RA±–RA4 and RB±–RB4 are put in a three-state mode
when ENR is a logic high.
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
floating 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 +±.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. NormallyF the MAX243’s
other receiver (+±.4V threshold) is used for the data line
(TD or RD)F while the negative threshold receiver is con-
nected to the control line (DTRF DTSF CTSF RTS, etc.).
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)F three-state (disabled)F 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.
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 negativeF left floatingF
or shorted to ground. The high output tells the serial
communications IC to stop sending data. To avoid thisF
the control lines must either be driven or connected
with jumpers to an appropriate positive voltage level.
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MAXIM [ MAXIM INTEGRATED PRODUCTS ]
