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The MAX239 has a receiver three-state control line, and the MAX223, MAX225, MAX235, MAX236, MAX240, and MAX241 have both a receiver three-state control line and a low-power shutdown control. Table 2 shows the effects of the shutdown control and receiver three state control on the receiver outputs.The receiver TTL/CMOS outputs are in a high-impedance, three-state mode whenever the three-state enable line is high (for the MAX225/MAX235/MAX236/MAX239–MAX241), and are also high-impedance whenever the
Shut down control line is high.
When in low-power shutdown mode, the driver outputs are turned off and their leakage current is less than 1µA with the driver output pulled to ground. The driver output leakage remains less than 1µA, even if the transmitter output is back driven between 0V and (VCC + 6V). Below
-0.5V, the transmitter is diode clamped to ground with1kΩ series impedance. The transmitter is also zener clamped to approximately VCC + 6V, with a series impedance of 1kΩ.
The driver output slew rate is limited to less than 30V/µs as required by the EIA/TIA-232E and V.28 specifications. Typical slew rates are 24V/µs unloaded and 10V/µs loaded with 3Ω and 2500pF.
RS-232 Receivers
EIA/TIA-232E and V.28 specifications define a voltage level greater than 3V as a logic 0, so all receivers invert. Input thresholds are set at 0.8V and 2.4V, so receivers respond to TTL level inputs as well as EIA/TIA-232E and V.28 levels.
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 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 activate 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, differing 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 +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 floating‚ 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.