MAX244EQH-D中文资料(15)

+5V-Powered, Multichannel RS-232

Drivers/Receivers

nominal 5kΩvalues. The receivers implement Type 1interpretation of the fault conditions of V.28 andEIA/TIA-232E.

The receiver input hysteresis is typically 0.5V with aguaranteed minimum of 0.2V. This produces clear out-put transitions with slow-moving input signals, evenwith moderate amounts of noise and ringing. Thereceiver propagation delay is typically 600ns and isindependent of input swing direction.

Shutdown—MAX222–MAX242

On the MAX222 MAX235 MAX236 MAX240 andMAX241 all receivers are disabled during shutdown.On the MAX223 and MAX242 two receivers continue tooperate in a reduced power mode when the chip is inshutdown. Under these conditions the propagationdelay increases to about 2.5µs for a high-to-low inputtransition. When in shutdown, the receiver acts as aCMOS inverter with no hysteresis. The MAX223 andMAX242 also have a receiver output enable input (ENfor the MAX242 and EN for the MAX223) that allowsreceiver output control independent of SHDN(SHDNfor MAX241). With all other devices SHDN(SHDN forMAX241) also disables the receiver outputs.

The MAX225 provides five transmitters and fivereceivers while the MAX245 provides ten receivers andeight transmitters. Both devices have separate receiverMAX220–MAX249

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. Thisis important for applications where systems are periodi-cally awakened to look for activity. Using low-powerreceive mode, the system can still receive a signal thatwill activate it on command and prepare it for communi-cation at faster data rates. This operation conservesand transmitter-enable controls. The charge pumpssystem power.

turn off and the devices shut down when a logic high isapplied to the ENT input. In this state, the supply cur-Negative Threshold—MAX243

rent drops to less than 25µA and the receivers continueThe MAX243 is pin compatible with the MAX232A, differ-to operate in a low-power receive mode. Driver outputsing only in that RS-232 cable fault protection is removedenter a high-impedance state (three-state mode). Onon one of the two receiver inputs. This means that controlthe MAX225 all five receivers are controlled by thelines such as CTS and RTS can either be driven or leftENRinput. On the MAX245 eight of the receiver out-floating without interrupting communication. Differentputs are controlled by the ENRinput while the remain-cables are not needed to interface with different pieces ofing two receivers (RA5 and RB5) are always active.equipment.

RA1–RA4 and RB1–RB4 are put in a three-state modeThe input threshold of the receiver without cable faultwhen ENRis a logic high.

protection is -0.8V rather than +1.4V. Its output goesReceiver and Transmitter Enable

positive only if the input is connected to a control lineControl Inputs

that is actively driven negative. If not driven, it defaultsThe MAX225 and MAX245–MAX249 feature transmitterto the 0 or “OK to send” state. Normally the MAX243’sand receiver enable controls.

other receiver (+1.4V threshold) is used for the data line(TD or RD) while the negative threshold receiver is con-The receivers have three modes of operation: full-speednected to the control line (DTR DTS CTS RTS, etc.). receive (normal active) three-state (disabled) and low-power receive (enabled receivers continue to functionOther members of the RS-232 family implement theat lower data rates). The receiver enable inputs controloptional cable fault protection as specified by EIA/TIA-the full-speed receive and three-state modes. The232E specifications. This means a receiver output goestransmitters have two modes of operation: full-speedhigh whenever its input is driven negative left floating transmit (normal active) and three-state (disabled). Theor shorted to ground. The high output tells the serialtransmitter enable inputs also control the shutdowncommunications IC to stop sending data. To avoid this mode. The device enters shutdown mode when allthe control lines must either be driven or connectedtransmitters are disabled. Enabled receivers function inwith jumpers to an appropriate positive voltage level.

the low-power receive mode when in shutdown.

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