1、附录 A MAX232-5V-Powered,Multichannel RS-232 Drivers/Receivers The MAX232 is a dual driver/receiver that includes a capacitive voltage generator to supply TIA/EIA-232-Fvoltage levels from a single 5-V supply. Each receiver converts TIA/EIA-232-F inputs to 5-V TTL/CMOS levels.These receivers have a typ
2、ical threshold of 1.3 V, a typical hysteresis of 0.5 V, and can accept 30-V inputs.Each driver converts TTL/CMOS input levels into TIA/EIA-232-F levels. The driver, receiver, andvoltage-generator functions are available as cells in the Texas Instruments LinASIClibrary.The MAX220MAX249 contain four s
3、ections: dualcharge-pump DC-DC voltage converters, RS-232 drivers,RS-232 receivers, and receiver and transmitterenable control inputs. Dual Charge-Pump Voltage Converter The MAX220MAX249 have two internal charge-pumpsthat convert +5V to 10V(unloaded) for RS-232 driveroperation. The first converter u
4、ses capacitor C1 to doublethe +5V input to +10V on C3 at the V+ output. Thesecond converter uses capacitor C2 to invert +10V to-10V on C4 at the V- output. A small amount of power may be drawn from the +10V(V+) and -10V (V-) outputs to power external circuitry(see the Typical Operating Characteristi
5、cs section),except on the MAX225 and MAX245MAX247, wherethese pins are not available. V+ and V- are not regulated,so the output voltage drops with increasing load current.Do not load V+ and V- to a point that violates the minimum5V EIA/TIA-232E driver output voltage whensourcing current from V+ and
6、V- to external circuitry. When using the shutdown feature in the MAX222,MAX225, MAX230, MAX235, MAX236, MAX240,MAX241, and MAX245MAX249, avoid using V+ and V- topower external circuitry. When these parts are shutdown, V- falls to 0V, and V+ falls to +5V. For applicationswhere a +10V external supply
7、is applied to the V+pin (instead of using the internal charge pump to generate+10V), the C1 capacitor must not be installed andthe SHDN pin must be tied to VCC. This is because V+is internally connected to VCC in shutdown mode. RS-232 Drivers The typical driver output voltage swing is 8V when loaded
8、 with a nominal 5k RS-232 receiver and VCC =+5V. Output swing is guaranteed to meet the EIA/TIA-232E and V.28 specification, which calls for 5V minimum driver output levels under worst-case conditions. These include a minimum 3k load, VCC = +4.5V, and maximum operating temperature. Unloaded driver o
9、utput voltage ranges from (V+ -1.3V) to (V- +0.5V). Input thresholds are both TTL and CMOS compatible.The inputs of unused drivers can be left unconnected since 400k input pull-up resistors to VCC are built in (except for the MAX220). The pull-up resistors force theoutputs of unused drivers low beca
10、use all drivers invert.The internal input pull-up resistors typically source 12A, except in shutdown mode where the pull-ups are disabled.Driver outputs turn off and enter a high-impedance statewhere leakage current is typically microamperes (maximum 25A)when in shutdown mode, in three-state mode, o
11、r when device power is removed. Outputs can be driven to 15V. The power supply current typically drops to 8A in shutdown mode. The MAX220 does not have pull-up resistors to force the outputs of the unused drivers low. Connect unused inputs to GND or VCC. The MAX239 has a receiver three-state control
12、 line, andthe MAX223, MAX225,MAX235, MAX236, MAX240,and MAX241 have both a receiver three-state control line and a low-power shutdown control. Table 2 showsthe effects of the shutdown control and receiver threestatecontrol on the receiver outputs.The receiver TTL/CMOS outputs are in a high-impedance
13、,three-state mode whenever the three-state enableline is high (for the MAX225/MAX235/MAX236/MAX239MAX241), and are also high-impedance whenever theshutdown control line is high. When in low-power shutdown mode, the driver outputs are turned off and their leakage current is less than 1A with the driv
14、er output pulled to ground. The driver output leakage remains less than 1A, even if the transmitter output is backdriven between 0V and (VCC + 6V). Below -0.5V, the transmitter is diode clamped to ground with 1k series impedance. The transmitter is also zener clamped to approximately VCC + 6V, with
15、a series impedance of 1k. The driver output slew rate is limited to less than 30V/s as required by the EIA/TIA-232E and V.28 specifications. Typical slew rates are 24V/s unloaded and 10V/s loaded with 3 and 2500pF. RS-232 Receivers EIA/TIA-232E and V.28 specifications define a voltagelevel greater t
16、han 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 andV.28 levels.The receiver inputs withstand an input overvoltage upto 25V and provide input terminating resistors withnominal 5k values. The recei
17、vers 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 outputtransitions with slow-moving input signals, evenwith moderate amounts of noise and ringing. Thereceiver prop
18、agation delay is typically 600ns and isindependent of input swing direction. Low-Power Receive Mode Thelow-power receive-mode feature of the MAX223,MAX242, and MAX245MAX249 puts the IC into shutdownmode but still allows it to receive information. Thisis important for applications where systems are p
19、eriodicallyawakened to look for activity. Using low-powerreceive mode, the system can still receive a signal thatwill activate it on command and prepare it for communicationat faster data rates. This operation conservessystem power. Receiver and Transmitter Enable The MAX225 and MAX245MAX249 feature
20、 transmitterand receiver enable controls. The receivers have three modes of operation: full-speedreceive (normal active) three-state (disabled) and lowpowerreceive (enabled receivers continue to function at lower data rates). The receiver enable inputs controlthe full-speed receive and three-state m
21、odes. Thetransmitters have two modes of operation: full-speedtransmit (normal active) and three-state (disabled). Thetransmitter enable inputs also control the shutdownmode. The device enters shutdown mode when alltransmitters are disabled. Enabled receivers function inthe low-power receive mode whe
22、n in shutdown. ShutdownMAX222MAX242 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.
23、5s 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 (EN for the MAX242 and EN for the MAX223) that allowsreceiver output control independent of SHDN (SHDNfor MAX241). With all ot
24、her 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 receiverand transmitter-enable controls. The charge pumpsturn off and the devices sh
25、ut down when a logic high isapplied to the ENT input. In this state, the supply currentdrops to less than 25A and the receivers continueto operate in a low-power receive mode. Driver outputsenter a high-impedance state (three-state mode). Onthe MAX225all five receivers are controlled by theENR input
26、. On the MAX245 eight of the receiver outputsare controlled by the ENR input while the remainingtwo receivers (RA5 and RB5) are always active.RA1RA4 and RB1RB4 are put in a three-state modewhen ENR is a logic high. The MAX247 provides nine receivers and eight driverswith four control pins. The ENRA and ENRB receiverenable inputs each control four receiver outputs. TheENTA and ENTB transmitter enable inputs each controlfour drivers. The ninth
