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SP236AET Datasheet(PDF) 10 Page - Sipex Corporation |
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SP236AET Datasheet(HTML) 10 Page - Sipex Corporation |
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10 / 17 page  Date: 8/3/04 + 5V Powered Multi-Channel RS-232 Drivers/Receivers © Copyright 2004 Sipex Corporation 10 of the charge pump. If the SP2XXB is powered up with SD low, then only the 2ms delay time will apply (refer to Figure 6). Under normal operation, both the wait time and delay time should be transparent to the user. With only the receivers activated, the device typically draws less than 5 µA (10µA max) sup- ply current. In the case of a modem interfaced to a computer in power-down mode, the RI (ring indicator) signal from the modem would be used to “wake up” the computer, allowing it to accept the data transmission. After the ring indicator signal has propagated through the SP2XXB receiver, it can be used to trigger the power management circuitry of the computer to power up the microprocessor and bring the SD pin to the SP2XXB low, taking it out of shutdown. The receiver propagation de- lay is typically 1 µs. The enable time for V+ and V- is typically 2ms. After V+ and V- have settled to their final values, a signal can be sent back to the modem on the DTR (Data Terminal Ready) pin signifying that the computer is ready to accept and transmit data. All receivers that are active during shutdown maintain 500mV (typ.) of hysteresis. Varying Capacitor Values As stated earlier, the capacitor values are some- what non-critical. Since they are an actual compo- nent of the charge pump circuitry, their value will affect its performance, which in turn affects the V OH and VOL levels. There is no upper limit for the value of any of the four capacitors; lower values will impact performance. C 1 and C2 are respon- sible for the charge accumulation and can be reduced to 1 µF; this will increase the output im- pedance of V+ and V–. Reducing these capacitor values will limit the ability of the SP2XXA/B to maintain the dc voltages needed to generate the RS-232 output levels. Capacitors C 3 and C4 can also be reduced to 1 µF; doing so will increase the ripple on V+ and V–. Typically each driver will require 1 µF of capaci- tance as a minimum to operate within all specified parameters; if five drivers are active in the circuit, then C 3 and C4 should be 5µF. In order to operate at these minimum values, the supply voltage must be maintained at +5.0V ±5%. Also, the ambient operating temperature must be less than 60 °C. The capacitor values must be chosen to suit the particular application. The designer must bal- ance board space, cost and performance to maxi- mize the design. The capacitors can be polarized or non–polarized, axial-leaded or surface-mount. As the size and value decrease, so does the cost; however, the value should be chosen to accom- modate worst-case load conditions. IBM Modem Port Interconnections Computer D R D R R D R D Transmitted Data (To Be Printed) Ready To Send Clear To Send Data Set Ready Signal Ground 4 2 5 6 7 Received Data (To Be Printed) Ready To Send Clear To Send Data Set Ready Signal Ground Modem R D Received Data 3 Received Data R D Data Carrier Detect 8 Data Carrier Detect D R Data Terminal Ready 20 Data Terminal Ready R D Ring Indicator 22 Ring Indicator SP239A SP237A INTERFACE EXAMPLE – A MODEM ON THE IBM PC SERIAL PORT The RS-232 standard defines 22 serial interface signals. These signals consist of ground lines, timing, data, control and test signals, plus a set of signals rarely used for a second data channel. Many of these signal lines are not used in typical RS-232 applications; in fact, the IBM® PC serial port is implemented using only nine pins. For example, consider the case of a PC using this nine pin port to communicate with a peripheral device such as a modem. We see the following activity on each of the RS-232 lines as the computer and modem are activated and commu- nicate with each other as well as the remote modem at the other end of the phone line. Signal Ground (GND) The Signal Ground pin acts as a reference for all the other signals. This pin is simply maintained at a 0V level to serve as a level to which all other signals are referenced. Both the PC and the modem will have this line connected to their respective internal ground lines. |
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