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SP504 Datasheet(PDF) 17 Page - Exar Corporation |
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SP504 Datasheet(HTML) 17 Page - Exar Corporation |
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17 / 32 page  Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 50-668-707 • www.exar.com SP504_02_2708 7 Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 50-668-707 • www.exar.com SP504_02_2708 Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 50-668-707 • www.exar.com SP504_02_2708 Receivers The SP504 has seven (7) independent re- ceivers which can be programmed for the different interface modes. Control for the mode selection is done via a 4–bit control word that is independent from the driver control word. The coding for the drivers and receiversisidentical.Therefore,ifthemodes for the drivers and receivers are supposed to be identical in the application, the control lines can be tied together. Like the drivers, the receivers are pre-ar- ranged for the specific requirements of the interface. As the operating mode of the receivers is changed, the electrical characteristics will change to support the requirements of clock, data, and control line receivers. Table 2 shows the mode of each receiver in the different interface modes that can be selected. There are three basic types of receiver cir- cuits — RS-232, RS-423, and RS-485. The RS-232 receiver is a single–ended input with a threshold of 0.8V to 2.4V. The RS-232 receiver has an operating voltage range of ±5V and can receive signals up to 20kbps. The input sensitivity complies with EIA-RS-232 and V.28 at +3V to -3V. The input impedance is 3kΩ to 7kΩ. RS- 232 receivers are used in RS-232 mode for all data, clock and control signals. They are also used in V.35 mode for control line signals such as CTS and DSR. TheRS-423receiversarealsosingle–ended but have an input threshold as low as ±200mV.Theinputimpedanceisguaranteed tobegreaterthan4kΩ,withanoperatingvolt- age range of ±7V. The RS-423 receivers can operateupto20kbps.RS-423receiversare usedinRS-449,EIA-530,EIA-530AandV.36 modes as Category II signals as indicated by their corresponding specifications. Thethirdtypeofreceiverisadifferentialwhich supports RS-485. The RS-485 receiver has aninputimpedanceof15kΩandadifferential threshold of ±200mV. Since the characteris- tics of an RS-422 (V.) receiver are actually subsets of RS-485, the receivers for RS-422 requirements are covered by the RS-485 receivers. RS-422 receivers are used in RS-449, EIA-530, EIA-530A and V.36 as Category I signals for receiving clock, data, and some controllinesignals. Thedifferentialreceivers can receive data up to 0Mbps. The RS-485 receivers are also used for the V.35mode. Unliketheolderimplementations of differential or V.35 receivers, the SP504 contains an internal resistor termination net- work that ensures a V.35 input impedance of 100Ω (±10Ω) and a short-circuit impedance of 150Ω (±15Ω). The traditional V.35 imple- mentations required external termination resistors to acheive the proper V.35 imped- ances. The internal network is connected via low on-resistance FET switches when the decoder is changed to V.35 mode. The termination network is transparent when all other modes are selected. The V.35 receiv- ers can operate up to 0Mbps. All receivers include a fail-safe feature that outputs a logic HIGH when the receiver inputs are open. For single-ended RS-232 receivers, there are internal 5kΩ pull-down resistors on the inputs which produces a logic HIGH ("") at the receiver outputs. The single-ended RS-423 receivers produce a logicLOW("0")ontheoutputwhentheinputs are open. This is due to a pull-up device connected to the input. The differential re- ceivershavethesameinternalpull-updevice on the non-inverting input which produces a logic HIGH ("") at the receiver output. The three differential receivers when configured in V.35 mode (RxD, RxC & SCT) do not have fail-safe because the internal termination resistor network is connected. Decoder The SP504 has the ability to change the interface mode of the drivers or receivers via an 8–bit switch. The decoder for the drivers and receivers is not latched; it is merely a combinational logic switch. The control word can be externally latched either HIGH or LOW to write the appropriate code into the SP504. The codes shown in Tables 1 and 2 are the only specified, valid modes for the SP504. Undefined codes |
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