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SP507CF Datasheet(PDF) 4 Page - Sipex Corporation |
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SP507CF Datasheet(HTML) 4 Page - Sipex Corporation |
4 / 30 page 4 SP505/6/7APN/03 SP505, SP506, SP507 Application Note © Copyright 2000 Sipex Corporation DCE Configuration to a DB-25 Serial Port The SP505, SP506 and SP507 can also be easily configured as a DCE in all serial communication applications. Figure 1 summarizes the usual signals used in synchronous serial communications. However when sourcing the signal by the DCE, the transceiver must be configured as a driver. The basic configuration shown in Figure 3 illustrates the connection to a DB-25 D-sub connector. Programmable DTE/DCE Configuration to a DB-25 Serial Port The SP505, SP506 and SP507 can also be conveniently configured so that the interface is programmable for either DTE or DCE. Extra attention must be paid to the direction of the signals since there may be bidirectional signals present. Figure 4 and 5 illustrate a connection to a DB-25 D-sub connector using the SP506 and SP507, respectively. When bidirectional signals are needed, this usually means a driver and receiver are half-duplexed together. In other words, the driver outputs are connected to the receiver inputs. This requires the driver outputs to be disabled and at a high impedance state. The receiver does not require a disable function as long as the inputs are high enough impedance so that the driver signals are not attenuated. A half-duplexed receiver without a disable function will still produce a signal at its output when the driver is active and communicating with the receiver at the other end of the cable. This signal can be ignored unless the receiver output is tied to the driver input. If this is the case, then the receiver output should a buffered with a latch or 2:1 mux in order to direct the driver input or receiver output into the HDLC device. The SP507 has additional receivers with enable lines for easier DTE/DCE implementation. The SP505, SP506 and SP507 can be configured on the equipment as either DTE or DCE to the DB-25 connector. For the illustration on Figure 4, DTE is used with the SP506. Since only a DB-25 connector is used as the equipment's serial port, daughter cables are still needed for the other connector types. In addition, to support DCE on this serial port, crossover cables are used. Thus, the equipment will need to provide a DTE V.35 cable and a DCE V.35 cable, for example. Crossover cables merely reroute the signals to the appropriate connector pin assignment. For DTE in V.35 mode, pins P and S are used for Transmit Data (ITU#103), and pins R and T are used for Receive Data (ITU#104). Pins P and S are connected to the driver outputs since they are sourced from the DTE. Pins R and T are connected to the receiver inputs since they are sourced for the DCE. To convert the serial port to a DCE configuration, the crossover cable swaps the signals to those pins. Specifically, the DB-25 will have pins 2 and 14 connected to the driver and pins 3 and 16 connected to the receiver. This is a normal DTE allocation. However, by the time these signals reach the other end of the cable to the ISO2593 V.35 connector, the pins 2 and 14 now go to R and T, respectively. Pins 3 and 16 on the DB-25 side now go to pins P and S, respectively. Therefore, pins R and T are now generating the data and thus, connected to the driver output. Similarly for pins P and S, now connected to the receiver inputs. The configuration on Figure 5 uses the SP507 in a popular DTE/DCE configuration. The TxC signal is half-duplex and bidirectional. The DCE_ST driver is active during DCE mode while the DTE_ST receiver is active during DTE mode. The STEN and SCTEN enable lines are connected together for common DCE/DTE control. Similarly with the RL/DCD pair and the LL/TM pair. The DCD signal is used for this driver labelled RL in this case. The Remote Loopback function is not available in this configuration. The same goes for the Test Mode function where the TM receiver is used for Local Loopback when in DCE mode. On-Board Programmable DTE/DCE Configuration (Without Crossover Cables) DTE/DCE programmability can also be achieved without using crossover cables. Instead, the selection can be designed in the circuitry. This requires a bidirectional serial port for all signals, not just TxC and DCD. An "on-board" solution would need to have circuitry allocated for DTE and circuitry allocated for DCE. The transceiver portion would need to address disable functions, low leakage currents, and specific timing issues when joined together in a half-duplex configuration. |
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