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 Abstract: The article has carried on the analysis and the discussion to the many kinds of agreement serial communications, for gathers the multi-agreement serial port chip which Linear Corporation produces, realizes the question as well as the actual WAN serial communication demand in view of the traditional serial port correspondence, proposed more than one kind of agreement serial interface’s design realizes the method.

    Key word: Multi-agreement serial port correspondence; Communication protocol; Transceiver; Coupling; Multi-agreement serial port chip LTC1546/LTC1544

Along with the communications network technology’s further development, more and more interconnection network equipment (for example router, switch, gateway, deposit installment) the serial interface (Wide Area Network) is designed in WAN can support many kinds of physical interface agreement or the standard. The WAN serial port agreement includes RS-232, RS-449, EIA-530, V.35, V.36 as well as X.21 and so on. Shown in Figure 1 is a simple serial communication connection schematic drawing. The figure shows, realizes the multi-agreement serial port correspondence key is the different transmission mode 熎 rubber which sends the coupling suan⒎Looks at the rubber long-tailed monkey 牶 tube bright electric signal to transform through the transceiver for the terminal can distinguish and process, has the TTL level signal.

1 traditional multi-agreement correspondence characteristic and question

1.1 “sub-board” way

In the WAN serial port application realizes the method is general for each physical agreement which needs provides an independent sub-board. Supports EIA-232, EIA-449 and the V.35 agreement system, usually needs three independent sub-boards as well as three different couplings. This method as a result of each kind of agreement request disposition together child board, therefore the system needs to the PCB sub-board, the transceiver chip, the coupling and so on to carry on the management, like this both waste resources, and will cause the supervisory work complication.

    1.2 general coupling ways

For the solution “the sub-board” the way shortcoming, may use together the master board and the general coupling. On a master board has many kinds of transceiver chips, may satisfy the multi-serial port agreement the request, and may use in common some general components, simultaneously may reduce the resources the waste. In the disposition, should pay attention to the question which are few because of coupling’s base pin brings, the good means are according to the signal, but is not assigns the base pin according to the agreement, namely for each signal distribution general base pin, no matter but how does its physical agreement define. If to EIA-232, EIA-449, EIA-530, V.35 and V.36, its TxD signal may link to the coupling same base pin. Namely the SD 煟 curtain 犘 lotus root handkerchief receives the base pin 2, SD 煟 long-tailed monkey 犘 lotus root handkerchief to receive the base pin 14. Then uses this to describe all agreement sending signal TxD to the base pin.

This method will similarly also bring a question, namely all transceiver’s must each other use in common I/O line to the general coupling’s base pin. For example, in a V.28 driver chip’s transmission data holding wire meets coupling DB-25 the base pin 2; At the same time, in a V.11 driver chip’s transmission data holding wire must meet to coupling’s base pin 2 and 14; But in the V.35 driver chip will transmit the data holding wire also to meet to coupling’s base pin 2 and 14. Thus, the general coupling’s base pin 2 will meet simultaneously has three holding wires, the base pin 14 meets has two holding wires. Thus, in this disposition, all drivers must have three condition characteristics, forbids the nonessential output. If transceiver not three condition characteristics, then needs to use a multi-channel multiplexer to choose the corresponding out-port. From this brings another question is the transceiver when forbids the use will produce the leakage electric current. If has chosen the V.28 agreement, its output voltage theoretical value is 15V. This time will be been forbid regarding the V.11 agreement’s driver, but will be in three condition time, its output leakage voltage must be low enough, can cause to be continually not affected in the identical coupling base pin’s V.28 agreement driver signal. If has the room partition switch between the transmitter and the receiver, then the switch must consider the leakage situation.

1.3 serial port DTE/DCE pattern cuts

The DTE/DCE cut may transform the electric cable through the choice different coupling to realize, like this, when realizes the DTE/DCE transformation may the greatest degree reduce the transceiver the complexity, but the shortcoming is needs to replace the electric cable, particularly the equipment lays aside the position inconvenient or DTE/DCE needs to cut frequently when this point outstandingly.

If maintains transmits the electric cable to be invariable, then may be two sets by supports separately the transceiver disposition DTE, the DCE way. But outputs the DTE transceiver’s driver with the DCE transceiver receiver input is connected, but outputs connected the receiver input end and the DCE transceiver’s driver. In order to control DTE or the DCE way, the driver or receiver’s output must be three condition. When the choice is the DTE way, the DCE chip forbids, its driver and the receiver are in three condition, vice versa.

Although this method has solved to electric cable’s frequent replacement question, but has caused the designed cost as a result of multipurpose set of transceivers is the enhancement greatly, and string oral plate’s volume also big many.

more than 2 agreement serial port correspondence realizes the principle

In the traditional design, usually should choose the corresponding receiving and dispatching chip in view of some kind of agreement, like regarding the RS-232 agreement, commonly used transceiver chips and so on DS-1488/DS-1489, MAX232 or SP208; But regarding the RS-449 agreement, then often uses transceiver chips and so on SN75179B, MAX488, MAX490. When simultaneously uses RS-232, RS-422 and the V.35 agreement, needs many transceiver chips to support the different agreement.

Now, some transceiver’s production manufacturer developed the multi-agreement transceiver chip. Sipex is the first household utensils delivers RS-232/the RS-422 software to be possible to choose agreement chip SP301 the company. This kind of chip may synthesizes RS-232 and the RS-422 transceiver electrical specification to a chip in realizes. And the SP50X serial products are most may support 8 kind of protocol standards. Other Manufacturer like Linear Corporation produces the LTC154x series, the LTC284x series chip also has the above function. The user may according to own need to choose the suitable chip.

Figure 2 is uses the separation the transceiver chip with to use more than a piece of agreement transceiver chip to realize the multi-agreement serial port correspondence correspondence card. May know from the chart, the former realizes the order of complexity must be bigger than the latter by far, concrete performance comparison like table 1 arranges in order.

Table 1 two methods realize the serial port correspondence performance comparison

In addition, compares with the separation transceiver chip, the multi-agreement transceiver enables the control to the driver and to outputs the leakage electric current processing to be much easier. When chooses some agreement through the software or the hardware method, the driver and receiver’s electrical parameter will adjust to the suitable size, the electric circuit interior the automatic control driver’s power output, receiver’s input threshold, the driver and receiver’s resistance value as well as each physical level agreement commonly used pattern scope.

Moreover, as a result of the exterior network termination to the V.35 demand, enables with the V.35 transceiver connection to look like other agreements to be so simple. When use separation transceiver chip, through uses the expensive relay switch resistance frequently when chooses other agreement connection the V.35 network termination separation, or the request user chooses a new interface standard to change a terminal module every time, like this both the waste resources and will cause the interface circuit will become complex, will thus not be one kind ideal realizes the method. But the multi-agreement serial port chip supplies automatically on the suitable terminal and the piece the switch conforms to V.10, V.11, V.28 and the V.35 electricity agreement, thus has solved the cable termination transformation problem.

3 based on LTC1546/44 multi-agreement correspondence

In order to explain that the multi-agreement serial port chip the principle of work, presently chip carries on the analysis take Linear Corporation’s LTC1546/1544 as the example.

3.1 LTC1546/LTC1544 performance

The LTC1546 chip is a 3 driver /3 receiver’s transceiver, its main feature is as follows:

* has the transceiver which the software may elect to be possible to support RS232, RS449, EIA530, EIA530A, V.35, V.36 and the X.21 agreement

* may provide on the piece the cable termination

* and the LTC1543 pin is compatible

* may complete complete DTE or DCE with the LTC1544 coordination

* work in the 5V single power source

* the perch area is small.

The LTC1544 chip is a 4 driver /4 receiver’s transceiver, its main feature includes:

* the software may elect the transceiver supports RS232, RS449, EIA530, EIA530A, V.35, V.36 and the X.21 agreement

* uses LTC1344The cable termination which may elect as the software

* uses LTC1543, LTC1544A either LTC1546 may realize complete DTE or the DCE port

* and LTC1543 similar work in the 5V single power source.

These two kind of chips use 28 lead wire SSOP surface to paste the seal, shown in Figure 3 for its pin arrangement.

May be composed set of complete softwares by LTC1546/LTC1544 to be possible to choose DTE or the DCE connection, meets uses in data net, the information service unit 煟 vast cherry winking 牶 sheds according to service unit (DSU) or in the data router, it supports many kinds of agreements, the cable termination may provide on the piece, therefore no longer needs the independent terminal design. And, LTC1546 each port’s half use for to produce with terminate the clock and the data signal suitably. LTC1544 uses for to produce the control signal and the local loop inverse signal (Local Loop-back, LL). The connection agreement through pattern choice pin M0, M1 and M2 decided that the concrete selector mode see Table 2.

Table 2 communication protocol pattern choice

May know by Table 2, if the port establishment is the V.35 pattern, the pattern choice pin must be M2=1, M1=0, M0=0. This time, regarding the control signal, the driver and the receiver will work in V.28(RS232) pattern; But regarding the clock and the data signal, the driver and the receiver will work in the V.35 pattern.

The pattern choice may with jump the line through the control circuit 熁 clam to meet the pattern pin to the place or Vcc 犂 word Zhiza foot M0, M1 and the M2 control, may also insert through the suitable interface cable to the coupling on realizes the external selection control. If selects the latter, when then puts aside the electric cable, the complete pattern pin does not connect, namely M0=M1=M2=1, this time LTC1546/LTC1544 enters the non-electric cable pattern. In this kind of pattern, the LTC1546/1544 supply current will drop to 500μA below, and LTC1546/the LTC1544 driver will output will be forced to enter the high-resistance condition. At the same time, LTC1546 R2 and the R3 receiver must use 103Ω the termination separately, but on LTC1546 and LTC1544 other receivers should receive through 30kΩ the resistance.

May enable in LTC1546 through the DCE/DTE pin in the driver 3/receiver 1, LTC1544 driver 3/receiver 1 and the driver 4/receiver 4; In the LTC1544 INVERT signal enables the function to driver 4/receiver 4. May through the following two method in one kind the LTC1546/LTC1544 establishment be DTE or the DCE working pattern: One kind has the suitable polar coupling to meet specially to DTE or the DCE end; Another kind is gives LTC1546/LTC1544 through the special-purpose DTE electric cable or the special-purpose DCE electric cable sending signal, simultaneously uses a coupling to constitute one kind both to suit DTE and to suit DCE the working pattern.

3.2 model applications

Figure 4 is one has the DB-25 coupling port and may by the establishment be DTE or the DCE working pattern multi-agreement serial port communication circuit, in the chart one side the LTC1546/LTC1544 chip is connected with the coupling, another edge joint to the HDLC chip, M0, M1, M2 and the DCE/DTE pin meets to the EPLD hardware control electric circuit realizes to the communication protocol and the working pattern choice. And DTE or the DCE working pattern needs to connect corresponding the electric cable to guarantee the correct signal transmission. For example, in the DTE pattern, the TxD signal 1 transmits through the LTC1546 driver to the pin 2 and 14. In the DCE pattern, the driver transmits the RxD signal to the pin 2 and 14.

In Figure 4, LTC1546 uses interior Rong Xingchong the electric pump to satisfy VDD and VEE. And, VDD is conforms to the V.28 positive electricity source voltage terminal, this end should connect 1F the electric capacity to arrive; VEE is the negative supply voltage terminal. A voltage multiplier produces approximately the 8V voltage in General VDD, but the voltage phase reverser will produce approximately - the 7.5V voltage on VEE. Four 1μF electric capacities are the tantalum which or the ceramic electric capacity the surface pastes installs, the VEE end electric capacity are smallest should for 3.3μF. All electric capacity pressure resistance should be 16V, simultaneously should lay aside as far as possible in LTC1546 neighbor reduces the EMI disturbance.

Figure 4 uses the LTC1546/LTC1544 chip to realize the multi-agreement serial port correspondence (DTE/DCE to be possible to elect)

    In the V.35 pattern, in LTC1546 switch S1 and S2 the breakover, simultaneously should connect a T network impedance, by 30kΩ the input impedance and the T network termination is parallel the receiver, but will obviously not affect the total input impedance, therefore regarding the user, under this kind of pattern circuit design and other patterns is completely same.

Because LTC1546 is 3 driver /3 receiver’s transceiver, LTC1546 is 4 driver /4 receiver’s transceiver, if therefore simultaneously uses RL, LL and the TM signal, then LTC1546/LTC1544 does not have the enough driver and the receiver. Therefore, available LTC1545 replaces LTC1544. LTC1545 is 5 driver /5 receiver’s transceiver, it can process the control signal which many may elect, like TM and RL.

All LTC1546/LTC1544 receiver has the expiration protection function under the complete pattern. If the receiver input floating or through a terminal resistance pipe nipple in the same place, that receiver’s output forever by the compulsion will be a logical high level.

4 concluding remark

Realizes the multi-agreement serial port correspondence method to be many, the different factory provides has the function various serial port chip. The designer may act according to own demand to choose. When designs supports each physical level agreement the complex DTE/DCE pattern, with uses many separations the primary devices to compare, chooses a monolithic multi-agreement serial transceiver to simplify the disposition, simultaneously designs the electric circuit will be also more nimble, is convenient and is simple.