Modem Max-2400. Scheme, description

Modem Max-2400. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Computers

Floppy disks are commonly used to exchange information between computers located at a distance from each other. This requires a lot of travel time. And if you need to contact another city? It often happens that at work you have to stay up for IBM in order to finish urgent work. After all, not everyone has a miracle of overseas technology at home. With such difficulty, we managed to assemble an 8-bit "Sinclair", but it turned out that it was not suitable for serious cases.

Main reason:

- incompatibility at the level of floppy disks. But text files can be transferred from IBM to "Sinclair" and vice versa - with the help of a small unit called a modem and a transcoder program. To begin with, the material of Nizhny Tagil authors is offered. The editors hope that the publication will inspire the authors to develop similar structures that meet international requirements for compatibility (Hayes-modem, AT commands, MHP protocols, etc.)

Recently, there have been quite a few developments in the field of computer communications, focused mainly on professional computers such as the IBM PC. At the same time, small PCs of the "Sinclair" type, popular in the amateur radio environment, remained outside the field of interest of large engineering firms, due to the small commercial benefits of developments for computers of this type. We tried to fill this gap, and developed software for transferring information between computers such as IBM PC and "Sinclair".

For the purpose of at least some unification on the part of the IBM PC, the Lexand-2400 modem, which has proven itself in operation on domestic telephone lines, developed by the specialists of the Central Scientific Research Laboratory "MENATEP", is used.

There are three options for connecting computers.

Option one - both subscribers use PCs of the "Sinclair" type. The baud rate is set to 600 baud, data conversion is disabled, the password system is disabled.

The second option is a PC of the "Sinclair" type, connected to an IBM PC (Fig. 1). The transfer rate is set to 600 baud, data conversion is enabled, the password system is enabled.

Modem Max-2400

Option three - both subscribers use a PC such as IBM PC. The baud rate is automatically set to 2400, 1200, or 600 baud depending on the state of the link. Data conversion is disabled, the password system is enabled.

From the side of the PC "Sinclair" it is possible to use a modem, the scheme of which is shown in fig. 2. Functionally, this is a simplified analogue of the Lexand TS-2400 modem taken as the basis. The modem is easy to manufacture and, subject to the nominal values of the parts, it practically does not require adjustment. The modem is controlled through port C of the parallel adapter KR580BB55, connected according to the "Pentagon" scheme. Power can be taken from the conclusions of the KR1818VG93 microcircuit.

Rice. 2 (click to enlarge)

Let us briefly dwell on the capabilities of the MAKS-2400 modem software.

Both programs (for PCs such as "Sinclair" and IBM PC) have the functions of a subscriber's address book, auto-dial on city and long distance communication lines, determining the caller's access level and a mailing list system for working in automatic mode. In addition, the program for IBM PC allows you to work in resident mode and has a built-in directory of city codes, CIS countries.

In addition to the standard functions required for the operation of any modern telecommunications system, several special functions have been introduced into the programs of MAKS modems, which allow increasing the capabilities of the system. For example, a function has been introduced to automatically determine the type of subscriber's computer, which is necessary to set the speed and conditions for data transfer.

In addition to automatic speed control, the modem protocol includes the function of automatically setting the size of the information frame. On lines with a high level of noise and interference, the modem can reduce the size of the information frame from 256 to 8 bits, which allows you to increase the noise immunity of the system, and without losing the resulting data exchange rate. Frame control sequence error checking and correction ensure error-free data transmission.

The modem has been tested on operating city and long-distance telephone lines. The test results showed its high resistance to external interference and the reliability of the transmitted data.

If the transfer-reception operation is successful, the temporary file is renamed to the desired name, and if the transfer is interrupted by mistake, this file is not deleted, which allows you to save some of the possibly necessary information.

The "MAKS-2400" modem, one of the "MAKS" modem family, is installed on IBM-compatible personal computers and provides reliable communication via telephone lines with other PCs (such as "Sinclair", "Corvette", "Poisk", IBM-compatible) if "MAKS" modems are also installed on them. In addition, the modem can work as a "Lexand TS-2400" modem.

The main program of the modem - "MODEM_MK" works in the resident mode. It uses the original information exchange protocol. "MODEM_MK" operates at 600-1200-2400 baud (with automatic baud rate selection and the ability to set the upper speed limit), allows you to receive data in auto answer mode or in simple telephone mode. The "Mailing List (Mail Request)" service mode has also been implemented for operation in computer networks.

The program fully simulates the operation of a telephone set with a memory (telephone directory) - a list of telephone numbers and names of subscribers, which is available at any time. The directory size is limited only by free disk space.

After starting, the program checks if the computer has a modem of the specified type, conducts a cycle of checks to fulfill some launch formalities, and displays a line with its name, version, date, and call conditions. After that, the resident is ready to work. The size of the program in memory is not more than 41 K.

After starting the program "MODEM_MK" can work with other programs and at any time it can be activated by pressing the key twice . The modem mode menu will appear in the upper left corner of the screen. Menu items are selected with function keys.

In F4 mode, using the marker movement keys, you can select the desired subscriber from the directory. By pressing the key, the program switches to dialing mode. Joint pressing of the left key and - view long-distance communication codes. Selecting the desired city - . Simultaneous pressing of the right key and - view international communication codes.

Switching between the Russian and Latin alphabets is carried out with the key . Up to 500 files can be sent or received in one communication session.

It is possible to work with sound: enable auto-answer and set the time - after how many calls the modem will pick up the phone to receive information; determine the number of attempts to auto-dial the subscriber's number, as well as the maximum data transfer rate.

The program is always ready to receive information in the current section if the handset is lifted. In auto-answer mode, the program itself picks up the phone and receives files in the mail section.

The program was tested on all computers available to the author (IBM PC / XT, IBM PC / AT, IBM PS-2 - with MS DOS 3.2 - MS DOS 5.0, DR DOS 6.0 operating systems. No specific incompatibility problems were found when working with all packages. Features of "MODEM_MK" operation are usual for all residents.

"MODEM_MK" is quite "correct" in working with programs that it overrides during initialization, which allows you to get the effect of background work of an overridden task. For example: BORLAND C++ translator, 3D STUDIO, ANIMATOR, etc. continue their work.

Authors: E. Antushev, M. Konevtsov, P. Sokolin; Publication: N. Bolshakov, rf.atnn.ru

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