Serial data communication connection over GSM data channel.

Bordea, Nicolae ; Sintea, Sorin ; Moraru, Aurel Sorin 等

1. INTRODUCTION

In many occasions we need to transfer data between serial isolated devices (controllers, programmable logic controllers or computers), located at long distance or in inaccessible locations. In these locations the only way to communicate is over GSM network (Tisal, J., 1999; Hammuda H., 1999). Also, all these devices are produced by third party producers and we do not have possibility to upgrade their software or add drivers (Bajenarescu T, 2000). In this case we need new equipment which adapt the signals and connect our isolated devices through GSM network. The equipment will ensure end to end communication connection protocol between three independent serial lines (two R[S.sub.2]32 lines and one RS485 line).

Until now, many manufacturers offer dedicated devices for communicating over GSM using modems connected through USB cables (Bajenarescu T, 2000). But we have a lot of old devices that not support USB connections. Also the modems request software driver or additional hardware modules implemented into connected device. Presented solution solves these requests by introducing a hardware protocol level between two communicating end points equipments.

2. GENERAL DESCRIPTION

The system proposed is composed by two communication devices. Each device is able to communicate with the pair device, and create a direct channel between each corresponding port (Fig. 1.). The device creates three parallel communication channels for each port. Attached to each port the device creates an internal 1024 Bytes buffer for data synchronization. For communication, each port status is show using 3 status LEDs (for data transmission, data receiving and buffer status/busy).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Additional the communication device have attached an Ethernet port. Over this port the operator can connect, using a notebook, with internal CPU unit data and configure the communication device parameters.

To ensure link with GSM network the communication device contains the GSM air connection modem.

The activity of all modules is synchronized by micro-controller unit. The microcontroller run dedicated software for implementation of communication protocol and link connection between unit and GSM modem.

2.1 Hardware Architecture

To solve all this requests (see Fig. 2) we are using an RCM-4010 Rabbit 8 bits CPU unit @ 59 MHz (Rabbit Semiconductor Inc, part of Digi International). This CPU is offering us possibility for connection of four serial USART connection ports (Rabbit Semiconductor Inc., 2006). Also the CPU has one Ethernet port on board for TCP/IP connection and implemented of one Web Server in system driver library. For saving data the CPU contain a data flash on board. Two USARTs from CPU board are connected with RS232 external devices (COM2 and COM3) using RS232 interface line adapters (see Fig. 2.) and one USART is connects RS485 devices (over COM1 connector) with our controller through RS485 interface line adapter. The connection with GSM modem is ensuring using low voltage serial port COM4 from CPU board (Rabbit Semiconductor Inc., 2006).

The Rabbit 4000 series CPU offer us possibility for parallel multitasking software programming through Dynamic-C compiler (Rabbit Semiconductor Inc., 2006).

Modem used for communication with GSM Network is Motorola G24-Lite (G24-L) modem. This modem implements Motorola G24 AT commands for communication and control.

On GSM network the modem supports GSM/CSD protocol for data transfer on quad band GSM networks (850/900/1800/1900 MHz) and GPRS multislot class 10 (Motorola Inc, 2008).

Modem, micro-controller and interfaces are integrated in the same compact enclosure (see Fig. 1). The module can be powered from 8-30V power supply. The external ports (communication ports and Ethernet port) are placed one the front of enclosure for easy access.

[FIGURE 3 OMITTED]

2.2 Software Architecture

All software is created based on parallel programming mechanism, ensured by Dynamic-C compiler language for Rabbit 4000 processors (Rabbit Semiconductor Inc, 2006). The software is divided in five threads, started by main application (see Fig. 3):

* Three of them are responsible with serial data processing incoming over R[S.sub.2]32 connections with local communication devices (named serialProcessl, serialProcess2, serialProcess3);

* One thread is responsible with processing of web interface (named webProcess);

* One thread is monitor communication with GSM modem, and monitor GSM communication line (for testing GSM signal strength, verifying GSM incoming connection), named gsmProcess;

* One thread is responsible with data delivery over GSM communication line (named gsmDataProcess). serialProcess thread, is the base thread which processing incoming data from R[S.sub.2]32 line (COM1, COM2 or COM3). This process is buffering incoming data from R[S.sub.2]32 controller's device driver. The process also drive synchronisation signals between controller and serial attached device. Also, the same process analyse one of the data receiving from GSM remote unit from data queues. If some data are found in attached queue, the data is extract from that queue and send through corresponding serial device driver, byte by byte.

gsmProcess thread is responsible with monitoring of GSM line. This thread monitors if is signalling a GSM connection from other threads. The process will force a connection with remote GSM unit, if GSM signal is able to ensure clear connection with the remote unit. When the connection with remote unit is established the process will send a signal in system. In case of incoming request from remote GSM unit, the process is responsible with ensure of correct answer to establish the connection with remote unit. After this, the process will signal in system that connection is ensured. All protocol syntax is described into G24-L modem device driver (Motorola Inc., 2008).

gsmDataProcess thread is responsible with processing of stream of data transferred through GSM communication line with remote unit. The activity of this process is divided into two main tasks. One task is involved with testing of serial receiving queue; in case of detecting at least one character, the thread will request a GSM connection with the remote unit over G24L modem. The other task of this thread is monitoring of incoming data stream from GSM network. When data are receiving, the task get data extract from data package the serial information data and transfer it to destination data buffer.

And at last is the webProcess thread. This thread is responsible with presentation of the status of the unit and configure of the unit on a web page. This web page can be accessed through TCP/IP layer over Ethernet port. Using this thread we can configure the following parameters:

* The communication USART parameters (speed, parity, even bit, serial word width, hardware protocol levels etc)

* The phone number of remote unit

* Buffer size for each channel (1 for direct connection)

All settings are saved into a local flash memory.

3. RESULTS

The link channel realised with this devices ensures a serial connection at 600-19200 bps speed between end equipments.

In domain 600-4800 bps speed, the connection protocol layer does not request data buffering. The data delay transmission (between data send and data recreated at end point) is about 2 ms + 1 / speed (where Speed is the data transmission speed).

When we are using the higher speed than 4800 bps, the data buffer is essential, and also the terminal must use hardware handshake signals to synchronize data transfer. The transmission delay is about 2 ms.

The connection time is depending by the calling time in GSM network. If you want to obtain good results (low time connection response) we recommend using an "auto connection" mode; in this case the communication devices will try to keep the data line always open.

4. CONCLUSIONS

Current equipment is created for an easy way of connection between two PLCs or controllers at long distance range using GSM network. The connection does not request any other equipments or software drivers.

This solution is very efficient when we do not have possibility of upgrading of equipments or software implemented into the controllers or PLCs. The adopted solution does not interpret the transferred information; it is presented as a protocol layer between the end points of communications.

In future the solution can be developed by sending data from PLC or custom controller to PC central point using GSM/CDMA or GSM/GPRS network infrastructure (Geier, J., 2005).

5. REFERENCES

Bajenarescu, T. (2000). Personal Communication Systems, Teora, ISBN 973-20-0236-0, Bucharest, Romania

Geier, J. (2005). Wireless Networks first-step, Corint, ISBN 973-653-709-9, Bucharest, Romania

Hammuda, H. (1999) Celular Mobile Radio Systems, Teora, ISBN 973-20-0004-X, Bucharest, Romania

Motorola Inc. (2008). Motorola G24-L and G24-LC Developer's Guide. Module Hardware Description., Motorola, 6802984C05-C, USA

Rabbit Semiconductor Inc. (2006-2007). RabbitCore RCM4000 User's Manual, Rabbit Semiconductors Inc, 019-0157, 070928-F, USA

Tisal, J. (1999). GSM: reseau et services, Teora, ISBN 973-200192-5, Bucharest, Romania