DEVICE OVERVIEW

MAX232 IC

MAX232 is an integrated circuit that converts signals from RS-232 serial port. The MAX232 is a dual driver/receiver and typically converts the RX, TX, signals. The drivers provide RS-232 voltage level outputs from a single 5V supply via on-chip charge pumps and external (typically 100 nF) capacitors. The receivers reduce RS-232 inputs (which may be as high as ± 25 V), to standard 5 V TTL levels. These receivers have a typical threshold of 1.3 V, and a typical hysteresis of 0.5 V and accept ±30-V inputs. The advantage of MAX232 is that it uses +5V power supplies same as the source voltage of PIC 16873A.

Feature for MAX232:

ü  Operate with single 5V supply

ü  Operate up to 120kbit/s

ü  Two drivers and two receivers

ü  ±30-V input levels

ü  Low supply current ,typical : 8 mA

ü  Designed to be interchangeable with MAX232

ü  Applications:TIA/EIA-232-F, Battery-Powered Systems, Terminals, Modems, Computers

                                                   Figure 1: MAX232 pin diagram

GSM Modem

                                               

A GSM modem is a wireless modem that works with a GSM wireless network. A wireless modem behaves like a dial-up modem. The main different between them is that a dial-up modem sends and receives data through a fixed telephone line while a wireless modem sends and receives data through radio waves.

Typically, and external GSM modem is connected to a computer through a serial cable. In order to operate, a GSM modem requires a SIM card from a wireless carrier like a GSM mobile phone. A GSM modem supports a common and extended set of standard AT commands. With the extended commands, the GSM modem can be used as:

ü  Reading, writing, and deleting SMS messages.

ü  Sending SMS messages.

ü  Monitoring the signal strength.

ü  Monitoring the charging status and charge level of the battery.

ü  Reading, writing and searching phone book entries.

The number of SMS messages that can be processed by a GSM modem per minute is very low, which is only about six to ten SMS messages per minute. The GSM net used by cell phones provides a low cost, long range, wireless communication channel for application that need connectivity rather than high data rates. Machinery such as industrial refrigerators and freezers, HVAC, vending machines, vehicle services and others could benefit from being connected to GSM system.

                                                              Figure 2: GSM Modem

RS232

RS232 is commonly used in computer serial port. The RS232 is a standard for serial binary data signal connecting between a DTE (Data Terminal Equipment-computer) and a DCE (Data Circuit-terminating Equipment-modem). This devices will be sending and receiving signals Generally, terminal and computers have male connectors with DTE functions, and modem have female connectors with DCE pin functions. Other devices may have any combination of connecter gender and pin definitions. Nowadays, many terminals were manufactured with female terminals but were sold with a cable with male connectors at each end.

To connect with GSM modem, the connection of RS232 cable pin diagram is as follows:

                                 Figure 3: RS232 null modem with loop back handshaking