Most PLCs use a special protocol for communications, and most manufacturers have their own proprietary protocol. Many also support public domain or published protocols such as Modbus.
To communicate with a PLC you need to send a serial string that includes the address you are working with and a command code that, for example, identifies if the communications is reading or writing data, and in some cases if the data is to be sent as bits or a decimal value, and so on. The communications string also includes a header character, the address of the PLC itself, an end of transmission character, and a checksum or other error checking system.
You need to identify the PLC brand and model, the communications protocol being used, the communications parameters (baud rate etc.) the serial communications type (RS232, RS485, etc.) and a lot of other data.
Hyperterminal is a very inadequate way to do this, because the entire string must be created, and then the response must be decoded. There are often timeout limitations in the PLC communications port, and some protocols are four pass - you send the request from the master, the slave acknowledges the request, the master asks for the data, and the slave sends the data. In this case, it is difficult to get the second command out fast enough from hyperterminal.
