Cable Connection

If you want to connect the embedded device directly to a computer, you must use the Null Modem driver. The figure belows shows an overview of a directly connected system.

However, RL-TCPnet's Null Modem driver supports two different procedures for establishing the null modem serial link with the computer:

• Windows Direct Serial Link. This is a non-modem link. This system requires the exchange of string tokens between the computer and the embedded device before the PPP link can be negotiated. The SLIP link needs no negotiation because SLIP is a simple protocol over a serial line.
  
  The Windows client sends the string "CLIENT". The Keil evaluation board must then send the string "CLIENTSERVER". When operating as a server, the Keil evaluation board must send the string "CLIENT". The Windows client must respond with the string "CLIENTSERVER".
  
  This string exchange must occur before Windows allows the PPP connection to proceed. This string exchange is peculiar to Windows and is not required by other hosts.
• Windows Standard Modem Link. In this system, the Null Modem driver simulates an external modem. The PPP source must also be modified to operate with the Windows direct serial driver. It requires the exchange of modem command strings between the computer and the evaluation board before the PPP link can be negotiated. The strings to be exchanged are shown in the table below.
  
  

  Windows Command	Reply	Description
  AT\r	OK\r	Attention Command
  ATE0V1\r	OK\r	Set No echo and Verbose Result reply
  AT\r	OK\r	Attention Command
  ATS0=0\r	OK\r	Disable Modem Auto Answer
  AT\r	OK\r	Attention Command
  ATE0V1\r	OK\r	Set No echo and Verbose Result reply
  AT\r	OK\r	Attention Command
  ATDTxxxxxxx\r	CONNECT\r	Dial Target number: xxxxxxx

  After the "CONNECT" string is received, Windows starts the PPP negotiations and user authentication.