Assignment #4

Clarifications, updated 28/5/00

The DTE sends whole packets to the DCE. Thus when only the RTS is reset (set to 0) the DCE waits to receive the whole packet and only then acknowledges with resetting the CTS.
If the DTR is reset as well before the whole packet has been received, this means that the DTE wants to terminate communication immediately, even in the middle of a packet.

Write a program that simulates the Modem Communication Protocol (by sending a file) between a DTE (processor) to a DCE (modem). The sender will be the DTE and the receiver the DCE. The program will be called dte2dce and receives from the user the following command line parameters:

Mode (dte or dce).
The COM (COM1, COM2, COM3 or COM4).
Word length (5 - 8).
Number of stop bits (1, 1.5, 2).
Parity (even, odd, or none).
Transfer rate (given the transfer rate you must compute the baud rate).
Name of text file to transfer (dte only).

For example the sender must run his program like this:

C:\>dte2dce dte COM1 8 1 odd 1200 ex4.c

The receiver must run his program like this:

C:\>dte2dce dce COM1 8 1 odd 1200

The program must get the base address of the COM from the BIOS (address 0x400).

DTE

After the transmission parameters have been set, the DTE establishes communication according to protocol.
It then reads the file line by line and sends each line as a "packet" to the DCE.
A packet is a block of bytes framed by a flag byte, the value of the flag byte is 0x7e (the '~' character). The packet is followed by the CRC byte. If the input file contains the '~' character it is exchanged with the string "/$/@/" (you may assume this string isn't in the input file).
Data is sent using the code you have written for the previous assignment. You must make sure of course before sending a byte that the DCE is ready to receive data (the CTS and DSR are set).
When the DTE isn't transmitting (reading from the file for example) the RTS must be reset, this
notifies the DCE that transmission is suspended.
After the file is sent the connection is severed (terminated) by resetting the DTR.
As in the previous assignment you must listen for errors that the DCE might detect and send, print them, and terminate transmission (according to protocol).

DCE

After the transmission parameters have been set, the DCE waits until the DTE establishes communication.
It then enters a loop where it receives "packets" from the DTE. The packets are stored in a buffer. When the buffer is full the DCE prints its contents to the screen. The DCE must reset the CTS when it is printing and set it when it is ready to receive more packets.
If there is an error in the data the DCE must let the DTE know what the error is (a bad CRC or a packet that doesn't start with the '~' char are also errors). Only the DTE can terminate communication by reseting the DTR. The DCE then resets the DSR.

It doesn't matter which side is started first, the DCE must wait for the DTE and vice-versa. Thus when a program exits it must make sure that all the control lines are reset.