XModem via TCP for Java

Question

I am sure that many of you experienced developers have read requests in the past concerning implmentation of Ward Christenen's XModem protocol over a TCP socket. If not, well... you are about to...
This is a major hack... but it is starting to come together... thanks to Fred Potter for his source code to start this project...
Objective:
Basically, I want to create a console application which accepts an incoming connection and starts the receive mode for a XModem file transfer. I am using CGTerm (for Commodore retrocomputing) but can test with HyperTerminal as well...
The user who connects to the server selects SEND and the FILE to send for a XModem file transfer... and the transfer begins...
The incoming blocks of 128 bytes are written to a file
After the transfer is over the server disconnects the client terminal.
Here is what I have so far:
import java.net.*;
import java.lang.*;
import java.io.*;
// X-Modem Server implementation via TCP/IP socket
public class XServer {
public static FileWriter fw;
public static void main(String[] args) throws IOException {
// define the file
try {   
fw = new FileWriter("filename.txt");
} catch (Exception e) {
System.out.println(e);
System.exit(0);
int port = Integer.parseInt(args[0]);
ServerSocket server = new ServerSocket(port);
System.out.println("X-Server v1.0 - waiting for connection");
Socket client = server.accept();
// Handle a connection and exit.
try {
InputStream inputStream = client.getInputStream();
OutputStream outputStream = client.getOutputStream();
new PrintStream(outputStream).println("Go to send file mode!"); // sent to client
System.out.println("Ready to receive file via X-Modem...");
* BEGIN TRANSFER HERE!
// set the debug flag
XModem.debug = true;
* Here we are instantiating a new InputStream that represents the remote
* file that we are receiving. In this single line we are attempting to
* start the flow.
* Behind The Scenes: We're sending a NAK across the serial line repeatedly
* until we finaly start seeing the data flow. If we don't see the data
* flow, then we throw an exception.
System.out.println("Sending NAK to start receive mode...");
InputStream incomingFile;
try {
incomingFile = new XModemRXStream(inputStream, outputStream);
} catch (IOException e) {
System.out.println("ERROR! Unable to start file transfer!");
e.printStackTrace();
return;
System.out.println("Starting file transfer...");
* Here we are reading from the incoming file, byte by byte, and printing out.
* Behind The Scenes: Internally, the read() method is handling the task of
* asking for the next data block from the remote computer, processing it (i.e.
* parsing, running checksums), and then putting it in an internal buffer. Not
* all calls to read() will request a new data block as each block contains at
* least 128 bytes of data. Sometimes you will only hit the buffer.
try {
for (;;) {
int c = incomingFile.read();
if (c==-1)
break; // End of File
// print character / byte
System.out.print(c+",");
// write to file
try {       
//System.out.print(".");
fw.write(c);
} catch (Exception e) {
System.out.println(e);
System.exit(0);
} catch (IOException e) {
System.out.println("error while reading the incoming file.");
e.printStackTrace();
return;
// done
System.out.println("File sent.");
new PrintStream(outputStream).println("");
new PrintStream(outputStream).println("transfer successful!");
} finally {
//client.close();
// save the file
try {   
fw.close();
System.out.println("file saved.");
} catch (Exception e) {
System.out.println(e);
System.exit(0);
* XModem keeps track of settings that the Receive and Transmit Stream classes will
* reference.
* <p>Copyright: Copyright (c) 2004</p>
* @author Fred Potter
* @version 0.1
class XModem {
public static boolean debug = false;
* XModemRXStream is an easy to use class for receiving files via the XModem protocol.
* @author Fred Potter
* @version 0.1
class XModemRXStream
extends InputStream {
// CONSTANTS
private static final int SOH = 0x01;
private static final int EOT = 0x04;
private static final int ACK = 0x06;
private static final int NAK = 0x15;
private static final int CAN = 0x18;
private static final int CR = 0x0d;
private static final int LF = 0x0a;
private static final int EOF = 0x1a;
// block size - DON'T CHANGE - I toyed with the idea of adding 1K support but the code is NOT there yet.
private static final int bs = 128;
// PRIVATE STUFF
private int ebn; // expected incoming block #
private byte[] data; // our data buffer
private int dataPos; // our position with the data buffer
private InputStream in;
private OutputStream out;
* Creates a new InputStream allowing you to read the incoming file. All of the XModem
* protocol functions are handled transparently.
* As soon as this class is instantiated, it will attempt to iniatate the transfer
* with the remote computer - if unsuccessful, an IOException will be thrown. If it
* is successful, reading may commense.
* NOTE: It is important not to wait too long in between calls to read() - the remote
* computer will resend a data block if too much time has passed or even just give up
* on the transfer altogether.
* @param in InputStream from Serial Line
* @param out OutputStream from Serial Line
public XModemRXStream(InputStream in, OutputStream out) throws
IOException {
this.in = in;
this.out = out;
// Initiate the receive sequence - basically, we send a NAK until the data
// starts flowing.
init:for (int t = 0; t < 10; t++) {
if (XModem.debug) {
System.out.println("Waiting for response [ try #" + t + " ]");
long mark = System.currentTimeMillis();
out.write(NAK);
// Frequently check to see if the data is flowing, give up after a couple seconds.
for (; ; ) {
if (in.available() > 0) {
break init;
try {
Thread.sleep(10);
catch (Exception e) {}
if (System.currentTimeMillis() - mark > 2000) {
break;
// We have either successfully negotiated the transfer, OR, it was
// a failure and timed out. Check in.available() to see if we have incoming
// bytes and that will be our sign.
if (in.available() == 0) {
throw new IOException();
// Initialize some stuff
ebn = 1; // the first block we see should be #1
data = new byte[bs];
dataPos = bs;
* Reads the next block of data from the remote computer. Most of the real XModem protocol
* is encapsulated within this method.
* @throws IOException
private synchronized void getNextBlock() throws IOException {
if (XModem.debug) {
//System.out.println("Getting block #" + ebn);
// Read block into buffer. There is a 1 sec timeout for each character,
// otherwise we NAK and start over.
byte[] buffer;
recv:for (; ; ) {
buffer = new byte[bs + 4];
for (int t = 0; t < 10; t++) {
System.out.println("\nReceiving block [ #" + ebn + " ]");
// Read in block
buffer = new byte[buffer.length];
for (int i = 0; i < buffer.length; i++) {
int b = readTimed(1);
// if EOT - don't worry about the rest of the block.
if ( (i == 0) && (b == EOT)) {
buffer[0] = (byte) (b & 0xff);
break;
// if CAN - the other side has cancelled the transfer
if (b == CAN) {
throw new IOException("cancelled");
if (b < 0) {
if (XModem.debug) {
System.out.println("Time out... NAK'ing");
out.write(NAK);
continue recv;
else {
buffer[i] = (byte) (b & 0xFF);
break;
int type = buffer[0] & 0xff; // either SOH or EOT
if (type == EOT) {
if (XModem.debug) {
System.out.println("EOT!");
out.write(ACK);
break;
int bn = buffer[1] & 0xff; // block number
int bnc = buffer[2] & 0xff; // one's complement to block #
if (
(bn != ebn) && (bn != (ebn - 1)) ||
(bn + bnc != 255)) {
if (XModem.debug) {
System.out.println("NAK'ing type = " + type + " bn = " + bn +
" ebn = " +
ebn + " bnc = " + bnc);
out.write(NAK);
continue recv;
byte chksum = buffer[ (buffer.length - 1)];
byte echksum = 0;
for (int i = 3; i < (buffer.length - 1); i++) {
echksum = (byte) ( ( (echksum & 0xff) + (buffer[i] & 0xff)) & 0xff);
if (chksum != echksum) {
out.write(NAK);
continue recv;
out.write(ACK);
if (ebn == 255) {
ebn = 0;
else {
ebn++;
break;
// We got our block, now save it in our data buffer.
data = new byte[bs];
for (int i = 3; i < (buffer.length - 1); i++) {
data[(i - 3)] = buffer;
dataPos = 0;
public synchronized int read() throws IOException {
// If at the end of our buffer, refill it.
if (dataPos == bs) {
try {
getNextBlock();
catch (IOException e) {
throw new IOException();
// If we're still at end of buffer, say so.
if ( dataPos == bs) {
return -1;
int d = data[dataPos];
if (d == EOF)
return -1;
dataPos++;
return d;
* A wrapper around the native read() call that provides the ability
* to timeout if no data is available within the specified timeout value.
* @param timeout timeout value in seconds
* @throws IOException
* @return int an integer representing the byte value read.
private int readTimed(int timeout) throws IOException {
long start = System.currentTimeMillis();
for (; ; ) {
if (in.available() > 0) {
return (in.read());
try {
Thread.sleep(10);
catch (InterruptedException ex) {
//if (System.currentTimeMillis() - start > timeout * 1000) {
if (System.currentTimeMillis() - start > timeout * 5000) {
return -1;
Here was the output...
Original file:
(Commodore CBM SEQ file exported to PC using DirMaster)
��
� �
� ��� �� �� ��� ��
� �� �� ���� �� ��� ��
� ��� ����������������������������������������������
�� ����� ������� ����� �� ����� ������ ����� ���
� �� ������ ������ ��� ��� �� ��� ���� �� ������
� � ���
����
� � ��OWERED BY �OLOR 64 ��� V8
�UNNING �ETWORK64 V1.26A
�
�UPPORTING 38400 �AUD �ATES
�����/����/�������
�
�ESTING �CHO-�ET V1 BETA
�
�EATURING �ESSAGES, �ILES,
�ET�AIL, AND �NLINE �AMES!
�YS�P: � � � � � � � � �
�
�RESS ANY KEY TO LOGIN\C�
The result when the file was uploaded and received by my XServer:
? ? ??OWERED BY ?OLOR 64 ??? V8
?UNNING ?ETWORK64 V1.26A
?UPPORTING 38400 ?AUD ?ATES
?ESTING ?CHO-?ET V1 BETA
?EATURING ?ESSAGES, ?ILES,
?ET?AIL, AND ?NLINE ?AMES!
?YS?P: ? ? ? ? ? ? ? ? ?
?RESS ANY KEY TO LOGIN\C?
The result is different!
Can someone help me along here... I have been trying to figure out how to do this for approx. a year or so... it has been a very slow process.
I could use a guru to help me out so I can write the upload and download routines for my Commodore BBS PETSCII Emulation Server.
Visit http://www.retrogradebbs.com for details.
Thanks.
Please help out a dedicated developer who is in over his head...
-Dave

Answer 1

Ok. Fair enough. What about general information about Xmodem. This is a hard project because of how obscure the legacy technology is that I am having to implement using Java and MySQL.
I have two major issues which I have to figure out how to troubleshoot and debug, if possible.
1. The 23+ blocks exception when a file is being received
2. The exception which is thrown immediately if I try to receive a binary file instead of an ASCII file.
I read that telnet is a 7-bit technology and that is why Xmodem, which is an 8-bit technology is not that popular as a viable protocol via telnet, whereas Kermit is, since it was developed for 7-bit systems, i.e. mainframes and minicomputers.
Is this correct?
If that is the case, why does www.serio.com have a viable X-Y-ZModem library available (for several hundred $$$ of course) which can be used with both RS-232 serial ports and TCP socket ports? Obviously, it can be done. They are the ONLY company with this library for sale for Java to do this. I cannot justify that $$$ amount for a mere hobby (writing the BBS emulation server for supporting Commodore PETSCII (CG) callers via CGTerm or a native C-64 terminal program using Jim Brain's TCPSER middleware, which emulates a Hayes modem via telnet for telBBSing/retrocomputing.
I really want to learn how to implement a file transfer protocol, since back in the 80s, I used Xmodem, Punter, Y/Z Modem, etc., a lot to upload and download files via modem at baud rates of 2400, 14.4, 19.2, and 38.4, respectively.
It's fun to learn how the old skool gurus of telecommunications technology did it. It is one thing to run a BBS which supports these technologies and features, and it is an entirely other thing to learn how to design and develop them yourself for implementation into a project such as I taken on.
It CAN be done. It WILL be done. However, I have just started my exhaustive research on how it needs to be done. I have read up as much as I could on XModem by Ward C., the father of the protocol.
But, I have no information to help me figure out why the communications are acting as they do so far.
Can someone please download the xserver.zip file on my website at:
www.retrogradebbs.com/projects/xserver.zip
Compile it. Run it. Connect using HyperTerminal, Netrunner, or another telnet terminal emulation program which supports Xmodem file transfers using WinSock.
See what happens. With finals due in the next two days, this project will have to be put on hold until after I submit my two final projects.
If anyone knows what needs to be done to support both ASCII and BINARY file transfers via Xmodem via a socket instead of a modem with RTS/CTS hardware flow control, please respond.
I know for a fact that this can be done.
- Dave