How do you get large amounts of data from one PC to another? Here's
an easy way, using a null modem (or bidirectional
parallel) cable and INTERLNK software bundled free with MS-DOS 6 and
above. One computer's disk drives appear as extra drives on the other machine.
Transfer data with COPY and XCOPY commands, Windows File Manager, and others.
This article describes how to build the cable,
connect the machines, install
the software, and copy files.
Copyright 1994, 1995, 1996, 1997, 1998 by Rose Business Systems Inc. All rights reserved.
This article may be redistributed provided that the article and this notice remain intact. This article may not under any circumstances be resold or redistributed for compensation of any kind without prior written permission from Rose Business Systems Inc.
If you have any questions about these terms, please send e-mail to cables@hamptons.com.
Do you frequently need to transfer large amounts of data from one PC to another? If so, you'll want to consider a high-speed connection between them so that one computer's disk and diskette drives and printers appear as extra resources on the second machine.
This sure beats the tedious method of backing up one machine (on dozens of diskettes) and restoring to the other. And it's a lot cheaper than a portable tape backup unit.
Unlike a regular local area network (LAN), there's no need to purchase expensive Network Interface Cards (NICs) -- just use the parallel or serial ports built into both computers.
Likewise, you won't need coaxial cable between the two machines. Instead, you can build a null-modem cable (or a special bidirectional parallel cable) using standard DB-25 and/or DB-9 connectors available from electronics parts retailers.
Typically, you'd use this method when you're upgrading a customer to a new computer (the "client") and you want to transfer the directories, programs, and data files from the old one (the "host").
At least one of the computers has to be running MS-DOS 6.0 or higher, or Windows 95.
Here's what you do.
Build (or buy) one of the following cables. If you have INTERSVR.EXE, the server program, already on the host machine, or you have a way to get it there (such as compatible diskette drives), we recommend the bidirectional parallel cable; it has a very fast transfer speed, and it doesn't interfere with modems and mouse drivers. It's a little harder to build, and it occupies the printer port, so choose this one only if you don't want to use the host's printer remotely. You can get away with using a three wire null modem cable if both machines have COM ports available.
However, if the host is running a version of DOS earlier than 6.0, and the two computers don't have mutually compatible diskette drives, you have to build the seven wire null modem cable just to transfer the server program from the client to the host. Also, the host computer must have an available COM1 or COM2 -- higher numbered COM ports won't work.
Bidirectional parallel cable: Two DB-25 male connectors, and cable with at least eleven conductors. If you can't find the multi-conductor cable, get thin stranded hookup wire and spiral cable wrap (Radio Shack #278-1638). Connect pins as follows:
PC #1 PC #2
2 <-----------------------> 15
15 <-----------------------> 2
3 <-----------------------> 13
13 <-----------------------> 3
4 <-----------------------> 12
12 <-----------------------> 4
5 <-----------------------> 10
10 <-----------------------> 5
6 <-----------------------> 11
11 <-----------------------> 6
25 <-----------------------> 25
Seven wire null modem cable: Two DB-25 or DB-9 female connectors (depending on the physical plugs you find connected to working COM ports on each computer), and cable with at least seven conductors. Connect pins as follows:
PC #1 DB-9 DB-25 DB-25 DB-9 PC #2
TD 3 2 <-----------------------> 3 2 RD
RD 2 3 <-----------------------> 2 3 TD
RTS 7 4 <-----------------------> 5 8 CTS
CTS 8 5 <-----------------------> 4 7 RTS
DSR 6 6 <-----------------------> 20 4 DTR
DTR 4 20 <-----------------------> 6 6 DSR
SG 5 7 <-----------------------> 7 5 SG
Three wire null modem cable: Same as the seven wire cable above; omit the RTS-CTS and DSR-DTR leads.
Typically, you'll want to transfer files from an older, slower PC to a newer, faster one that will be replacing it. The older machine, or "host", may be running an earlier version of DOS -- say, MS-DOS 3.3. If the two computers have compatible diskette drives, just copy INTERSVR.EXE from the newer PC's \DOS directory, via diskette, to the older machine.
If you haven't the luxury of compatible diskette drives, INTERSVR can install a copy of itself on a remote machine. Connect the two computers via the seven wire null modem cable. On the client, enter the command:
INTERSVR /RCOPY
The program will prompt you for the COM port number of the other (host) machine. Highlight either COM1 or COM2 and press Enter.
On the host machine, enter the commands:
MODE COM1:2400,n,8,1,p
CTTY COM1
Replace 'COM1' with 'COM2' above as necessary.
The INTERSVR program will copy itself to the host machine.
You can specify options, such as a maximum data transfer rate, which disk drives to omit, and which COM or LPT port to use for transfer. Enter
INTERSVR /?
for a list of all the options.
On the host machine, enter the command:
INTERSVR
The host is now ready to act as a file server to the client PC.
You can specify which COM or LPT port to use for data transfer, the number of host drives to address, and other options. Enter
HELP INTERLNK.EXE
for a list of all the options.
On the client machine, enter the command:
EDIT C:\CONFIG.SYS
... or use your favorite text editor to add a line to the CONFIG.SYS file to read as follows:
DEVICE=C:\DOS\INTERLNK.EXE
Also, if a line containing the text "LASTDRIVE" is already in the file, raise the drive letter it specifies, if necessary, so that you can add as many extra drives as there are drives on the host. Usually, that's three. For example, if the client computer already has diskettes A: and B:, a hard disk C:, and a RAM disk D:, LASTDRIVE must be at least G: to accommodate the new E:, F:, and G: drives, which are known on the host as A:, B:, and C:.
In this example, the line looks like this:
LASTDRIVE=G
If LASTDRIVE is already set high enough, you don't need to change it.
Save the file, exit the editor, and reboot the computer (Ctrl-Alt-Del).
If the connection is working properly, the server screen will display
the additional drive letters that now appear as additional drives on the
client.
The host computer's disk and diskette drives and printers are now available as additional drives (and, maybe, printers) on the client computer. For example, if the host has drives A:, B:, and C:, they may become known to the client as E:, F:, and G:. Programs such as the Windows File Manager, the DOS Shell, Xtree (tm), and ProFinder (tm) will let you copy to and from, move, rename, and delete files, on host drives interchangeably with your local drives.
To copy an entire directory, including all subdirectories and files, from the host's hard disk to the client's, substitute the correct drive letters and directory name in the following commands:
MD C:\WP51
XCOPY G:\WP51\*.* C:\WP51 /S
When you're finished using the host's disks, you can stop the server program by pressing Alt-F4 on the host keyboard.
On the client computer, edit CONFIG.SYS and delete the INTERLNK command line. (It's generally OK to leave LASTDRIVE set to a high value, like Z. The cost in memory is very small.) Save the file, exit the editor, and reboot.
Some folks like to leave INTERLNK permanently installed on one computer.
If you do, you might consider adding the /AUTO switch to the DEVICE= line
in CONFIG.SYS. That way the program will unload itself immediately (and
release its memory) if it doesn't find a copy of INTERSVR at the other
end of the cable at boot time.
Q: Are these the same cables LapLink uses?
A: Yes; also Brooklyn Bridge. These are commercial software products with
more features than Interlnk. Windows 95's Direct Cable Connection
also uses these cables.
Q: How do I get the Interlnk software and how much does it cost?
A: It's free, included in every copy of MS-DOS 6 and above.
Q: Does it work under Windows 95?
A: Yes. Either of the two computers can be running Windows 95. However,
INTERLNK and INTERSVR will not communicate with Direct Cable Connection
(DCC), a free add-on to Windows 95.
Q: Does it work under older revisions of DOS?
A: Intersvr does. You need at least DOS 6.0 -- or Windows 95 --
to run Interlnk on the client.
Q: What is the transfer rate?
A: The maximum possible, as limited by: the processing speed of both computers;
the presence of any buffering on the serial ports (run the program MSD
from a DOS prompt and examine the COM ports; a 16550 is buffered; an 8250
is not); the choice of a parallel vs. a serial path (parallel is generally
faster); the cable construction and length (shielded is better; shorter
is better).
Q: What is the maximum cable length?
A: It varies with the cable construction. Since the transfer protocol is
error-correcting, you probably won't see data errors as such. Instead,
you'll observe a slowdown in data transmission.
Q: Why can't the client computer "see" the server's CD-ROM
and network drives?
A: Interlnk uses a "redirection interface" to make the server's
drives visible on the client machine. MS-DOS accesses network drives and
many CD-ROM drives via redirection, and you can't redirect a drive twice.
The same holds, unfortunately, for SCSI devices that aren't directly BIOS-supported.
Q: How much memory does the INTERSVR software take up on the host?
A: All of it! Forget about using the host for any other purpose while it's
running INTERSVR. Unlike INTERLNK, which is "resident" and shares
the machine with user programs, INTERSVR requires a dedicated computer.
Q: It doesn't work. Why?
A: Check that you built the cable correctly. Check that you included DEVICE=C:\DOS\INTERLNK.EXE
in the CONFIG.SYS of one computer, and rebooted. Check that
the other computer does not have the device statement active,
and is running INTERSVR from the command line.
OS/2: Neither INTERLNK nor INTERSVR works under native OS/2. The INTERLNK device driver is incompatible with OS/2, and the INTERSVR program crashes the DOS box. You may be able to run one or the other in a Virtual DOS Machine (VDM) under OS/2. I haven't tried it, and if you have, I'd appreciate hearing of your experiences.
Windows 95 alert: A new OEM release of Windows 95 allows large disk volumes to have 32-bit file allocation tables (FATs). Older operating systems, including all the DOS releases and earlier ones of Windows 95, don't support this file structure. If you want to connect a 32-bit FAT machine to an older one, the older one must be the host.
Old DOS: Recently, I was stymied trying to transfer files from an old 286 running MS-DOS 3.30. Any attempt to read the 286's disk from the client computer, running DOS 6.2 and Interlnk, resulted in a DOS error message "Invalid media type". I tried booting the 286 from a DOS 6.2 diskette, without Interlnk; the error showed up again. I theorize that the media type byte in the partition header really was bad, but DOS 3.30 isn't particular about it. (I transferred the data another way and reformatted the partition. Then Interlnk worked correctly.)
CD-ROM: In most cases a CD-ROM drive on a host machine cannot be made visible to the client. To get around this problem, either:
You can save the time and uncertainty of building your own cables by purchasing factory-made ones. If your retail store is out of them, we'll cheerfully ship the following on receipt of your check or money order.
#219: 4-head 7-wire null modem cable (female DB-9 & DB-25 each end) US$17.95 #310: Bidirectional parallel cable (male DB-25 each end) $14.95 #248: 6-head cable, use for parallel or serial $29.95
Cables are guaranteed to work, or return them for a refund. Priority Mail postage to U.S. locations is included; NY and NJ addressees add your local sales tax. Add US$5.00 to Canada or Mexico; $10.00 for other international destinations. Prices are valid through December 31, 1998.
Visit our price list and order form or write to:
ROSE BUSINESS SYSTEMS INC.Internet: cables@hamptons.com