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Transfer Points

Build Your Null Modem Adaptor for Moving Files Between Computers


by Dan Keen and Robert Murray

Published June 1986

80 Micro, CW Communications/Peterborough Inc.

Many owners of Tandy MS-DOS computers face the Job of transferring Model I/III/4 files to their new machines. transfer methods require extra hardware or software that can cost serious money. But for a few dollars, we built a null modem device, which allowed us to move files from a Mode1 4 to a Tandy 1000 via their serial ports (see the Photo]. Since TRS-DOS 6 has a COMM utility and the 1000 comes wish DeskMate, we didn't need additional communications software. You could, of course, use a similar setup to move files between any two computers.

Making the Connection

To port files, your Tandy 1000 must have the RS-232C upgrade sold by Radio Shack (stock number 25-1006, $99).

The Null modem is an adapter that you connect to one of the computers, it crosses send and receive lines so the two machines communicate. To build your own, you'll need male and female 25-pin D-subminiature connectors (part numbers 276-1547 and 276-1548 respectively] or the equivalent. The Table shows the wiring configuration. We tied pins 6, 8, and 20 together and the device still worked.

Only nine pins have to be wired on each connector. Don't use ribbon cable which is awkward for crossing connections. Use solid wire instead of stranded for better support. The wire path should be flexible the connectors won't dislodge if the computers are accidentally bumped. Figure 1 and Figure 2 show the protocols you should use to configure DeskMate and SETCOM. Pressing the clear and 8 keys together gives you a menu shoaling the COMM settings.