Many users now own or have access to more than one computer and find one may have certain advantages over another for creating and editing data files. At times it is easier to create a file on one system but another may offer more flexibility for printing or storing these files. Sometime we may just have used one for a long time and have many files saved when along comes a new system with different storage medium or incompatable formats on the same medium. To save the time of re-entering the data again it is much simpler to just send the file to the other computer.
This procedure can be quite simple if both computers have a serial port and communications software. The data rate can be increased to whatever speed the computer can handle. To use the serial port a small inexpensive piece of hardware must be added “THE NULL MODEM”.
The null modem is nothing more than a cable with some signal wires crossed to “fool” the serial port into thinking that it’s communicating with a serial device. The only electrical connection between the two computers can be accomplished by three wires, GROUND or COMMON, TRANSMIT DATA and RECEIVE DATA. However the serial port and the communications software need some other conditions before it can establish the connection. It is therefore necessary to jumper outbound signals back to the inbound signals. For my version of the null modem I will show the pin numbering for the 25 pin D-shell connector. Your pin configuration may differ but connecting the same signal wires as shown will produce the same end results.
THE NULL MODEM SYSTEM A SYSTEM B TRANSMIT DATA (pin 2)------------------RECEIVE DATA (pin 3) RECEIVE DATA (pin 3)------------------TRANSMIT DATA (pin 2) SIGNAL GROUND (pin 7)------------------SIGNAL GROUND (pin 7) REQ. TO SEND (pin 4)--] [--REQ. TO SEND (pin 4) CLEAR TO SEND (pin 5)--] [--CLEAR TO SEND (pin 5) DATA TERM. RDY(pin 20)-] [--DATA TERM.RDY(pin 20) DATA SET. RDY (pin 6)--] [--DATA SET RDY (pin 6) DATA CARR DET (pin 8)--] [--DATA CARR DET (pin 8)
Each system will activate the REQ TO SEND and the DATA TERM RDY line and the jumpers will activate CLEAR TO SEND, DATA SET RDY and DATA CARR DET. If your system does not have all of the above lines present on the serial port then they can be omitted on the null modem.
Both systems must be operating at the same speed (baud rate)and parameters (8,n,1). Up and down loading is now done to transfer the files, use simple buffer transfers or XMODEM if error checking is desired.
Ed.Note: Dave uploaded this article from his IBM-PC to the local BBS, TCCS as a text file. I then downloaded it, “NULMODEM.TXT” via Xmodem and Mterm (Casby’s Loader V Program) saved the file to tape, ran it through Casby’s Unloader program, loaded it into my Mscript, editted it, and printed it out. Start to finish about 35 minutes, and I didn’t have to get the wife to shovel the driveway.