The connection between the control's tape reader and the internal processor was interrupted by a microprocessor based device which emulated the paper tape reader's signals, but which had a serial port connected to the DNC system. In these cases, a device known as a Behind The Reader or BTR card was used. Older controls frequently had no port capable of receiving programs such as an RS-232 or RS-422 connector. No time was lost due to broken tapes, and if the software was correctly used, an operator running incorrect or out of date programs became a thing of the past. Depending on program size, for the first time operators had the opportunity to modify programs at the DNC terminal. The host software would be responsible for tracking and authorising NC program modifications. For example, DLog offered an x86 based terminal, and NCPC had one based on the 6809. DNC companies offered machine tool links using rugged proprietary terminals and networks. The Host computers would frequently be Sun Microsystems, HP, Prime, DEC or IBM type computers running a variety of CAD/CAM software. The focus in the 1980s was mainly on reliably transferring NC programs between a host computer and the control. Many companies were still punching programs on paper tape well into the 1980s, more than twenty-five years after its elimination in the computer industry.
NC controls had paper tape readers precisely for this purpose. Programs had to be walked to NC controls, generally on paper tape. Controls of this type are very widely used in industries with significant sheet metal fabrication, such as the automotive, appliance, and aerospace industries. Wireless DNC is also used in place of hard-wired versions.
DNC networking or DNC communication is always required when CAM programs are to run on some CNC machine control. However, if this provision is not possible, some software companies provide DNC applications that fulfill the purpose. Usually, the manufacturer of the control provides suitable DNC software. If the computer is connected to a number of machines it can distribute programs to different machines as required. On some CNC machine controllers, the available memory is too small to contain the machining program (for example machining complex surfaces), so in this case the program is stored in a separate computer and sent directly to the machine, one block at a time.
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