CNC data interfaces

It is not surprising that CNC Data interfacing is a source of confusion in many plants. The many names by which CNC data interfaces are specified have been a contributing factor. Some of the more common terms are: data port, serial port, communications interface, behind-the-tape-reader interface (BTRI), DNC interface, and parallel port, to name a few. These terms are often combined with vague definitions and written into equipment specifications. For example, the following definitions are taken from actual specifications: * "A serial data port is required for future connection into a DNC or FMS system." * "A serial or parallel data port is required. A BTRI is acceptable." * "The control must have MAP communications compatibility." * "An RS-232 interface is required." * "A DNC interface is required."
The ambiguity of the above statements allows the CNC vendor to comply without defining what capability will be provided. For example, an RS-232 interface can include a very sophisticated protocol that provides error checking and status reporting capability, or it may not have any of these capabilities.
One way to minimize confusion is to use the International Standards Organization (ISO) seven-layer network architecture for local area networks (LANs). Even though this architecture is a model for open systems interconnection (OSI) for LANs, it can also provide a common base of understanding for serial interfaces as well. The seven layers consist of: (1) physical, (2) data link, (3) network, (4) transport, (5) session, (6) presentation and (7) application.

When specifying a serial data interface there are three functions to be considered: (1) the connection, (2) the data-handling protocol and (3) the capability to be provided. The ISO seven-layer structure is used in the following description to explain these three functions:
Connection--This is the most basic part of the specification. It is the way the data is to be transmitted. Examples of the connections for serial interfaces are RS-232, RS-422 and RS-423. The speed or baud rate should also be stated at this level. Keep in mind however that even though each of the above standards provides a choice of baud rates, the distance which the data is to be transmitted becomes a limiting factor on achievable rates. Also, if error-checking protocol is to be employed, speed is usually sacrificed.
Protocol--Is a set of rules governing the exchange of messages between two communicating processes. For LANS like Manufacturing Automation Protocol (MAP) there is a protocol assigned for each of the seven ISO layers. For serial interfaces, the protocol normally only deals with starting and stopping the data flow, performing error checking if required and initiating the retransmission of a segment of data when there is an interruption. Example of protocols for serial interfaces are RS-491, RS-484, KERMIT, DDCMP and SNA. These may be isolated to ISO layer 2 functions or spread across multiple layers.
Capability--This layer is one that is most often ignored in users specifications. At this level the user should state what capability he expects the interface to have. Examples are: upload/download part programs, upload/download tool data, upload/download probe data, upload machine status, handle cycle start/stop over the data line, upload specified machine fault messages, and upload specific CNC fault messages.
In order to create an adequate interface specification it is necessary to define your requirements accurately. Include only the capability that is required since "standard" interfaces can add as much as $20,000 to the basic price of the control. Custom-designed interfaces can cost $100,000 or higher. This fact-of-life is the reason to stay with existing standard interfaces.
The above discussion was intended to address only the most significant points of serial data interfaces in CNCs. It is worth noting that all of the capability expected at the CNC data interface must also be implemented in the host computer if there is to be meaningful communications. It therefore becomes of utmost importance for the user to coordinate the communications requirements at both ends of the data line so that the proper interface tools exist when the time comes to use them.