RS-232 protocol

Introduction

The protocol RS232 defines the signals used in communication, and the hardware to transfer signals between devices.
The time diagram of the typical signal used to transfer character ‘A’  (ASCII: 6510 or 0x41) from device A to device B is given in fig.1 and would appear on the upper line TX ‐> RX between devices

The standard defines voltage levels V(0) to be at least +5V at the transmitting end of the line TX, and can be degraded along the line to become at least +3V at the receiving and of the line. Similarly voltage level V(1) must be at least ‐5V at TX, and at least ‐3V at RX. The standard also defined the upper limit for these voltages to be up to ±15V. Logic high is transferred as V(0)

The connectors are typically so‐called D9 connectors, and the electric wiring in between two connectors at devices A and B is shown in Fig. 2, for two female type connectors at both devices.

RS-232C INTERFACE IN CNC MACHINE

Data transfer between two electronic devices (computers and controls) requires a number of settings that use the same rules for each device. Since each device may be manufactured by a different company, there must be a certain independent standard that all manufacturers adhere to. The RS-232C is such a standard – the letters RS stand for ‘Recommended Standard’. Almost every CNC system, a computer, a tape puncher and tape reader, has a connector (known as a port) that is marked RS-232C or similar. This port exists in two forms, one with a 25 pin configuration, the other with a 25 socket configuration. One with the pins is known as the DB-25P connector, one with the socket as DB-25S connector (male/female respectively). Figure3 illustrates the typical layout.

RS-232C port on the CNC unit is usually a standard feature and uses the DB-25S type (the letter S means it is a socket type). An external computer, usually a desktop computer or a laptop, together with a suitable cable and communications software is also needed to transfer CNC programs. External devices use mainly the DB-25P type connector (the letter P means it is a pin type). CNC program is sent to the system memory and is stored there as long as needed to run the job.

Although terms such as Transmit (or Send) and Receive are more common in software, some CNC systems use the terms Punch (which is equivalent to Send) and Read (which is equivalent to Receive). These terms go back to the days of punched tape.

Some DNC software also allows a useful feature called ‘drip-feeding’, which is a method used when the program is too large to fit into the CNC memory.

TERMINOLOGY OF COMMUNICATIONS

Communications have their own terminology. There are many terms, but five terms are commonly used in CNC:

•  Baud Rate
•  Parity
•  Data Bits
•  Start Bit
•  Stop Bit

 Baud Rate

Baud rate is the data transmission speed. It is measured as the amount of data bits per second, written as bps. Baud rates are only available in fixed amounts. Typical rates for older Fanuc controls are 50, 100, 110, 200, 300, 600, 1200, 2400, 4800 and 9600 bps. Modern controls can have the baud rate set to 2400, 4800, 9600, 19200, 38400, 57600 and 76800 bps. In terms of time, the higher the rate, the faster the transmission.

A single bit transferred at 300 bps will take 0.03333 of a second, but a single bit transferred at 2400 bps will take only 0.00042 of a second. In practice, it takes about 10 bits to transfer one character (see Stop Bits section below), so at 2400 bps setting, the transmission will be at a rate of about 240 cps (characters per second). 4800 bps is a good setting once everything is working well. Higher settings are necessary for ‘drip-feed’ methods.

 Parity

Parity is a method of checking that all transmitted data were sent correctly. Just imagine what would happen if some characters or digits of a CNC program were not transferred correctly or not transferred at all. Parity can be even, odd, or none, and even is the most common selection for CNC communications. This is similar to punched tape parity.

 Data Bits

A bit is an acronym for Binary digit and is the smallest unit that can store information in a computer. Each binary digit can have a value of either one (1) or zero (0). One and zero represent the ON and OFF status respectively, so a bit is something like a toggle switch that can be turned on and off as needed. In the computer, every letter, digit, and symbol used in a CNC program is represented by a series of bits, eight bits to be precise, that create a unit called a byte.

 Start and Stop Bits

To prevent loss of data during communication, each byte is preceded by a special bit called the start bit, which is low in voltage level signal. This signal is sent to the data receiving device and informs it that a byte of data is coming next.

A bit similar to the start bit, but at the end of the byte, has exactly the opposite meaning. It sends a signal to the receiving device that the byte has ended or stopped being transmitted. This bit at the end of a byte is called the stop bit. Because the start and stop bits go together, they are often teamed up together as the stop bits and set the devices to two stop bits.

Many terms exist in communications. With growing interest, this is a very rich field to study.

DATA SETTING

Data used for communications must be set properly before data transfer can begin. The setting at one end (computer or the CNC system) must match the setting at the other end. For baud rate, consult the machine manual – a good start is at 2400 bps. Newer models have a higher default. Typical software setting is done through the configuration at the computer end and through the CNC system parameters at the CNC end. Settings at both ends must match. Typical Fanuc settings are:

•  4800 bps baud rate
•  Even parity
•  7 data bits (seven data bits)
•  2 stop bits (two stop bits)

Proper connection depends mainly on the configuration of connecting data cables.

CONNECTING CABLES

The most common cable for communication between a CNC machine and a computer is a shielded and grounded cable, containing several small wires (at least eight), each one enclosed in a colored plastic sleeve. Main purpose of making a communication cable is to connect the CNC port (usually 25 sockets) with the computer port (usually 25 pins), using a properly configured cable. Always use a cable of high quality. Shielded cables can reach farther distances and are generally better choice to withstand interferences during data transmission. Wires are identified by their gauge value, for example a 22-gauge or a 24-gauge wire is a good choice for communications.

The 25-pin port has each pin or socket numbered (see the first page of this chapter) and individual wires of the cable have to be connected to proper numbers at each end. It is quite common to ‘cross ‘the wires between each end. Typical crossing would be between the pin number 2 and the socket number 3, and a pin 3 and socket 2. Some numbered positions have to be connected at the same end of the cable. This is called ‘jumping’.

 Null Modem

A very common cable wiring that is used in general communications is called a null modem. Connection of the two ends follows a certain standard, shown in Figure 52-5. Each number represents the pin or socket on the DB-25 connector. Note the jumps between connections 6 and 8 at both ends. Figure 52-6 shows the same null modem configuration in a graphic way. This is a very popular method showing cable configurations.

PIN DB-25P	SOCKET DB-25S
1	1
2	3
3	2
4	5
5	4
7	7
6 and 8	20
20	6 and 8

Figure 52-5 Null modem pin connections

Null modem pin connections

Figure 52-6 Graphic representation of null modem connections

 Cabling for Fanuc and PC

GE FANUC controls use a serial port to transfer data between the control and a computer or other device. There are a number of reasons why the communications would not be working properly.

Check your cable against the following diagrams. RS232 is limited to communicating at distances of < 50 Ft. For testing purposes connect a short cable directly from the computer to the machine.

NOTE:
On all Fanuc controls, pin 25 of the RS-232 port is supplied with +24v from the control for use with specific GE Fanuc devices (ex. Handy File). DO NOT connect this pin through to your PC or it may be damaged

Regardless of what cable configuration will be used, a good communication software that will run the whole operation is also needed. Some companies use a software specially designed for CNC work, others purchase very inexpensive communications software, sold by the majority of computer stores.

Example CNC Parameters

Fanuc 10M

Setting screen:
Punch code- ISO
TV CHECK=0 OFFUnder Service page: 
0000 = xxx01000 (ISO)
0020 = 2 (Input device Foreground)
0021 = 2 (Output device Foreground)
0022 = 2 (Input device Background)
0023 = 2 (Output device Background)
5001 to 1 (Device Select)
5002 to 2 (RS232 Device Select)
5110 to 3 (Device type DC Codes used)
5111 to 1 (1 stop bit) 
5112 to 10 (4800 baud) 11=9600, 9=2400, 8=1200 7=600 6=300