RS 232 Interface. RS 232 is the Serial interface on the PC. Three major wires for the Serial interface: Transmit Pin 2 Receive Pin 3

Transcription

1 RS 232 Interface RS 232 is the Serial interface on the PC Three major wires for the Serial interface: Transmit Pin 2 Receive Pin 3 Note: SR510 switches pins 2,3 internally HP Func. Gen. Requires a null modem cable Computer Ground Pin 7 (25 pin connector) Pin 5 (9 pin connector) Tx Rx Rx Tx Device Gnd Gnd Transmit connects Week to 6 Receive material

2 RS 232 Standard When is a constant variable? While many aspects of the RS 232 port is standardized, there can be variations in Pin assignments, data formats, transmission speed, etc. RS232 might require more effort to work than GPIB Example: In trying to get two computers to play against each other in a video game through the serial communications port, we found out that time width of data bit different for different computers! Example: Lock in has 25 pin connector and only requires pins 2,3,7 to be connected. Function Generator has a 9 pin connector and requires a null modem cable. Experimentation/ Oscilloscope to monitor data transmission a must!!

3 RS232 Settings Computer and device must have the same settings for Baud Rate Parity Stop bits Baud Rate data transmitted in bits/second Parity Check against faulty data transfer. If used, 8 (typically) data bits sent plus parity bit. Responsibility of device to check parity Stop Bits denotes end a data string. Use 2 stop bits with SR510. (Recommended)

4 RS232 Data format (SR510) Data Format: Time Bit 0: Start bit Voltage Bit 1 8: ASCII code of Data Bit 9 : Parity Bit Bit : Stop bits Start Bit LSB ASCII Data is sent LSB first (eg. ASCII A= ) MSB Parity & Stop Bits Logic of Data is inverted. Control lines use positive logic Each ASCII character requires about 12 bits to be transmitted or about 833 s for character. A string of G 22<cr> would require about 4.2msec to be sent.

5 ASCII CODE

6 RS232 with Control Lines Analogous to GPIB with Control lines (although control lines not required in RS232) some devices require them. All you need is a handshake. CTS Clear to Send (pin 5 on SR510) Set high (Asserted) by instrument to tell computer that instrument is ready to receive data DTR Data Terminal Ready (pin 20) Set high by computer to tell instrument that Computer is ready to receive data.

7 Particulars of SR510 If control lines not connected, the SR510 assumes that PC is always ready to receive data If DTR line is low, no data transferred from SR510. Remember, view data with an oscilloscope to test to data format, sense of logic, time duration of data, etc. to make sure computer/ device are compatible. Misc: Some devices have an ECHO, Echo, echo, echo feature When data is received by device, copy is echoed back on RS232. By comparing data sent with echo, you can check for errors.

8 Xon/ Xoff Handshaking Common problem in serial communications: both sender and receiver keeping up with data transmission. I/O serial buffer of limited size. The computer ignores new data until you have read enough data out of the buffer to make room for new information. Handshaking helps prevent this buffer from overflowing. With handshaking, the sender and the receiver notify each other when their buffers fill up. The sender can then stop sending new information until the other end of the serial communication is ready for new data. Two kinds of handshaking in LabVIEW: Hardware or Software: Hardware: Control lines Software Handshaking XON/XOFF XON/XOFF: When the receive buffer is nearly full, the receiver sends XOFF ( <Ctrl S> [decimal 19]) to tell the other device to stop sending data. When the receive buffer is sufficiently empty, the receiver sends XON (<Ctrl Q> [decimal 17]) to indicate that transmission can begin again. Do not use XON/XOFF with binary data transfers because <Ctrl Q> or <Ctrl S> may be embedded in the data, and the devices will interpret them as XON and XOFF instead of as data.

9 Serial Port Parameters flow control BUNDLE contains the following parameters. Input XON/XOFF : Default False Input HW Handshake: On PC, this parameter corresponds to Request To Send (RTS) handshaking. Default value: FALSE Input alt HW Handshake: On the PC, this parameter corresponds to Data Terminal Ready (DTR) handshaking. Default value: FALSE Output XON/XOFF: Default value: FALSE Output HW Handshake: On the PC, this parameter corresponds to Clear to Send (CTS) handshaking. Default value: FALSE Output alt HW Handshake: On the PC, this parameter corresponds to Data Set Ready (DSR) handshaking. Default value: FALSE XOFF byte: is the byte used for XOFF (^S). XON byte: is the byte used for XON (^Q). Parity Error Byte: If the high byte is non zero, the low byte is the character that is used to replace any parity errors found when parity is enabled. Default value: 0

10 Serial Port Setup Parameters Communications port VISA Resource Name C O M 1 Com 1, Com2, etc. Use CREATE CONSTANT Buffer size: the size (in bytes) of the input and output buffers the VI allocates for communication. Baud rate is the rate of transmission.default value: 9600 Data bits is the number of bits in the incoming data. (between 5 and 8). Default value: 8 Stop bits is 0 for one stop bit, 1 for one and a half stop bits, or 2 for two stop bits. Default value: 1 bit Parity is 0 for no parity, 1 for odd parity, 2 for even parity, 3 for mark parity, or 4 for space parity.default value: 0

11 Serial Port Parameters Enable Termination Char prepares the serial device to recognize termination char. If TRUE (default), recognize the termination character. If FALSE, the serial device does not recognize the termination char. termination char calls for termination of the read operation. The read operation terminates when the termination char is read from the serial device. 0xA is the hex equivalent of a linefeed character (\n). Change the termination char to 0xD for message strings that terminate with a carriage return (\r). timeout sets the timeout value for the write and read operations. VISA resource name specifies the resource to be opened.

12 Serial Port Read/ Write COM1 VISA Configure Serial Port VISA Write Set Volt 0.2 INPUT Terminals: Port number : VISA RESOURCE NUMBER wired from output of serial port setup VI string to write is the data to be written to the serial port. If the number of characters in string to write is greater than the buffer size specified in Serial Port Init, the number of characters equal to the buffer size will be written.

13 C O M 1 by te c o un t (0) 10 0 V I S A R ea d rea d bu ff er Input Terminals Port number: Com 0, 1, etc. Requested byte count specifies the number of characters to be read. If you want to read all of the characters currently at the serial port, first execute the Bytes at Serial Port VI to determine the exact number of bytes ready to be read. Then use the byte count output of that VI as the requested byte count input to the Serial Port Read VI. Output Terminals string read returns the bytes read in string read. Serial Port Read/Write COM1 COM1 VISA Configure Serial Port 80 VISA Read VISA Configure Serial Port Instr Bytes at Port VISA Read read buffer read buffer

14 Control of Serial Port Once LabVIEW opens a serial port, it is inaccessible to other applications until you quit LabVIEW To free the port, use the CLOSE VISA SESSION to free the port for other applications. C O M 1 V IS A C lo se

15 Testing out the RS232 Use HyperTerminal (Windows XP) Programs>>Accessories>>HyperTerminal Set up Communications : Com 1, Baud, 8 Bits, no parity, 2 stop bits. With SR510 Lockin, can use RS232 terminal echo mode (switch 6 on SW2) to echo commands to terminal Type commands in Hyperterminal to read/write data to SR510. (Set up ASCII for linefeeds, local echo, etc.) NOTE: Do not need flow control with EITHER lockin or function generator Lockin.ht Hyperterminal Configuration Files: functgen.ht

16 Testing out the RS232 Use National Instruments MAX program Set up Communications : Com 1, Baud, 8 Bits, no parity, 2 stop bits. For Function Generator, use 9600 Baud Meas urement & Automation.lnk Use BASIC I/O Menu and then WRITE and READ responds from interface.

17 Example Programs RS232 Lock in Amplifier RS232 Example Function Generator RS232 Example You need Null Modem cables for Function generator so use Gateway Computers (9 pin connector)