Serial interfaces (RS232 and USB) for instrument control

Transcription

1 Serial interfaces (RS232 and USB) for instrument control

2 Serial interface RS232 for instuments Alternative to GPIB (IEEE 488) Mainly cheap models without GPIB + Advantage: for several decades standard interface in most PCs availability - Potential problem: reimplementation to instruments with GPIB (as far as possible keeping parts of old firmware while introducing RS232) - Recently: less widely available replaced by USB)

3 Serial interface RS232 Original standart: EIA-RS232-C (1969) Today: more recent version of standart: EIA/TIA- RS232-F, ANSI/TIA/EIA-574, etc Original connector: 25-pin DB-25 Original use: interconnection between computer and modem (DTE and DCE) PROBLEM: frequent deviations in implementation details by individual manufacturers

4 RS232 DTE RS232 PSTN DCE DCE RS232 DTE DTE, Data Terminal Equipment = computer, PC DCE, Data Circuit-Termination Equipment (somatimes: Data Communicatiuon Equipment) = modem PSTN, Public Switched Telephone Network

5 RS232 Today variant: usually 9-pin connector DB CD, Carrier Detect 2 - RxD, Received Data 3 TxD, Transmitted Data 4 DTR, Data Terminal Equipment Ready 5 SG, Signal Ground 6 DSR, DCE Ready (Data Set Ready) 7 RTS (*), Request to Send 8 CTS, Clear to Send, modem signals readiness 9 RI, Ring Indicator, incoming call (*) signal 7 today usually redefined as RFR (see below) Fig.:

6 RS232 cable interconnection a) connection DTE-DCE: cable 1:1, connector extender Use: usually: connection PC - modem rarely: PC - instrument (not typical, rare solution)

7 RS232 cable interconnection b) connection DTE-DTE: today much more typical SG TxD RxD TxD RxD Interconnection by crossover cable ( null modem ): Minimally: TxD -> RxD and vice versa (at least SW handshake) Usually also: CTS -> RTS (RFR) and vice versa (HW handshake) Use: PC - instrument

8 SW Handshake (DTE-DTE) Software hanshake: exchange of special characters Xon, Xoff - text transmission only - not for binary data (random occurrence of Xon or Xoff in data) SG TxD RxD TxD RxD Minimal implementation: 3 wires

9 HW Handshake (DTE-DTE) Hardware hanshake: exchange of enabling signals RTS-CTS Originally (DTE-DCE interface): request for transmission, clearance for transmission, i.e. single-sided data flow control. DTE cannot stop receiving. Today (DTE-DTE interface) redefinition of RTS signal: RFR Ready for receiving. Enables mutual data flow control. Sometimes DSR and DTR signals also used. SG TxD RxD CTS TxD RxD CTS RTS RTS Minimal implementation: 5 wires

10 U +15V Signal levels RS232 WARNING: inverse logic SPACE, Log. 0 +3V (receiver), +5V (transmitter) Forbidden zone -3V (receiver), -5V (transmitter) t MARK, Log. 1-15V Conversion from microprocessor logic levels (TTL 0-5V): e.g. MAX232 chip, etc

11 Transmission in frames Quiet state -> Start bit -> N bits data -> (Parity if used) -> Stop bit(s) = Quiet state (Logic:) Quiet Start LSB MSB Par Stop Quiet Resynchronization of receiver clock (Voltage:) + - Start LSB MSB Par Stop

12 Frame composition Every frame starts with 1 Start bitem (transition from quiet state to log. 0 for resynchronization of receiver clock) Follows N bits of data (typically 7 or 8, rarely other count), first the least significant bit (LSB), last the most significant bit (MSB) Follows one bit of parity (if enabled) Choice: None, Even, Odd, Mark (always 1), Space (always 0) Follows 1 (or more) Stop bits (transition back to quiet state) MUST agree frame format setting of transmitter and receiver, including data rate (Baud rate) Otherwise -> framing error reported by receiver (unexpected values measured at key control bit intervals)

13 Baud rate Baud rate = Symbol rate (rate of encoding impulses at physical layer) Not the same as effective data bit rate Generally (not for RS232) 1 symbol may encode multiple bits (QAM-type modulation etc ) for RS232 1 pulse = 1 bit, BUT: additional start bit, parity, stop bit(s), inter-frame delay, Standart baudartes: 75, 110, 300, 1200, 2400, 4800, 9600, 19200, 38400, 57600,

14 Waveform examples 9600Bd i.e. 1 bit (symbol) = 104 us, 8 bit, No Parity Character = 0x0F = bin = 15dec S Stop SPACE, Log. 0 MARK, Log. 1

15 Waveform examples 9600Bd i.e. 1 bit (symbol) = 104 us, 8 bit, No Parity Character = 0xAA = bin = 170dec S Stop SPACE, Log. 0 MARK, Log. 1

16 Waveform examples 2400Bd i.e. 1 bit (symbol) = 417 us, 8 bit, EVEN Parity Character = 0xFF = bin = 255dec S P=0 Stop SPACE, Log. 0 MARK, Log. 1

17 Waveform examples 2400Bd i.e 1 bit (symbol) = 417 us, 8 bit, EVEN Parity Character = 0x01 = bin = 01dec S P=1 Stop SPACE, Log. 0 MARK, Log. 1

18 Efforts of standartization in measurement systems IEEE 1174 standart 2000, revision 2009 BUT: Minimal real use among manufacturers Defines: HW interface: 9-pin RS232 (EIA-574) Emulation of special messages IEEE over RS232 Implementation of IEEE (message protocols, common commands, etc.) over RS232 Purpose: maximal protection of existing firmware in instruments using (i.e. minimal changes enforced by migration to RS232)

19 IEEE 1174 IEEE message emulation e.g.: &SRQ CRLF = Service Request &POL = Serial Poll &GET = Group Execute Trigger &DCL CRLF = Device Clear (Minimal real support of manufactureres) IEEE support: Fig.: Standard IEEE 1174 Mainly defined solution od nonstandard situations (messgae congestion, deadlock, )

20 Programming RS232 communication LabView and LabWindows/CVI: Using RS232 library Using VISA library - recommended Similarly in MATLAB Instrument Control Toolbox Eventually: directly in Win32 API

21 Example program in CVI - RS232 Serial port setting (COM1, 9600Bd, )

22 Example program in CVI - VISA VISA resource: ASRL1 = COM1 Attribute settings Baudrate, N bits, Parity

23 USB measuring instruments Wide availability of USB in computers => Motivation for USB support in instruments Possible replacement of GPIB (IEEE 488) Faster data rate than RS232 and GPIB More devices on USB bus (127 via hubs) PROBLEM: retaining maximum of firmware in instruments (from IEEE support) => standartization USB class Test and Measurement

24 USB + Data rate 1. USB 1.1 (1998): 12 Mbit/s 2. USB 2 (2000): 280 MBit/s (35 MByte/s) 3. USB 3 (2008): 4 Gbit/s First osciloscopes: USB host for USB flash discs (screenshot and data storage, replacement of floppy discs) Later: also communication and control from PC (as replacement or supplement of GPIB)

25 Definition of USB class Test and Measurement Device USB standart defines device classes: Audio class, Battery charging, Imaging class, IrDA, Printer Class,, Test & Measurement Class,, Video Class Purpose: retaining maximum of instrument firmware from 488.2, replacement of lower levels ( > USB) See:

26 USB Discovery in PC Here: osciloscope Agilent DSO1012A Like all USB: automatic detection and driver load

27 VISA and USB Problem: Unlike e.g. GPIB, VISA resource descriptor for USB is higly sensitive to completeness (serial number, etc.) Thus, full specification of target device is necessary: even more than for GPIB it is good practice to make the program flexible, i.e. import the VISA resource string from GUI or a.cfg file + VISA resource can be Copy & Paste-ed from MAX + can be looked up using VISA function vifindrsrc (,"?*INSTR",,, ); Supported in VISA and integrated SW tools (NI MAX, Agilent IO Control)

28 Detection in NI MAX VISA resource can be Copy & Paste-ed

29 Tools in MAX:

30 Basic communication USB in MAX Identification query - Response Record in NI I/O Trace

31 Example program in CVI - VISA

32 Example program in LabView