Serial interfaces (RS232 and USB) for instrument control

Serial interfaces (RS232 and USB) for instrument control

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)

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

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

RS232 Today variant: usually 9-pin connector DB-9 1 - 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.: http://www.camiresearch.com/data_com_basics/rs232_standard.html

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

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

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

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

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 1 0 1 Conversion from microprocessor logic levels (TTL 0-5V): e.g. MAX232 chip, etc

Transmission in frames Quiet state -> Start bit -> N bits data -> (Parity if used) -> Stop bit(s) = Quiet state (Logic:) 1 0 1 1 Quiet Start LSB MSB Par Stop Quiet Resynchronization of receiver clock (Voltage:) + - Start LSB MSB Par Stop

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)

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, 115200

Waveform examples 9600Bd i.e. 1 bit (symbol) = 104 us, 8 bit, No Parity Character = 0x0F = 0000 1111bin = 15dec S 1 1 1 1 0 0 0 0 Stop SPACE, Log. 0 MARK, Log. 1

Waveform examples 9600Bd i.e. 1 bit (symbol) = 104 us, 8 bit, No Parity Character = 0xAA = 1010 1010bin = 170dec S 0 1 0 1 0 1 0 1 Stop SPACE, Log. 0 MARK, Log. 1

Waveform examples 2400Bd i.e. 1 bit (symbol) = 417 us, 8 bit, EVEN Parity Character = 0xFF = 1111 1111bin = 255dec S 1 1 1 1 1 1 1 1 P=0 Stop SPACE, Log. 0 MARK, Log. 1

Waveform examples 2400Bd i.e 1 bit (symbol) = 417 us, 8 bit, EVEN Parity Character = 0x01 = 0000 0001bin = 01dec S 1 0 0 0 0 0 0 0 P=1 Stop SPACE, Log. 0 MARK, Log. 1

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 488.1 over RS232 Implementation of IEEE 488.2 (message protocols, common commands, etc.) over RS232 Purpose: maximal protection of existing firmware in instruments using 488.2 (i.e. minimal changes enforced by migration to RS232)

IEEE 1174 IEEE 488.1 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 488.2 support: Fig.: Standard IEEE 1174 Mainly defined solution od nonstandard situations (messgae congestion, deadlock, )

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

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

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

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 488.2 support) => standartization USB class Test and Measurement

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)

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 (488.1 -> USB) See: http://www.usb.org/developers/devclass_docs#approved

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

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)

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

Tools in MAX:

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

Example program in CVI - VISA

Example program in LabView