Industrial Communications First Test

Industrial communications first test

DDC = direct digital control

DCS = distributed control systems

Why digital: more data, multidrop (many devices on same com. media), robustness (accurate values transferred), variety of protocols.

Stanards: - cost, + quality; interoperability (primary concern!); TAG (Technical Advisory Group – national standards body), IEC (International Electrotechnical Commission), ISO, ISA (Instrumentation Society of America), CEN, IEEE, EIA, TIA

Factory automation: ideal network types for simple I/O focus on low overhead and small data packets. Examples: Seriplex, CANbus, AS-I (actuator-sensor interface) – Sensor buses/bit level buses.Advanced: DeviceNet, SDS, CANopen – device buses, byte-level buses.Process automation: contin. regulatory control; nets: Foundation Fieldbus, Profibus PA, HART.

SCADA (supervisory control and data acquisition) – defines a comp. system used for gathering and analyzing real time data coming from industrial processes → operator interface is important – HMI.

Business/Enterprise level – big network, LAN/WAN, several protocols used at the same time. Low cost per node. Control level – LAN, broadcast/point2point messaging between automation nodes; protocols → backbone forPLCs, SCADA, HMI. Ethernet → low cost. Device/Field level: field-level means com. nets that link indust. field devices (sensors, actuators, controllers), ‘networking of I/O’. Uses HART (FSK – Frequency Shift Keying). HART uses a superimposed digital signal (at a low level) on top of the 4-20 mA (built upon 4-20mA Current Loop (CL)). Compatible with existing analog devices, but enables usage of digital signals. Device and field buses are almost on the same level. Standards issues due to control system supplier competition.Bit/sensor level – simple buses. Small overhead, large cycle times.

Criteria for choosing a best-fit network: Redundancy, efficiency (how hard is to send a message, how many msg’s for RW op-s, how much the host computer has to do?), speed (bandwidth – raw speed of data within a channel), determinism (e.g. when using Ethernet due to CSMA/CD performance cannot be guaranteed), distance, length of messages, cabling (tw.p or fiber-opt?), vendor support, maintainability; how to choose: focus on application, consider the costs, access the net connectivity, understand hidden changes and impacts; evaluate interoperability.

Peculiarities: environment? EMI? Type of data (time critical?), dependability (fail-safe? process error situations? avoid downtime? decentralization? error recovery? origin of data? priorities of messages? fast changing technology)

Requirments: dependability (handle errors and emergencies, fault tolerance), autonomous operation (decentralization – contradiction with master/slave), time-tagged data, distributed information of global time (via say local real time clock), guaranteed delivery time, real-time traffic (traffic of packets in com. channel should be independent of presence of errors; broadcast messages are important), datagrams (~ connectionless), manageability (the com. system needs to be able to reconfigure itself according to various situations), scaleability.

Reqs @ field level: very short resp. time, tolerance for harsh environments, long distance, power distribution; Reqs @ control level: short response times, tolerance for harsh environments, very high availability (MTBF – many years), security, power backup, net management. Automation integration can be achieved through extensive use of standards, e.g. MAP.

MAP(for interoperability) – manufact. automation protocol – to overcome com. problems between multi-vendor automation devices (developed by GM and Boeing). MAP3.0 specs published in 1984 → FullMAP (flexibility for com. stations, not good for real-time), MiniMAP (reduced OSI stack, suitable for time critical), EPA (merge MiniMAP/FullMAP).

MAP at higher levels: more complex info, long distance, async timing; lower levels: simpler info, short distances, sync timing. MAP generated the token bus protocol and a new message exch. protocol MMS – Manufacturing Message Spesification ISO9506, most suitable at lowest levels, used client-server model, VMD – virtual manufact. device. Protocols of MAP: ISO TP4 transp. – reliable conn.-oriented service, 3way handsh.

Integration of manufacturing enterprise

MRP – Material requirements planning, MRPII – manufacturing resource planning, ERP – enterprise resource planning (on-line response times).

MES – manufacturing execution systems; developed to provide infrastructure for info (real-time/on-line plant floor and logistics info) on Manufacturing Enterprise Collaboration.→ carry out the plan. MES:

MIS – manufacturing intelligence system; the goal is to gather info and prepare it for presentation and analysis.

Transmission impairments– noise, attenuation distortion, delay distortion. BER – bit error ratio, affected by bandwidth, SNR, transmission media and distance environment. S/N(db)=10log10(S/N). Noise types: Thermal noise (white noise), atmospheric noise, intermodulation (different frequencies on same medium), crosstalk, power noise, transients (impulse noise), noise coupling: EMI, inductive (current), capacitive (voltage), RFI, common impenance (different circuits share common wires), conducted noise (via transm. noise by wires) – normal mode between signal line and circuit reference (diff. voltage, cannot be distinguished from the transducer signal), common mode noise between signaling circuit and ground (picked up on both leads from ground).

Handling noise: differential signals + twisted signal leads; current signals (vs. voltage signals), proper GND and earthing, layout, routing of cables, shielding, NSF (noise suppr. filters), galvanic isolation.

Cable spacing 5cm-1.2m. Place cables over AC lines only at correct angles. Shielding – connect to GND only @ one end (normal circumstances).

Wiring: plain pair, shielded pair, coaxial cable, twisted pair (magnetic fields cancellation, most common – grade 5 UTP cable, lightweight, easy to pull & terminate, but susceptible to EMI). STP – improve signaling rate being heavier and more difficult to manufacture (shield attenuates electrical fields). Fiber-optic – best solution, expensive.

RS232 – intended primarily for DTE-DCE links, adopted for char. oriented peripherals.

RS232C – volt. levels +/-15V

RTS – DTE→DCE, CTS (clear t.s.)– backwards. DSR (data set ready) – DCE to DTE; DTR (data terminal ready – DTE to DCE, means DTE is ready to accept data from DCE).

232C vs. 232D

Transmit example: (DTE→DCE): DTR, RTS, wait for DSR, wait for CTS, transmit the data. Receive: (DCE→DTE) DTR, wait for DSR, receive data.

Min RS232 signals: async: TD, RD, SG (signal gnd); sync: TD, RD, SG, TC, RC, XTC (external transmit clock).

RS232 problems: unbalanced transmission (common-mode noise), top speed: 20Kbps, max distance 15 m between DTE and DCE.

Differential transmitter: generates 2 signals of equal, opposite polarity for each bit. Reciever: sensitive only to difference between 2 signals @ its inputs – noise is thus absorbed by both wires and doesn’t affect receiver + good common mode rejection.

RS-422,-485:balanced → driver produces 1 signal that flows thru 2 lines (2...6 V across A and B). ‚Enable’ connects the driver to its output terminals.

‚disabled’ → third state (tristate).

485 vs. 232: 485 is balanced, uses dif. signaling on pair of wires – small voltage detection thresholds, common mode rejection – improves performance over longer distances (1200m, any rate below 9600 baud); uses driver enable/disable – multidrop (up to 32 driver/receiver pairs can share a net).

A RS485 bus behaves like a transm. line, therefore must be terminatedto avoid reflections (120Ohm res between A and B at each end of the bus).Two Wire or Four Wire: 422 needs a dedicated pair of wires for each signal, whereas 485 allow a single pair of wires in half-duplex (reduced cable cost). Four-wire with 485: one node a master node, others – slaves. Tristate control may be achieved using a RTS signal (0→tristate, 1→driver on). Termination: to match impedance of a node to the impedance of transm. line, otherwise transm. signal is not completely absorbed by the load and reflected back to the transmitter (term. increases load on the drivers and installation complexity). Biasing: in order to maintain the proper idle voltage state in the line (using pull-up (B) and pull-down (A) resistors) – to maintain a minimum of 200mV between B and A data lines. Underbiasing → decreased noise immunity or even complete data failure. 485 driver control: 1. use a control line (e.g. RTS handshake line) to enable/disable the driver (may have timing related issues); 2. ASDC – Automatic Send Data Control. Needs special circuitry that senses that data is being transmitted. Preferred method because it reduces software overhead and simplifies programming.

Error detection and correction: EDC (error detection codes):parity, checksums, CRC; ECC (correction c.): hamming, reed-solomon. Hamming distance – # of different bits in code words. CRC is widely used in practice (ATM, HDLC), performance: - can detect for r bits/frame frame len. < 2r-1: all patterns of 1,2,3 err.; all burst errors of r or fewer bits; random large # of err. with prob. of 1-2^-r. Correction: AQR (Automatic Repeat reQuest) with seq. numbers, acks, nacks, sacks & timers → methods: stop&wait ARQ (1/2 duplex); sliding-window: go-back-n; selective repeat.

Error correction: FEC (e.g. hamming): extra bits to detect & correct – large overhead, cannot recover from huge errors, used in simplex transm., & where transm. times are long. BEC – only to detect, simple, effective, least expensive.

Hamming:

Flow control: 1. point-to-point (softw. flow ctrl. Xon/Xoff based on ASCII → not suitable for binary/raw data transfers, h/w flow ctrl RTS/CTS signal lines);

2.End-to-End flow ctrl – ACKs/NAKs (stop&wait, sliding window : go-back-n, selective repeat). Stop&wait works in half duplex or when receiver buffer is limited in size (1 frame). Efficiency=tframe/(2tprop+tframe). Sliding window: transm. can send # blocks with seq.# with no ACK (# determined by window size).

Go-back-N: retransmit all unACKed frames from the last ACKed frame upon a receipt of an out-of-sequence frame; Selective-repeat: retransmit only corrupted frames, transmission order is allowed to change.‚Inflight’ data amount ideally =bandwidth*delay [product ~ BDP]; Flowrate=RWin/RTT – if receiving app can’t keep up; RWin<BDP then receiver limits flow, otherwise chan. properties (RTT, b/width).