Industrial communications first test Criteria for choosing a best-fit network: at lowest levels, used client-server model, VMD
Redundancy, efficiency (how hard is to send a – virtual manufact. device. Protocols of MAP:
DDC = direct digital control message, how many msg’s for RW op-s, how ISO TP4 transp. – reliable conn.-oriented
DCS = distributed control systems much the host computer has to do?), speed service, 3way handsh.
Why digital: more data, multidrop (many (bandwidth – raw speed of data within a
devices on same com. media), robustness channel), determinism (e.g. when using
(accurate values transferred), variety of Ethernet due to CSMA/CD performance cannot
protocols. be guaranteed), distance, length of messages,
Stanards: - cost, + quality; interoperability cabling (tw.p or fiber-opt?), vendor support,
(primary concern!); TAG (Technical Advisory maintainability; how to choose: focus on
Group – national standards body), IEC application, consider the costs, access the net
(International Electrotechnical Commission), connectivity, understand hidden changes and
ISO, ISA (Instrumentation Society of America), impacts; evaluate interoperability.
CEN, IEEE, EIA, TIA Peculiarities: environment? EMI? Type of Integration of manufacturing enterprise
Factory automation: ideal network types for data (time critical?), dependability (fail-safe?
simple I/O focus on low overhead and small process error situations? avoid downtime?
data packets. Examples: Seriplex, CANbus, decentralization? error recovery? origin of
AS-I (actuator-sensor interface) – Sensor data? priorities of messages? fast changing
buses/bit level buses. Advanced: DeviceNet, technology)
SDS, CANopen – device buses, byte-level Requirments: dependability (handle errors and
buses. Process automation: contin. regulatory emergencies, fault tolerance), autonomous
control; nets: Foundation Fieldbus, Profibus PA, operation (decentralization – contradiction with
HART. 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 MRP – Material requirements planning, MRPII
(~ connectionless), manageability (the com. – manufacturing resource planning, ERP –
system needs to be able to reconfigure itself enterprise resource planning (on-line response
according to various situations), scaleability. times).
SCADA (supervisory control and data Reqs @ field level: very short resp. time,
acquisition) – defines a comp. system used for tolerance for harsh environments, long
gathering and analyzing real time data coming distance, power distribution; Reqs @ control
from industrial processes → operator interface level: short response times, tolerance for harsh
is important – HMI. environments, very high availability (MTBF – MES – manufacturing execution systems;
Business/Enterprise level – big network, many years), security, power backup, net developed to provide infrastructure for info
LAN/WAN, several protocols used at the same management. Automation integration can be (real-time/on-line plant floor and logistics info)
time. Low cost per node. Control level – LAN, achieved through extensive use of standards, on Manufacturing Enterprise Collaboration. →
broadcast/point2point messaging between e.g. MAP. carry out the plan. MES:
automation nodes; protocols → backbone for
PLCs, SCADA, HMI. Ethernet → low cost. MAP (for interoperability) – manufact.
Device/Field level: field-level means com. nets automation protocol – to overcome com.
that link indust. field devices (sensors, problems between multi-vendor automation
actuators, controllers), ‘networking of I/O’. Uses devices (developed by GM and Boeing).
HART (FSK – Frequency Shift Keying). HART MAP3.0 specs published in 1984 → FullMAP
uses a superimposed digital signal (at a low (flexibility for com. stations, not good for real-
level) on top of the 4-20 mA (built upon 4-20mA time), MiniMAP (reduced OSI stack, suitable for
Current Loop (CL)). Compatible with existing time critical), EPA (merge MiniMAP/FullMAP).
analog devices, but enables usage of digital MIS – manufacturing intelligence system; the
signals. Device and field buses are almost on goal is to gather info and prepare it for
the same level. Standards issues due to control presentation and analysis.
system supplier competition. Bit/sensor level –
Transmission impairments – noise,
simple buses. Small overhead, large cycle
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
MAP at higher levels: more complex info, long (current), capacitive (voltage), RFI, common
distance, async timing; lower levels: simpler impenance (different circuits share common
info, short distances, sync timing. MAP wires), conducted noise (via transm. noise by
generated the token bus protocol and a new wires) – normal mode between signal line and
message exch. protocol MMS – Manufacturing circuit reference (diff. voltage, cannot be
Message Spesification ISO9506, most suitable distinguished from the transducer signal),
common mode noise between signaling circuit
and ground (picked up on both leads from ASDC – Automatic Send Data Control. Needs
ground). special circuitry that senses that data is being
Handling noise: differential signals + twisted transmitted. Preferred method because it
signal leads; current signals (vs. voltage reduces software overhead and simplifies
signals), proper GND and earthing, layout, programming.
routing of cables, shielding, NSF (noise suppr. Error detection and correction: EDC (error
filters), galvanic isolation. detection codes): parity, checksums, CRC;
Cable spacing 5cm-1.2m. Place cables over ECC (correction c.): hamming, reed-solomon.
AC lines only at correct angles. Shielding – Hamming distance – # of different bits in code
connect to GND only @ one end (normal words. CRC is widely used in practice (ATM,
circumstances). HDLC), performance: - can detect for r
bits/frame frame len. < 2r-1: all patterns of 1,2,3
Wiring: plain pair, shielded pair, coaxial cable, err.; all burst errors of r or fewer bits; random
twisted pair (magnetic fields cancellation, most large # of err. with prob. of 1-2^-r. Correction:
common – grade 5 UTP cable, lightweight, AQR (Automatic Repeat reQuest) with seq.
easy to pull & terminate, but susceptible to numbers, acks, nacks, sacks & timers →
EMI). STP – improve signaling rate being methods: stop&wait ARQ (1/2 duplex); sliding-
heavier and more difficult to manufacture window: go-back-n; selective repeat.
(shield attenuates electrical fields). Fiber-optic –
best solution, expensive. Error correction: FEC (e.g. hamming): extra
Differential transmitter: generates 2 signals of bits to detect & correct – large overhead,
RS232 – intended primarily for DTE-DCE links, equal, opposite polarity for each bit. Reciever: cannot recover from huge errors, used in
adopted for char. oriented peripherals. sensitive only to difference between 2 signals simplex transm., & where transm. times are
@ its inputs – noise is thus absorbed by both long. BEC – only to detect, simple, effective,
wires and doesn’t affect receiver + good least expensive.
common mode rejection. Hamming:
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
RS232C – volt. levels +/-15V
Flow control: 1. point-to-point (softw. flow ctrl.
Xon/Xoff based on ASCII → not suitable for
‚disabled’ → third state (tristate). binary/raw data transfers, h/w flow ctrl
485 vs. 232: 485 is balanced, uses dif. RTS/CTS signal lines);
signaling on pair of wires – small voltage
detection thresholds, common mode rejection –
improves performance over longer distances
(1200m, any rate below 9600 baud); uses
RTS – DTE→DCE, CTS (clear t.s.)– driver enable/disable – multidrop (up to 32
backwards. DSR (data set ready) – DCE to driver/receiver pairs can share a net).
DTE; DTR (data terminal ready – DTE to DCE, A RS485 bus behaves like a transm. line,
means DTE is ready to accept data from DCE). therefore must be terminated to avoid
2.End-to-End flow ctrl – ACKs/NAKs
reflections (120Ohm res between A and B at
232C vs. 232D (stop&wait, sliding window : go-back-n,
each end of the bus). Two Wire or Four Wire:
selective repeat). Stop&wait works in half
422 needs a dedicated pair of wires for each
duplex or when receiver buffer is limited in size
signal, whereas 485 allow a single pair of wires
(1 frame). Efficiency=tframe/(2tprop+tframe). Sliding
in half-duplex (reduced cable cost). Four-wire
window: transm. can send # blocks with seq.#
with 485: one node a master node, others –
with no ACK (# determined by window size).
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
Transmit example: (DTE→DCE): DTR, RTS, transm. signal is not completely absorbed by
wait for DSR, wait for CTS, transmit the data. the load and reflected back to the transmitter
Receive: (DCE→DTE) DTR, wait for DSR, (term. increases load on the drivers and
receive data. installation complexity). Biasing: in order to
maintain the proper idle voltage state in the line Go-back-N: retransmit all unACKed frames
Min RS232 signals: async: TD, RD, SG (signal from the last ACKed frame upon a receipt of an
(using pull-up (B) and pull-down (A) resistors) –
gnd); sync: TD, RD, SG, TC, RC, XTC (external out-of-sequence frame; Selective-repeat:
to maintain a minimum of 200mV between B
transmit clock). retransmit only corrupted frames, transmission
and A data lines. Underbiasing → decreased
RS232 problems: unbalanced transmission noise immunity or even complete data failure. order is allowed to change. ‚Inflight’ data
(common-mode noise), top speed: 20Kbps, 485 driver control: 1. use a control line (e.g. amount ideally =bandwidth*delay [product ~
max distance 15 m between DTE and DCE. RTS handshake line) to enable/disable the BDP]; Flowrate=RWin/RTT – if receiving app
driver (may have timing related issues); 2. can’t keep up; RWin<BDP then receiver limits
flow, otherwise chan. properties (RTT, b/width).