Communication lines

OSI model
• Open Systems Interconnection (OSI) model
(ISO/IEC 7498-1)
• Physical layer – specifies electrical and physical properties (cable)
• Pins, voltages, impedance, modulation, timing, topology
Source: homepages.uel.ac.uk

OSI model
• Open Systems Interconnection (OSI) model
(ISO/IEC 7498-1)
• Physical layer –
Transmits raw bit stream over physical cable
• IEEE 802, RS232,
RS422, RS485, I 2 C,
SPI,…
Source: homepages.uel.ac.uk

OSI model
• Open Systems Interconnection (OSI) model
(ISO/IEC 7498-1)
• Data link layer – specifies network data frame (packet), checksum, source and destination address, and data
• E.g. Ethernet MAC,
RS232
Source: homepages.uel.ac.uk

OSI model
• Open Systems Interconnection (OSI) model
(ISO/IEC 7498-1)
• Network layer – routing, directing datagrams from one network to another
• E.g. IP addresses
Source: homepages.uel.ac.uk

RS232 (EIA232)
• Dates from 1969 (RS-232-C)
• Last standard is TIA/EIA-232-F from 1997
• Defines physical and data link layer
• Single transmitter and receiver
TX RX
Log. 0 : +5 to +15 V
Log. 1 : -15 to -5 V
Noise immunity: min. 2 V
Log. 0 : > +3 V
Log. 1 : < -3 V

RS232 (EIA232)
TX RX signal
• Length RS-232-C = 15 m
• RS-232-F defines max. load capacity 2500 pF
C
C
S
S
C
M
C
RX
~20 pF shield

RS232 - Cable capacity signal
C
C
S
S
C
M
C
RX
~20 pF shield
Max. length: L max
= 2500 / C total
C total
= C
M
+ C
S
C
S
~ 0,5 C
M unshielded cable
C
S
~ 2 C
M shielded cable e.g. Belden 1700A TP: 78,7 Ω/km,
45,9 pF/m

RS232 - Communication protocol
Log. 0 : +5 to +15 V
Log. 1 : -15 to -5 V start
0 0 0 data: 11010101b (213dec)
(parity)
•
• Odd
Even
0 idle
1 b0 b1
LSB
1 1 1 1 b2 b3 b4 b5 b6 b7
MSB
1 stop
1,1.5,2 idle

RS232 - Asynchronous communication
Fixed comm. speed: tx + rx same (tolerance ~3%)
RX TX start
0 0 0 0
(parity) idle
1 b0 b1
1 1 1 1 b2 b3 b4 b5 b6 b7
1 stop idle

RS232 - Communication speed
RS-232-F limits to 30 V/µs, max 4% of bit time => max. theoretical speed 200 kbit/s 0
ΔV
1 1
Δt

RS232 - Communication speed
RS-232-F standard defines speeds:
50,75,110,150,300,600,1200,2400,4800,9600,19200 bit/s
Common speeds above standard definition:
28800, 38400, 57600, 115200 bit/s
Higher speed
= lower distance
Baud rate [Bd] Max length [ft] Max length [m]
19 200
9 600
4 800
2 400 source: www.hw.cz
50
500
1 000
3 000
15
150
300
900

Dev. 1
RS232 - Signals
TxD RxD
RxD TxD
RTS
DCD
Request to send
Data Set Ready
CTS
CTS
DTR
Clear to send
Data terminal ready
RTS
Data Carrier Detect
DCD
DSR
DSR DTR
GND
Dev. 2

source: www.arcelect.com
RS232 - Connectors
9 pin

RS232 - Connectors 25 pin source: fjkraan.home.xs4all.nl
source: www.solentcables.co.uk

RS232 - Summary
• 1 transmitter, 1 receiver
• Common ground
• Typically 8 bit, no parity, 1 stop bit (8N1)

RS-422 (EIA-422)
• ANSI/TIA/EIA-422-B or ITU-T
Recommendation T-REC-V.11
• Uses differential signaling + GND
• 1 transmitter + 10 receivers
• 10 Mbit/s (12 m), 100 kbit/s (1200 m)
• Max. 1200 m
• Standard does not define protocol and pins

Differential signaling source: www.root.cz
source: www.root.cz

RS-422 source: meteosat.pessac.free.fr
source: www.scantec.de
Log. 0: V
A
– V
B
≥ +0.2 V
Log. 1: V
A
– V
B
≤ -0.2 V

RS-485
• ANSI/TIA/EIA-485 (1998)
• Uses differential signaling
• 32 transmitter + 32 receivers
• 10 Mbit/s (12 m), 100 kbit/s (1200 m)
• Max. 1200 m
• Standard does not define protocol and pins

RS-485 signaling
Source: www.consultants-online.co.za
Log. 0: V
A
– V
B
≥ +0.2 V
Log. 1: V
A
– V
B
≤ -0.2 V source: www.sealevel.com

RS-485 devices
Source: www.root.cz
Internally each node can have a transmitter and receiver, they are switched into high-impedance mode when not used

source: www.alciro.org
Half duplex

source: www.alciro.org
full duplex

source: hw.cz
Grounding

source: hw.cz
Maximal speed

source: hw.cz
RS-422 vs. RS-485

USB
• Universal serial bus (1995)
• Differential signaling (DATA + , DATA -) + power source: en.wikipedia.org
source: www.se7ensins.com

USB specifications
• Max. 127 devices
• Superspeed – 5 Gbit/s (USB 3.0 - 2008)
• High Speed - 480Mbits/s (USB 2.0 - 1999)
• Full Speed - 12Mbits/s
• Low Speed - 1.5Mbits/s

USB signaling
• Uses NRZI (Non Return to Zero Invert) signaling host
D+
D-
• Differential „1“:
D+ > 2.8 V, D- < 0.3V
• Differential „0“:
D- > 2.8 V, D+ < 0.3V
device
• Differential „1“:
D+ greater than D-
• Differential „0“:
D+ less than D-

source: www.tek.com
USB signaling

Speed identification
• Full speed device source: www.beyondlogic.org

Speed identification
• Full speed device source: www.beyondlogic.org

Non Return to Zero Invert signaling
• "One" is represented by a transition of the physical level.
• "Zero" has no transition.
source: en.wikipedia.org
source: www.jbmelectronics.com

Ethernet – physical layer
• Standard IEEE 802.3
• Max. length 100m
• 3 Mbit/s to 100 Gbit/s
• Differential signaling
• Signals TX+, TX-, RX+, RXsource: techpubs.sgi.com

Ethernet – physical layer source: hw-server.com
Logic levels (10BaseT - output)
• Log. 1 > +0.7 V
• Log. 0 < -0.7 V source: www.interfacebus.com
source: sigalrm.blogspot.com

Ethernet – Manchester encoding
• Encoding helps clock recovery source: en.wikipedia.org

Ethernet – physical layer source: flickr.com
Ethernet data, showing MLT-3 encoding of bits. (used in e.g. 100BASE-TX)

Ethernet – Data Link Layer - frame synchronization source: communities.netapp.com

Ethernet – Data Link Layer - frame addresses source: communities.netapp.com

Ethernet – Data Link Layer - frame source: communities.netapp.com
identifies what higher-level network protocol is being carried in the frame (example: TCP/IP)

Ethernet – Data Link Layer - frame source: communities.netapp.com
data

Ethernet – Data Link Layer - frame source: communities.netapp.com
Control checksum
Cyclic redundancy check