Arctic RS-485 / RS-422
RS- 485
• Also known as RS-485 Half Duplex, RS-485 2-wire
• same pair is used to transmit and receive data
• only one device can transmit on the bus simultaneously
because the bus is shared
• in theory there could be multiple masters and also
slaves could speak with each other but typically used on
”single master – multiple slaves” setup communicating
with request-reply type of protocol
MASTER
+
SLAVE
+
-
SLAVE
+
-
SLAVE
+
-
B
A
REQUEST
REPLY
RS- 422
• Also known as RS-485 Full Duplex, RS-485 4-wire
• separate pairs for transmit and reception
• only one device can command the bus, slaves can’t speak
with each other
• device can transmit and receive simultaneously (full duplex)
• typically used on ”single master – multiple slaves” setup
MASTER
TX
+
RX
-
+
SLAVE
TX
-
+
REPLY
REQUEST
SLAVE
RX
-
+
TX
-
+
RX
-
+
-
Voltages
• RS-485 and RS-422 use differential (balanced)
signalling providing good noise immunity
• Generally non-inverting line is called positive, +, A or Y
• Generally inverting line is called negative,-,B or Z
• If voltage between A-B (Vdiff) is >200mV receiver
detects state ”1” and if A-B is below -200mV receiver
detects state ”0”
• Usually transmitters drive >1,5V for state ”1” and <-1.5V
for state ”0” leaving good margin to 200 mV trigger level
A
Vdiff = A-B
B
Loading
• Each device causes capacitive and resistive load to bus
• If the load is too big the minimum ”200 mV” signal
difference may not be reached or the signal waveform
becomes too disturbed to detect the start bit
• Also termination and biasing loads the bus
• Also the length of the cable and serial speed affects the
performance
• Because the implementation between devices vary a lot
(e.g. some has internal termination) the only sure way is
to measure the differential waveform (at both ends of the
bus) with oscilloscope
• Note that the RS-485 has both ”transmit” and ”receive”
phases, both have to exceed the 200 mV limit
Grounding
• Allthrough data transmission is balanced grounding may
be required if the distance between devices is long
and/or they don’t othervise share the common ground
• The most common reason for RS-485/422 circuit
damages is the excessive potential difference between
the devices
• When separate grounding is required use 100R resistor
to connect each remote device to common ground wire.
The resistor should be rated for at least 0.5 Watts.
MASTER
SLAVE
+
SLAVE
SLAVE
+
-
+
-
+
-
GND
B
A
Termination
• Usually short lines do not require termination
• Usually small data rates (<115200 bps) do not require
termination
• The purpose of the termination is to cancel/attenuate the
reflections on the bus  the value of termination resistor
should match the characteristic impedance (Z0) of the
cable
• Typically twisted-pair type of cables have Z0 100-120 
(e.g. CAT-5 has 100 )
• Termination is most effective on the receiving end
• Note that the termination causes plenty of load to the
bus, increases idle state power consumption and should
be used only when required
• ”AC termination” does not increase idle state power
consumption but is not as effective as pure resistive
termination
Termination placement
MASTER
+
SLAVE
+
RS-485
SLAVE
+
-
-
SLAVE
+
-
MASTER
TX
RS-422
+
RX
-
+
SLAVE
-
TX
+
SLAVE
RX
-
+
TX
-
+
RX
-
+
-
Arctic Termination
• Arctic internal termination is ”AC termination”
• The termination is available between RS2 pins 2 and 8
(RS-422 receive pins) when DIP switch 4 is ”ON”
• If termination is required on RS-485 (2-wire) mode pins
2-7 and 3-8 must be connected manually
PIN 2
DIP 4
10 nF
120R
PIN 8
Biasing
• When the tranmitters are not active the line may float
causing e.g. detection of false start bits
• The purpose of biasing is to tie the bus to known state
when idle
• Most modern RS-422/485 receivers have internal ”failsafe” biasing
• However if termination is used the internal ”fail-safe”
biasing may not be enough and external biasing is
required
• Placement of biasing is not that sensitive as termination
A
> 200 mV when idle
B
Arctic Biasing
• Arctic biasing uses 3.3V voltage and 560R resistors
• The biasing is available between RS2 pins 2 and 8
(RS-422 RX pins) when DIP switchs 1 and 3 are ”ON”
• If biasing is required on RS-485 (2-wire) mode pins 27 and 3-8 must be connected manually
3V3
DIP 1
DIP 3
560R
PIN 2
PIN 8
560R
Arctic DIP Switches and RS2
pinout
•
DIP switches control the operation of RS2
Number
Function
State
Explanation
1
RS-232/RS-485
0=RS-232, 1=RS-485
Selects RS-port operation
2
FULL/HALF
0=full, 1=half
Selects between full-duplex (4-wire) and
half-duplex (2-wire) RS-485 operation
3
BIAS
0=off, 1=on
RS-485 biasing
4
TERMINATION
0=off, 1=on
RS-485 termination
•
RS2 pinout (standard male DB-9)
•
RS2 in RS-422/485 mode
Pin
1
2
3
4
5
6
7
8
9
RS-485 FD (4-wire) RS-485 HD (2-wire)
RXD+ (in)
TXD- (out)
TXD/RXD- (out/in)
GND
GND
TXD+ (out)
RXD- (in)
TXD/RDX+ (out/in)
Arctic RS- 485 howto
•
•
•
•
•
•
•
Set DIP switches 1 and 2 ”ON” which causes RS2 to be on RS-485
(Half-Duplex) mode
If biasing is required set DIP switch 3 to ”ON” and manually connect
Arctic RS2 pins 2-7 and 3-8 together
If termination is required set DIP switch 4 to ”ON” and manually
connect Arctic RS2 pins 2-7 and 3-8 together
Connect Arctic pin 7 (TXD/RXD+) to other device non-inverting pin
(usually marked positive,TXD/RXD+,+,A or Y)
Connect Arctic pin 3 (TXD/RXD-) to other device inverting pin (usually
marked negative,TXD/RXD-,-,B or Z)
Connect Arctic pin 5 (GND) to common ground wire (with 100R
resistor in series if the distanse is long and devices do not othervise
share common ground)
NOTE! When using RS-422/RS-485 Arctic RS2 handshaking must be
set to ”none” in application configuration
ARCTIC
DEVICE
2
7
+/A/Y
3
-/B/Z
8
Arctic RS- 422 howto
•
•
•
•
•
•
•
•
•
Set DIP switch 1 ”ON” and DIP switch 2 ”OFF” which causes RS2 to be on
RS-422 (Full-Duplex) mode
If biasing is required set DIP switch 3 to ”ON”
If termination is required set DIP switch 4 to ”ON”
Connect Arctic pin 7 (TXD+, output) to other device non-inverting receiver
pin (input, usually marked RX+)
Connect Arctic pin 3 (TXD-,output) to other device inverting receiver pin
(input, usually marked RX-)
Connect Arctic pin 2 (RXD+, input) to other device non-inverting transmitter
pin (output, usually marked TX+)
Connect Arctic pin 8 (RXD-,input) to other device inverting transmitter pin
(output, usually marked TX-)
Connect Arctic pin 5 (GND) to common GND wire (with 100R resistor in
series if the distanse is long and devices do not share common ground)
NOTE! When using RS-422/RS-485 Arctic RS2 handshaking must be set to
”none” in application configuration
ARCTIC
DEVICE
7
RX+
3
RX-
2
TX+
TRANSMITTER
RECEIVER
RECEIVER
TRANSMITTER
8
TX-