Document No. 485OFS1200 - pg. 1/3
B&B Electronics March 2000
This product designed and manufactured in USA of domestic and imported parts by
S u sp e ct N o d e
Tru s te d N o d e
4 8 5 O F S
4 8 5 O F S
S u sp e ct N o d e
R S -4 8 5 N e tw o rk
Figure 1. Typical Setup
RS-485 to RS-485 Optical Isolator with
Fail-Safe Protection
CE
CE
CE
CE
Model 485OFS
Protect Your System From:
Shorted
Nodes
Open
Connections
Damaged
Transceivers
Power Loss on a Node
Software
Lock-ups
Ground
Loops
Power Supply Voltage Surges
Transient Voltage Spikes
Description:
The 485OFS prevents a single damaged or locked-
up RS-485 device from taking down the whole network.
It protects your RS-485 network from most typical
failures. Any device without valid data is automatically
removed from the network. The 485OFS fully isolates
data signals and ground. A typical setup using the
485OFS is shown in Figure 1.
The 485OFS works with both two wire half duplex
and four wire full duplex systems. Driver enable control
is automatic with B&B's Send Data Control circuit. The
485OFS uses terminal blocks on each side of the
device. It supports Transmit Data (A) and (B), Receive
Data (A) and (B), Protective Ground, and Signal
Ground.
Connection:
To connect the 485OFS to a four wire system, connect the outputs of the device to the inputs of the 485OFS and vice-versa as shown
in Figure 2. To connect the 485OFS to a two wire system, the transmitter and receiver on each side must be tied together. Refer to Figure
3 for connection to a two wire system.
Proper operation of any RS-485 system requires the presence of a signal return path. The 485OFS isolates the signal grounds on
either side of the unit from each other, so both sides must be tied to their respective signal ground lines. The RS-485 Standard
recommends that a separate internal conductor be used for this instead of the shield. To protect the RS-485 network from transient
suppression generated at the node, the protective ground (PG) terminal must be tied to a good frame (chassis, green wire, or earth)
ground. The Protective Ground on the network side of the 485OFS should not be tied to earth ground. The RS-485 network signal ground
should be tied to earth ground at one point. Refer to B&B's RS-422/485 Application Note for more information on network design and
grounding.
No wire type or maximum run length is listed in the RS-485 Standard. However, the RS-422 Standard, which is very similar,
recommends number 24AWG twisted pair telephone cable with a shunt capacitance of 16 picofarads per foot and no more than 4000 feet
of distance.
S IG N A L G N D 1
D ATA O U T (A ) -
D ATA O U T (B ) +
4 8 5 O F S
P R O T. G N D 1
D ATA IN (A ) -
D ATA IN (B ) +
+ 1 2 V D C
G N D
W H I T E M A R K E D W I R E
S O L ID B L A C K W IR E
S IG N A L G N D
T D B ( + )
T D A (- )
R D A ( -)
R D B ( + )
Earth G round Connection
o n N o d e S ide O N LY
S U S P E C T
R S - 4 8 5 N O D E
S IG N A L G N D 2
D ATA IN (A ) -
D ATA IN (B ) +
P R O T. G N D 2
D ATA O U T (A ) -
D ATA O U T (B ) +
S IG N A L G N D
R D B ( + )
R D A ( -)
T D A (- )
T D B ( + )
Earth G round connection
should be m ade at one
location on netw ork side
P R O T E C T E D
R S - 4 8 5 B U S
Figure 2. Four Wire Connections
Document No. 485OFS1200 - pg. 2/3
B&B Electronics March 2000
This product designed and manufactured in USA of domestic and imported parts by
Table 1.
Approximate Timeout Values
Baud
Rate
Time
(ms)
Resistor
R17 & R25
(Ohm)
Capacitor
C6 & C9
(
F)
Jumper
Position
JP2 & JP4
300
36.3
330K
0.1
-
600
17.6
160K
0.1
-
1200
9.0
820K
0.01
-
2400
4.73
430K
0.01
A
4800
2.42
220K
0.01
B
9600
1.43
130K
0.01
C
19.2K
0.62
56K
0.01
D
38.4K
0.30
27K
0.01
E
57.6K
0.18
16K
0.01
-
115.2K
0.090
8.2K
0.01
-
230.4K
0.047
4.3K
0.01
-
460.8K
0.024
2.2K
0.01
-
Note: Baud rates greater than 38.4 kbps will not require a resistor
change unless turn-around time is less than 0.30 ms in a half-duplex
system.
Timeout = 1.1 x R x C
Where R is in Ohms and C is in Farads
All timeouts +/- 20%
Operation:
When no data is being transmitted through the 485OFS, the receivers are enabled on both sides of the device. As data is received on
one side of the 485OFS, the opposite driver is enabled and the data is passed through. If the 485OFS does not receive a data transition
for at least one character time, the driver is disabled. This timeout period is factory preset for one millisecond to accommodate a baud rate
of 9,600 bits per second. The timeout period can be changed to any value between 0.26 and 4.16 milliseconds by removing the cover and
moving the jumpers JP2 and JP4 inside the 485OFS.
The preset jumper settings available on the 485OFS should accommodate almost all systems, but a different timeout can be achieved
by putting JP2 and JP4 in position E and changing the values of the capacitors C6 and C9 and resistors R17 and R25. Spaces for
through-hole replacement of these components are also provided. They are labeled C15, C16, R26, and R27. The jumper positions for
various baud rates, as well as the resistor and capacitor values for time-out periods beyond this range are given in Table 1. See Figure 4
and Figure 5 for the location of the timing components.
Jumpers JP1 and JP3 determine whether the receivers will be disabled when transmitting (half-duplex) or always enabled (full-
duplex). As a general rule, JP1 and JP3 should be in the half-duplex position for two-wire operation and in the full-duplex position for four-
wire systems. See Figure 4 for the location of jumpers JP1-JP4 on the PC board.
S IG N A L G N D 1
D ATA O U T (A ) -
D ATA O U T (B ) +
4 8 5 O F S
P R O T. G N D 1
D ATA IN (A ) -
D ATA IN (B ) +
+ 1 2 V D C
G N D
W H I T E M A R K E D W I R E
S O L ID B L A C K W IR E
S I G N A L G N D
T X /R X A ( - )
T X /R X B ( + )
Earth G round C onnection
o n N o d e S ide O N LY
S U S P E C T
R S - 4 8 5 N O D E
S IG N A L G N D 2
D ATA IN (A ) -
D ATA IN (B ) +
P R O T. G N D 2
D ATA O U T (A ) -
D ATA O U T (B ) +
Earth G round connection
should b e m ade at one
location on netw ork side
P R O T E C T E D
R S - 4 8 5 B U S
S I G N A L G N D
T X /R X A ( - )
T X /R X B ( + )
Figure 3. Two Wire Connections
Figure 4. PCBD Top View
Figure 5. PCBD Bottom View
Document No. 485OFS1200 - pg. 3/3
B&B Electronics March 2000
This product designed and manufactured in USA of domestic and imported parts by
Specifications:
Isolation:
2.000 volts RMS for 1 min. optical isolation of data
lines and ground
Surge Suppression:
6.5V working peak voltage, bi-directional
over voltage suppressor
600W peak power dissipation
3,000 pF maximum capacitance
Data Rates:
Up to 460.8 kbps
Temperature Rating:
0
C to +70
C
Power Requirements:
9-14 VDC @ 60mA (idle state),
170mA (Full-duplex data with 62 Ohm load)
Dimensions:
3.8 x 2.4 x 1.0 in (9.7 x 6.1 x 2.5 cm)
FCC Approved Class A
FIGURE 6. Circuit Diagram
DECLARATION OF CONFORMITY
Manufacturer's Name:
B&B Electronics Manufacturing Company
Manufacturer's Address:
P.O. Box 1040
707 Dayton Road
Ottawa, IL 61350 USA
Model Number:
485OFS
Description:
RS-485 Optical Isolator/Repeater
Type:
Light industrial ITE equipment
Application of Council Directive:
89/336/EEC
Standards:
EN 50082-1 (IEC 801-2, IEC 801-3, IEC 801-4)
EN 50081-1 (EN 55022)
Michael J. Fahrion, Director of Engineering
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