Contemporary Controls ARCNET SBX20 Series User manual

SBX20
ARCNET® Network Interface Module for Intel iSBX

Compatible
INSTALLATION GUIDE
INTRODUCTION
The SBX20 series of ARCNET network interface modules (NIMs) links
single-board computers (SBCs) with the ARCNET local area network.
ARCNET is classified as a token-bus local area network (LAN) operating at
2.5 Mbps while supporting 255 nodes. Interfacing ARCNET to a host
computer usually requires a NIM which plugs into the host computer’s bus.
In the case of single-board computers which have no bus, a NIM solution is
not possible. However, many SBCs have an on-board expansion slot which
conforms to Intel’s iSBX Bus Specification. Using the SBX20 ARCNET
NIM, SBCs can now be linked to the ARCNET LAN via the iSBX
expansion socket.
The SBX20 ARCNET NIM is a compact, low power CMOS design, which
conforms to the single-wide iSBX Multimodule Board Outline with
variation (2.85" x 3.70"; 72mm x 93mm). However, the coaxial and fiber
connectors extend beyond the board outline and raise the maximum
component height of the SBX20 ARCNET NIM to 0.500" (12mm) instead
of the specified 0.400" (10mm) height.
There are several versions of the SBX20 ARCNET NIM. The SBX20-CXS
supports coaxial star configurations requiring external active or passive
hubs. The SBX20-CXB supports a multidrop or coaxial bus configuration
usually requiring no hubs. Other versions include the SBX20-FOG
which supports fiber optic cable with either ST or SMA connectors. The
SBX20-TPB supports multidrop twisted-pair cabling. The SBX20-485
supports backplane mode EIA-485 cabling, while the SBX20-485D supports
non-backplane mode EIA-485. The SBX20-485X supports transformer
coupled EIA-485.

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SPECIFICATIONS
Environmental
Operating temperature: 0°C to +60°C
Storage temperature: -40°C to +85°C
Data Rates
2.5 Mbps, 1.25 Mbps, 625 kbps, 312.5 kbps, 156.25 kbps
Dimensions
2.85H" x 3.7"W x 0.5"D
(72mm x 93mm x 12mm)
Shipping Weight
1 lb. (.45kg)
iSBX Bus Specification
142686-002 dated 3/81
Power Requirements
Model +5V -12V
SBX20-CXS 200mA 20mA
SBX20-CXB 200mA 50mA
SBX20-FOG-SMA 300mA N/A
SBX20-FOG-ST 300mA N/A
SBX20-TPB 200mA 50mA
SBX20-485 200mA N/A
SBX20-485D 200mA N/A
SBX20-485X 200mA N/A

INSTALLATION
The SBX20 can be installed in any SBX compatible, 8 or
16-bit, expansion bus. To install the SBX20, remove the cover of the
computer exposing the motherboard and bus connectors. Care should be
taken when installing the SBX20 because both it and the exposed
microcomputer motherboard are sensitive to electrostatic discharge. To
prevent inadvertent damage, touch the metal case of the internal power
supply to discharge yourself then proceed to remove the SBX20 from its
protective ESD package. The SBX20 is shipped complete with a ½" round
nylon standoff. The standoff snaps into place on the solder side of the
SBX20 through a hole located near the center of the module. The SBX20
can then be inserted into the system by applying a downward even pressure
until it stops and is firmly seated into the connector and the standoff snaps
into place on the motherboard. Installation is completed by replacing the
computer’s cover.
Register Map
The SBX20 requires 16 contiguous I/O address locations in order to access
the COM20020 register and node ID switch. Because several locations are
reserved, it is important not to address another device to these locations.
The register map is shown in Table 1.
I/O Read Write
Address Register Register
Base + 0 Status Interrupt Mark
Base + 1 Diagnostic Status Command
Base + 2 Address Pointer High Address Pointer High
Base + 3 Address Pointer Low Address Pointer Low
Base + 4 Data Data
Base + 5 Reserved Reserved
Base + 6 Configuration Configuration
Base + 7 Test ID/..../Next ID Test ID/..../Next ID
Base + 8 Node ID Switch Reserved
Base + 9 Node ID Switch Reserved
Base + A Reserved Reserved
Base + B Reserved Reserved
Base + C Reserved Reserved
Base + D Reserved Reserved
Base + E Reserved Reserved
Base + F Reserved Reserved
Table 1. Register Map
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Interrupts
Both SBX interrupt request lines MINTR0 and MINTR1 are supported.
Interrupts can be invoked at jumper location E3. To enable an interrupt,
insert a jumper across a pair of posts corresponding to the desired interrupt.
Only one interrupt can be selected; therefore, only one jumper is supplied. If
no interrupts are desired, remove all jumpers at E3. The default interrupt
setting is MINTR0.
Indicator Lights
There is a dual LED located on the SBX20. The yellow LED indicates that
the SBX20 is being accessed via its I/O address. The green LED indicates
that the SBX20 is receiving ARCNET traffic from the network.
Node ID Switch
Although not always necessary with the COM20020, the SBX20 provides a
separate input port that reads an 8-bit DIP switch (SW1) located near the P1
connector. This switch is intended to serve as a node ID switch, although it
can serve as a general purpose switch if desired. The node ID switch has no
connection to the COM20020 ARCNET controller chip.
The most significant bit (MSB) is switch position 1, and the least significant
bit (LSB) is switch position 8. A switch in the open position (off position or
towards the edge of the printed circuit board) introduces a logic “1.” Figure
1 shows the node ID switch. In this example, the switch is set to
hexadecimal address AF.
Figure 1. Node ID Switch

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FIELD CONNECTIONS
The SBX20 is available in several transceiver options. Each transceiver,
which is matched to a particular cable type, is identified by a three-digit
suffix appended to the model number. The capabilities of each transceiver
differs.
-CXS Coaxial Star
There are generally two types of coaxial cables that are used with the -CXS
transceiver, RG-59/u and RG-62/u. RG-59/u is 75 ohm cable which does not
precisely match the impedance of the transceivers used on the NIM. This
cable will work, but communication distances are reduced compared to
RG-62/u because of the higher attenuation of RG-59/u cable. We
recommend RG-62/u because it is a better match to the transceivers and a
full 2000 foot segment distance can be achieved using this cable. Both
cables support male BNC connectors which the -CXS NIM accommodates.
In a two-node system, simply connect the two -CXS NIMs together using
RG-62/u coaxial cable. The length of cable cannot exceed 2000 feet.
If more than two NIMs are used on a network, either an active or passive
hub is required. With passive hubs, a maximum of four NIMs can be
interconnected. Unused ports on the passive hub must be terminated with a
93 ohm (nominal) resistor. The maximum length between a passive hub port
and a NIM is 100 feet.
Active hubs provide overall better performance than passive hubs since
greater distances can be achieved along with a degree of isolation. Connect
each NIM to a port on the hub using RG-62/u coaxial cable. This length of
cable cannot exceed 2000 feet nor can the length of cable between two
cascaded hubs exceed 2000 feet. However, up to ten hubs can be cascaded
thereby providing an overall cable length of 22,000 feet. Unused ports on
active hubs need not be terminated.
Figure 2. Bus segments can be extended
through active hubs.

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-CXB Coaxial Bus
For hubless systems, the -CXB transceiver can be used. NIMs are
interconnected with RG-62/u cables and BNC Tee connectors. Each -CXB
NIM represents a high impedance connection in both the powered and
unpowered states. Therefore, passive termination must be applied to both
ends of a bus segment. Use BNC style 93 (nominal) ohm resistors at each
end. The maximum segment length is 1000 feet and the maximum number
of NIMs that can be connected to a segment is eight.
To extend a bus segment beyond 1000 feet, an active hub is required. If the
hub port is of the -CXS type, connection can be made if a few simple rules
are followed. Only connect the hub at the end of a segment. Do not connect
the hub to the middle of a segment since the hub port is not of the high
impedance type. Do not terminate the end which attaches to the hub port
since a -CXS port effectively terminates the end of a bus segment. Simply
remove the BNC Tee connector and terminator from the segment end and
attach the cable directly to the hub port. The opposite segment end still
requires termination if no hub connection is being made.
Figure 3. Active hubs can be cascaded
for greater distances.
Passive hubs cannot be used with -CXB NIMs and we only recommend the
use of RG-62/u coaxial cable for segments. Contemporary Controls (CC)
also supplies BNC terminators and Tee connectors. Ask for BNC-TER and
BNC-T respectively. One note of caution, there is a minimum cable length
between nodes (6 feet). Do not violate this specification since unreliable
operation may result.
-FOG Fiber Optics
Fiber optic cable is typically available in three sizes, 50/125, 62.5/125, and
100/140. With this size fiber, multimode operation will be experienced
requiring the use of graded index fiber. The larger the size, the more energy
that can be launched and, therefore, the greater the distance. CC’s
technology operates over the 850 nm range. Two connectors are supported:
the SMA and the ST. The conventional SMA connector offers a threaded

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termination. When this type of connection is tightened excessively, the fiber
optic cable can break resulting in unreliable communications. Also, the
degree of tightening affects the optimum throughput of the cable and causes
attenuation. Depending upon the degree
of tightening, fiber alignment can vary
from the optimal coupling, thereby
introducing some attenuation. The ST
connector alleviates both of these
problems but is generally more expensive
than the SMA connector. The ST provides
a bayonet style connector similar in
operation to a BNC coaxial cable
connector.
Fiber optic connections require a duplex
cable arrangement. Only star and
distributed star topologies are
supported. Two unidirectional cable paths provide the duplex link. There
are two devices on each NIM. One device, colored light gray, is the
transmitter and the other, dark gray, is the receiver. Remember that “light
goes out of the light (gray).” To establish a working link between a NIM and
another NIM or a hub to a NIM, the transmitter of point A must be
connected to a receiver at point B. Correspondingly, the receiver at point A
must be connected to a transmitter at point B. This establishes the duplex
link which is actually two simplex links. Fiber optic cable is available
paired for this purpose. Usually the manufacturers’ labeling is only on one
cable of the pair which is handy for identifying which of the two cables is
which. Establish your own protocol for connecting cable between hubs and
NIMs in the field using the manufacturers’ labeling as a guide. However,
remember that to connect point A to point B requires a paired fiber optic
cable and that the light gray connector at one point must connect to a dark
gray connector at the other point.
Optical Power Budget
When specifying a fiber optic installation, attention must be paid to the
available optical power
budget. The power budget is
the ratio of the light source
strength divided by the light
receiver sensitivity
expressed in dB. This value
must be compared to the
link loss budget which is
based upon the optical cable
and optical connectors. The
link loss budget must be less than the power budget. The difference is called
the power margin which provides an indication of system robustness.
Figure 4. Fiber Optic
Option (-FOG)
Table 2. The power budget varies with
the fiber core size.

Transmitter power is typically measured at one meter of cable and,
therefore, includes the loss due to at least one connector. The outputs vary
so CC tests each device to ensure that a minimum output power is achieved.
The output power also varies with core sizes. In general, larger cores launch
more energy.
Receiver sensitivity also varies so again CC tests for the least sensitive
receiver. The difference between the weakest transmitter and least sensitive
receiver is the worst case power budget which CC specifies. Realized power
budgets will exceed this value since the probability of the worst case
transmitter being matched with the worst case receiver is remote. However,
CC recommends using the stated power budgets for each core size.
Link Loss Budget
Fiber optic cable attenuation is usually specified by the cable manufacturer.
Use this figure to determine the maximum distance of the fiber link. It is
necessary to include losses due to cable terminations. Connectors usually
create a loss of from 0.5 to 1 dB. For example, assume a 1500 meter run of
62.5 cable which the cable manufacturer specifies as having a cable
attenuation of 3.5 dB per 1000 meters. The cable loss will be 5.25 dB.
Assuming two connector losses of 0.5 dB each, the link loss budget would
be 6.25 dB which is within the 10.4 dB power budget specified by CC. The
5.15 dB difference represents a high degree of margin. A 3 dB margin is
what is typically recommended.
Overdrive
Overdrive occurs when too little fiber optic cable is used resulting in
insufficient attenuation. To correct this condition, a jumper can be removed
in each fiber optic transceiver to reduce the gain sufficiently to allow for a
zero length of fiber optic cable to be installed between a transmitter and
receiver. This is potentially a problem with 100 micron cable. By removing
the jumper at E1, a 2 dB reduction in output power is achieved.
-TPB Twisted-Pair Bus
The -CXB transceiver can be modified to drive a balanced cable system
with the addition of some parts. This configuration is called -TPB and it
supports unshielded twisted-pair cable such as IBM type 3. Dual RJ-11
connectors replace the single BNC connector in order to support the popular
modular plug connectors. For convenience, a four position screw terminal
connector is also provided. Follow the connector pin assignments in Tables
4 and 5 when using this connector or when mixing cable types. Wiring
between NIMs is accomplished in a daisy-chain fashion with point-to-point
cables connecting the various NIMs to create a bus segment. The end NIMs
will have one vacant RJ-11 socket which is to hold the RJ-11 style 100 ohm
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Figure 5. TPB NIMs are connected in a daisy-chain fashion with
terminators inserted at both end NIMs.
terminator required to terminate the end points of the bus segment. When
terminating the screw terminal connector, install a 100 ohm, ¼ watt resistor
across the unused terminals. Use twisted-pair cable and observe polarity.
Modular plugs must be installed on this cable such that they do not invert
the signals. Most satin cable does not twist the pairs nor maintain signal
polarity. Do not use this cable. To test for the proper cable connections, hold
both ends of the cable side by side with the retaining clips facing the same
direction. The color of the wire in the right-
most position of each plug must be the same
if there is no inversion of the cable. If this
is not the case, the cable is inverted. Up to
eight -TPB NIMs can be connected to one
segment which cannot exceed 400 feet in
length.
The overall distance of a twisted-pair
network can be expanded beyond 400 feet if
hubs are used. Use a hub port that supports
a balanced twisted-pair signal (-TPS) or
use a BALUN. CC recommends a Mux
Labs
BALUN
available
from CC (under the part number
BALUN) connected to a -CXS port on a
MOD HUB expansion module. The
BALUN converts the balanced twisted-
pair signals to single-ended signals
suitable to the -CXS port. A more direct
approach is to use a MOD HUB
expansion module with a -TPS port. The
-TPS port has an internal BALUN and
provides an identical RJ-11 connection as
the BALUN. Unfortunately, the signal
sense is inverted from the -TPB module so
that an inverted cable connection is
Figure 6. Modular Jack
Numbering Orientation
1 2 3 4
Figure 7. Screw Terminal
Connector Numbering
Orientation

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required to either the BALUN or -TPB port. Connect either of these devices
to one of the end NIMs on the segment. This requires the removal of the
terminator at that end. The BALUN or -TPS port provides the termination.
-485 DC Coupled EIA-485 Backplane Mode
The SBX20-485 supports DC-coupled, EIA-485, backplane mode
communications. One four-position screw terminal connector and two RJ-11
connectors are supplied on each NIM which are bussed together so as to
provide a convenient daisy-chain connection
and cabling type flexibility. Refer to Figures
6 and 7 for connector pin assignments. The
recommended cable for EIA-485
communication is IBM Type 3, unshielded
twisted-pair with a maximum segment
length of 900 feet. Up to 17 NIMs are
supported on each 900 foot segment. The
COM20020’s unique backplane mode of
operation is supported with this SBX20
transceiver option.
Termination
Each end of an EIA-485 bus segment must
be terminated with the characteristic cable impedance. A terminating
resistor is provided and can be invoked by installing the E4 jumper on post 1
and 2. Refer to Figure 8. To operate the SBX20-485 unterminated (in the
middle of a bus segment) simply install the E4 jumper on posts 2 and 3.
Bias
In addition to the termination, it is also necessary to apply bias to the
twisted-pair network so that when the line is floated differential receivers
will not assume an invalid logic state. The SBX20-485 utilizes a distributed
biasing scheme where the total bias is comprised of bias networks on each
module. The degree of bias is dependent on the number of nodes on the
twisted-pair network. There are two
groups of precision bias resistors (Rb) of
equal value on each board. One resistor
group is tied to the +5V line while the
other is tied to ground. Each resistor has
a jumper associated with it. When
jumpers are installed, the resistors tied to
+5V are connected to the (+) signal line
while the grounded resistors are
connected to the (-) line. This voltage
drop will bias the differential receivers
into the “1” state when no differential
Table 3. Backplane Mode,
DC-Coupled EIA-485
Option (-485)
Jumper
E2/E5 # of Nodes
1-2, 3-4, 5-6 2-5
3-4, 5-6 6-15
5-6 16-30
Figure 8. Backplane
Mode, DC-Coupled
EIA-485 Option (-485)