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StarLAN With The DP839EB
Evaluation Board
National Semiconductor
Application Note 498
Wesley Lee
OVERVIEW
Because of the identical packet structures between Star­
LAN (IEEE 802.3 1base5) and Ethernet (10base5), the
DP8390 Network Interface Controller (NIC) will operate in all
versions of IEEE 802.3 based networks. To evaluate the
DP8390 in StarLAN applications, the DP839EB Evaluation
Board can be used with a "daughter card" that replaces the
EthernetlCheapernet front end with a Star LAN front end.
The StarLAN front end consists of an RS-422/485 type
transceiver and a 1 Mbitl s Manchester encoder/decoder
(EN DEC), as shown below. The 82C550A, manufactured by
Chips and Technology, and the MK5035N, manufactured by
Mostek corporation, can provide the required ENDEC functions
for the NIC.
CABLING
Since a significant number of StarLAN networks are expected
to use existing twisted pair telephone wiring, DTEs will be
connected to wall outlets, which in turn, will be connected to
wiring closets where the StarLAN hubs will be located. The
cabling used typically will consist of 26-22 gauge, unshielded
twisted pairs with maximum cable length approximately
250 meters (800 tt) from Hub to DTE. If 5 levels of hub are
used, the network may extend up to 2.5 Km.
TRANSCEIVER
The transceiver connects to two twisted pair phone wires,
one for transmit, the other for receive and is isolated by two
pulse transformers. Some pulse transformers also provide
rise time limiting to reduce EMI. The transceiver circuitry is
based on the DS8923 dual receiver/driver combination.
Two of the receivers are used to provide receive and
squelch functions.
RECEIVER/SQUELCH
Since the cabling may be bundled together and routed close
to heavy electrical equipment, squelch circuitry is necessary
to reject signals generated from crosstalk between adjacent
wires and impulse noise from large equipment. Proper noise
immunity may be implemented using a second-order Butterworth
filter with a 2 MHz cutoff and setting a 600 mV
squelch level. Because RS-422 receivers typically have
200 mV threshold levels, these inputs must be skewed to
600 mV. This may be implemented by using a resistor ladder
which holds the inputs 600 mV apart (see Squelch Adjustment).
When an incoming signal exceeds the 600 mV
threshold, the receiver is enabled.
As shown in the Squelch Level Adjustment figure, two receivers
are used for the receive/squelch function. One receiver
sets the 600 mV input threshold and is used by the
ENDEC to drive its internal squelch circuitry; the other receiver
presents the actual unskewed data to be decoded.
TRANSMITTER
The transmitter is comprised of one RS-422 driver provided
in the DS8923 dual line driver-receiver package. The driver
is enabled using the external transceiver output of the Manchester
ENDEC, which is asserted coincident with the first
bit of valid data and is de-asserted two bit times following
the last bit. This allows generation of the 2-bit idle signal,
marking the end of the packet.
82C550A INTERFACE TO THE NIC
The 82C550A interfaces to the DP8390 via 5 inverters to
provide the proper polarity of CRS, COL, TXE, RXC, and
LBK. The normal mode (MODE = 1) is selected to allow an
external transceiver to be used. The squelch level input,
/RxDI must be connected to pin 1 of the DS8923 to attain
StarLAN Front End for the DP8390
Isolation
Transceiver Block
1 Mb/s
ENDEC
Controller
RX Pair
DP8390
NIC
TX Pair
TLlF/9S46-1
1-149