DN85 - Interfacing to Apple LocalTalk ® Networks

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Interfacing to Apple LocalTalk ® Networks
Design Note 85
Robert Reay
external –5V supply. The bipolar chips also draw large
supply currents, making them undesirable for batterypowered applications. A better solution is to use the
LTC1323CS24 which provides a complete low power
serial I/O interface while generating it’s own –5V supply
as shown in Figure 1.
LocalTalk Overview
Of the many connection technologies available for linking an AppleTalk ® network, LocalTalk is one of the most
common because it is designed to connect local work
groups using inexpensive and easily configurable plugand-play cabling. LocalTalk is laid out in a bus topology
with all devices joined in a line with no circular connections. LocalTalk conforms to the EIA RS422 electrical
standard to provide a balanced differential voltage signal transmitting at 230.4kbs over a maximum distance
of 300 meters with up to 32 devices connected to a
twisted-pair network. The balanced configuration provides good isolation from ground noise currents and is
not susceptible to fluctuating potentials between system grounds or common-mode electromagnetic interference (EMI).
The LTC1323 uses the differential driver output stage of
Figure 2. The driver swings between ground and VCC
when a differential LocalTalk load is connected between
TXD + and TXD – while meeting the EIA RS422 differential voltage swing requirement of ±2.0V into 100Ω.
When a single-ended load is connected from TXD + to
ground or TXD – to ground, the driver will swing between VEE and VCC while meeting the EIA 562 voltage
swing requirement of ±3.7V into 3kΩ. Because the
differential LocalTalk load requires a large current drive,
the charge pump that generates VEE would be unreasonably large if the driver were to swing to VEE with a
differential load. By limiting the current flowing into VEE
to 15mA, the charge pump can easily be integrated into
the chip.
LTC1323
The original LocalTalk hardware design uses an
AM26LS32 chip for the receivers and an AM26LS30
chip for the drivers of a LocalTalk port. The drawback of
the design is that it requires two chips per port and an
LTC1323
0.33µF
2
CHARGE PUMP
CPEN 3
TXD 4
21
19 TXD +
DX
RXEN 8
17 RXI
RX
RX
16 RXI
15 RXD –
14 RXD +
RXDO 11
12
5Ω TO 10Ω
1µF
20 TXD –
18 TXO
RXO 10
5Ω TO 10Ω
EMI FILTER =
0.33µF
100pF
DX
S/D 7
RXO 9
1µF
23
22
TXI 5
TXDEN 6
5V
24
1
RX
EMI FILTER
EMI FILTER
EMI FILTER
EMI FILTER
8
7
6
EMI FILTER
5
4
3
EMI FILTER
2
1
EMI FILTER
13
DN85 FIG.1
Figure 1. Single Supply LocalTalk Port
AppleTalk and LocalTalk are registered trademarks of Apple Computer, Inc.
09/15/94/85
and LTC are registered trademarks and LT is a trademark of Linear Technology Corporation.
For applications where the single-ended control signals
are not required, such as PhoneNet ®, the LTC1323CS16
can be used as shown in Figure 3. The differential driver
is still able to drive a single-ended load to VEE when not
connected to a LocalTalk network.
EMI Filter
Most LocalTalk applications use an electromagnetic interference (EMI) filter consisting of a resistor-capacitor T
network between each driver and receiver and the connector (Figure 5). Unfortunately, the resistors significantly
attenuate the driver’s signal applied to the cable. Because
LTC491
the LTC1323 and LTC491 are single supply drivers, the
For PhoneNet-type applications where the differential driver
resistor values should be reduced to 5Ω to 10Ω to insure
is not required to drive a single-ended load below ground,
enough voltage swing on the cable. In most applications,
the LTC491 can be used as shown in Figure 4. Only RS485
removing the resistors completely does not cause an
drivers will work in this application. RS422 drivers will not
increase in EMI as long as a shielded connector and cable
work because they load the cable when the chip power is are used. With the resistors removed, the only DC load
removed.
becomes the primary of the LocalTalk transformer. This
will increase the DC standby current when the drivers are
active, but does not adversely affect the drivers because
they can handle a direct short circuit indefinitely. For
VCC
maximum swing and EMI immunity, a ferrite bead and
capacitor could be used.
D1
D4
P1
P2
TXD+
D2
LTC491
D3
D6
D5
RXDO
RXEN
N1
5V
0.1µF
14
TXD–
N2
N3
N4
2
12
11
3
EMI FILTER
4
TXD
5
10
EMI FILTER
RXD +
RXD –
8
7
5
4
2
9
6
3
1
6
15mA
7
TXD –
DN85 FIG.2
TXD +
VEE
Figure 2. LTC1323 Differential Drive Output Stage
LTC1323CS16
0.33µF
2
5V
CHARGE PUMP
1µF
5Ω TO 10Ω
5Ω TO 10Ω
15
14
4
Figure 4. PhoneNet Application
16
1
DN85 FIG.4
100pF
0.33µF
1µF
13
TXDEN
TXD
S/D
RXEN
RXDO
3
12
11
5
6
7
8
9
10
EMI FILTER
EMI FILTER
EMI FILTER
EMI FILTER
RXD –
RXD +
TXD –
TXD +
100pF
8
7
6
5
4
3
2
1
FERRITE BEAD
FERRITE BEAD
100pF
DN85 FIG.5
DN85 FIG.3
Figure 3. PhoneNet Application
PhoneNet is a registered trademark of Farallon, Inc.
Linear Technology Corporation
Figure 5. EMI Filters
For literature on our Interface Products,
call 1-800-4-LINEAR. For applications help,
call (408) 432-1900, Ext. 453
LT/GP 0994 180K • PRINTED IN THE USA
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 LINEAR TECHNOLOGY CORPORATION 1994