DEMO MANUAL DC1851A
LTC2872
Dual Multiprotocol Transceiver
with Integrated Termination
DESCRIPTION
Demonstration circuit 1851A showcases the LTC
®
2872
RS232/RS485 dual multiprotocol transceiver with inte-
grated termination. Separate supply inputs power the
interface and logic sections, permitting operation of the
logic control and I/O down to 1.7V. LEDs indicate when
supplies are present. Control pins are accessible via tur-
rets for external connections, or they may be set high or
low by means of a jumper. All logic I/O and interface I/O
is routed to turrets. The RS485 interface lines are also
available at a terminal block for convenient connection of
a transmission line. Because the LTC2872 contains half-/
full-duplex switching, it is unnecessary to jumper A and
B to Y and Z for half-duplex operation.
Design files for this circuit board are available at
http://www.linear.com/demo
L,
LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
PERFORMANCE SUMMARY
PARAMETER
Input Supply
Maximum Data Rate
CONDITIONS
V
CC
Operating
V
L
Operating
RS232
RS485
Specifications are at T
A
= 25°C
MIN
3.0
1.7
500
20
TYP
MAX
5.5
V
CC
UNITS
V
V
kbps
Mbps
dc1851af
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DEMO MANUAL DC1851A
QUICK START PROCEDURE
Overview
The LTC2872 dual multiprotocol transceiver contains two
identical channels, each comprising two RS232 drivers,
two RS232 receivers, and one RS485 driver and receiver.
The two channels are largely independent, sharing only
loopback and duplex control and a split supply generator.
Operating power is derived from the V
CC
input, while a
separate logic I/O supply pin (V
L
) permits the LTC2872
to adapt to any logic level ranging from 1.7V up to V
CC
.
Another key feature is that RS232 and RS485 inputs and
outputs may be interconnected without regard to loading
or voltage limitations. Allowing that only one interface
type is enabled at any given moment, input and output
characteristics of the selected standard are preserved
without violating the limits of the disabled sections. The
LTC2872 also includes selectable RS485 terminations, as
well as switching for half- and full-duplex modes, permit-
ting full digital control and eliminating the need for external
jumpers or relays to change between modes.
DC1851A includes:
• Bananas and turrets for supply connections along the
upper edge of the board.
• Green LEDs to indicate the presence of V
CC
and V
L
. Note
that the V
L
LED, D1, unavoidably dims and extinguishes
at V
L
< 2V.
• Turrets for logic I/O along the left edge of the board.
• Turrets for RS232 and RS485 interface connections
along the right edge of the board.
• Terminal blocks J5 and J6 for RS485 connections.
• Turrets and jumpers for selecting modes along bot-
tom edge of board. There are no pull-up or pull-down
resistors on the control lines; known states must be
asserted by jumpers or by applying a definitive logic
level to the turrets.
• 10k pull-up resistors on driver inputs DY1, DZ1, DY2
and DZ2. They assert a known logic-1 state when these
turrets are left open circuit.
In summary, DC1851A features a turret for each and every
LTC2872 logic, interface and supply pin. In addition the
supply pins have banana jacks, and the control pins are
connected to jumper blocks to establish their states.
Components
Components on DC1851A are divided into four basic
groups:
• R1-R11: 100Ω resistors included in series with the con-
trol signal turrets to limit fault current arising from any
accidental misadventures with the turrets or jumpers.
• D1, D2, R16, R17: V
CC
and V
L
supply indicator LEDs
and current limiting resistors.
• R12-R15: 10kΩ pull-ups to V
L
on DY1, DZ1, DY2 and
DZ2.
• C1-C5, L1: supply generation and bypassing compo-
nents for the LTC2872 (U1).
LTC2872 Operation
Each channel of the LTC2872 contains two RS232 drivers,
two RS232 receivers, one RS485 driver and one RS485
receiver. The 485/232 control lines enable either the RS485
or RS232 elements for each channel.
The RS485 transceiver includes a special half-/full-duplex
switching feature. In full-duplex mode, the receiver inputs
are connected to the A and B pins, while the driver is
connected to Y and Z. In half-duplex mode, the receiver
inputs are moved to Y and Z so that there is no need to
jumper A-B and Y-Z when switching between half- and
full-duplex modes.
RS485 terminations are included in the LTC2872. The
terminations are controlled by a termination enable control
line, TE485.
dc1851af
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DEMO MANUAL DC1851A
QUICK START PROCEDURE
Jumpers
Jumper blocks are included to set the state of each control
pin to VL or GND. If the shorting jumper is set in the EXT
position, the control pin is connected to its associated tur-
ret and external signaling may be applied. Failure to select
one of the three positions results in a floating control pin
whose state is indeterminant. The turret is disconnected
and there are no internal or on-board pull-ups to establish
the logic state if the jumper block is open.
As shipped, jumpers on DC1851A are set to select
full-duplex RS485 mode with terminations enabled for
Channel 1, and RS232 mode for Channel 2.
See the data sheet Pin Functions and Function Tables for
a complete description of the control pin functions and
truth tables. A brief summary is given here:
Shared Control
JP5, LB:
Loopback Enable. A logic high loops driver input
signals immediately back to associated receiver outputs.
Loopback is interrupted when a receiver is disabled. Default
position low, loopback disabled. Controls both channels
simultaneously.
JP6, H/F:
RS485 Half-Duplex Select Input. A logic low
selects full-duplex operation where the RS485 receiver
responds to signals on the A and B pins. A logic high selects
half-duplex operation where the RS485 receiver responds
to singals on the Y and Z pins. Default position low, full
duplex enabled. Controls both channels simultaneously.
H/F has no effect on a channel in RS232 mode. Note that
it is also possible to operate in half duplex by selecting
full-duplex mode and simply connecting A to Y and B to Z.
JP7, FEN:
Fast Enable. A logic high enables fast enable
mode; the internal supply generator remains operational
regardless of the state of the driver, receiver and termina-
tion enables. Default position high, fast enable enabled.
In this state, the internal supply generator remains opera-
tional in the event all drivers, receivers and terminations
are disabled. If shut down, the supply generator requires
about 200μs to power up again.
Independent Control
JP1, JP11,
RXEN:
Receiver Enable. A logic low enables
the receivers. Default position low, receivers enabled.
JP2, JP10, DXEN:
Driver Enable. A logic high enables the
drivers. Default position high, drivers enabled.
JP3, JP9, TE485:
RS485 Termination Enable. A logic high
enables 120Ω terminations across A-B and Y-Z. Default
JP3 position high, RS485 terminations enabled. Note
that in RS232 mode, TE485 is overridden and the RS485
termination for that channel is automatically disabled.
Default JP9 position low.
JP4, JP8, 485/232:
RS485/RS232 Mode Select. A logic
high enables the RS485 mode. Default Channel 1 RS485,
Channel 2 RS232.
Probe Pads
Probe pads are included on the bottom of the board for
examination of the VCC, VDD, VEE, SW and CAP pins. A
GND probe pad is located nearby.
J5, J6
Terminal blocks J5 and J6 are included for easy connec-
tion of RS485 cables. For half-duplex operation, only Y
and Z need be connected. Terminal block connections are
hardwired to turrets A, B, Y and Z.
How to Operate DC1851A
Operation is straightforward: as shown in Figure 1, con-
nect a V
CC
supply of 3V to 5.5V, and a V
L
supply of 1.7V
up to V
CC
. There are no special supply sequencing restric-
tions, although if V
L
> V
CC
the logic I/O signaling may not
operate properly. Note that D1, the V
L
supply indicating
LED, will extinguish at V
L
< 2V. Apply logic and interface
signals to the desired inputs. Inputs may be driven with
the supplies removed, without damage to the LTC2872.
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DEMO MANUAL DC1851A
QUICK START PROCEDURE
The LTC2872 may be made to self-oscillate as a ring
oscillator by connecting the drivers and receivers in one
all-encompassing loop, and adding one inversion. The
inversion may be introduced with a logic inverter, or by
cross-connecting the RS485 driver and receiver in full-
duplex mode (connect Y to B and Z to A). The connection
arrangement shown in Figure 2 forms a ring oscillator
when used with the default jumper conditions. With V
CC
=
V
L
= 3.3V, the frequency of oscillation is typically 260kHz.
–
LOGIC SUPPLY, V
L
1.7V TO V
CC
+
–
MAIN SUPPLY, V
CC
3.0V TO 5.5V
+
LOGIC
SIGNALS
I/O
SIGNALS
dc1851 F01
CONTROL LINES
Figure 1. Basic Test Setup. As Shown, Channel 1 is Configured for Full-Duplex RS485, While Channel 2 Is Configured for 2 + 2 RS232.
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DEMO MANUAL DC1851A
QUICK START PROCEDURE
3.3V SUPPLY
≈60mA
–
+
dc1851 F02
Figure 2. Ring Oscillator Connection Permits Examination of All Waveforms
Without the Need for a Signal Generator. Oscillation Frequency ≈260kHz
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