2

dc1851af

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 VCC input, while a

separate logic I/O supply pin (VL) permits the LTC2872

to adapt to any logic level ranging from 1.7V up to VCC.

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 VCC and VL. Note

that the VL LED, D1, unavoidably dims and extinguishes

at VL < 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: VCC and VL supply indicator LEDs

and current limiting resistors.

• R12-R15: 10kΩ pull-ups to VL 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.