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dc1851af
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.
