5
AN1248.1
May 30, 2006
jumper
(1)
) are high due to the A1 and B1 5k
Ω
input
resistors to GND.
To switch the Rx output states, install the A1 and B1 Rx bias
jumpers, “J10”
(2)
and “J9”
(3)
. The “RXBIAS-VCC”
(15)
jumper now drives the Rx input voltages to V
CC
, which
switches RA and RB low. Removing the “RXBIAS-VCC”
jumper and connecting a power supply between the
“RXBIAS” banana jack and GND, allows this supply to set
the Rx input voltages, in case the user wants to evaluate the
Rx switching points.
To disable the Rx outputs via the active high RXEN pin,
remove “J10”
(2)
and “J9”
(3)
, and switch the “(RXEN)”
(11)
jumper to the “LOW” position (make sure the “RXEN1”
(10)
jumper is in the “VH” position). To disable the Rx outputs via
the active low RXEN1 pin, move jumper “RXEN1” to the
“VH” position (make sure the “(RXEN)” jumper is in the
“LOW” position).
Return the “RXEN1” and “(RXEN)” jumpers to the “VH”
position, remove the “RXBIAS” power supply, and reinstall
the “RXBIAS-VCC”
(15)
jumper.
Driver Tests
The RS-232 Tx outputs, Y and Z, are high (
≈
6.5V) and low
(
≈
-7V), respectively, in the default configuration. To switch
the output states, simply switch the “DY1 (DZ/SLEW)”
(13)
and “DZ1/DE1 (DY)”
(14)
jumpers (note Rev. A label errors)
to the opposite states. To evaluate the loaded driver output
voltages, configure the port for “external loopback”, as
described previously (i.e., install jumpers
(5)
and
(6)
). Each
driver output is now loaded by an Rx input resistor (
≈
5k
Ω
),
and the output voltages still exceed
±
6.2V.
The driver outputs can be disabled by moving the
“(DEN)”
(8)
jumper to the “LOW” position.
Remove the “external loopback” jumpers when finished,
switch the “DY1(DZ/SLEW)” and “(DEN)” jumpers to the
“VH” position, and move the “DZ1/DE1 (DY)” jumper back to
the “LOW” position.
Internal Loopback
To configure the ISL41387 for internal loopback mode,
simply move the “(ON)”
(12)
jumper to the “LOW” position,
while ensuring that the “(DEN)”
(8)
and “(RXEN)”
(11)
jumpers are set to “VH”. Note that I
CC
increases due to the
enabling of the loopback receivers. RA is now low due to Tx
output Y feeding back to Rx A, while RB is high due to Z
feeding back to Rx B. You can repeat the previous Rx
switching tests to confirm that the external Rx input pins now
have no affect on RA nor RB.
Low Power SHDN
The ISLX1387 enter the SHDN mode when
“(ON)”
(12)
= “LOW”, and the Tx and Rx are disabled
(“(DEN)”
(8)
= “LOW”, “(RXEN)”
(11)
= “LOW”, and
“RXEN1
(10)
= “VH”), and the already low supply current
drops to as low as 5µA. SHDN disables the Tx and Rx
outputs, and disables the charge pumps, so V+ collapses to
V
CC
, and V- collapses to GND.
All but 5µA of SHDN I
CC
current is due to the on-chip SPB
pull-up resistor (~20µA/resistor), so SHDN I
CC
varies
depending on the ISLX1387 configuration. To evaluate the
lowest SHDN I
CC
, ensure that the “SPB”
(9)
jumper is in the
“VH” position.
Single Tx and Single Transceiver Modes
RS-232 mode also offers the options of selecting a single Tx
mode, or a single Tx/Rx pair mode. To evaluate the single Tx
mode, set the “(ON)”
(12)
and “(RXEN)”
(11)
jumpers “LOW”
and “(DEN)”
(8)
and “RXEN1”
(10)
jumpers to “VH”. To
evaluate the single Tx/Rx mode, set the “(ON)” and “(DEN)”
jumpers “LOW” and either the “RXEN1” jumper to “LOW” or
the “(RXEN)” jumper to “VH”.
Basic AC Evaluation
Before starting, ensure that the jumpers are back in the
default RS-232 positions. Note that the RS-232 data rate is
fixed, so the speed select pins have no effect.
Receiver Tests
Add jumper “A1”
(1)
to engage the 50
Ω
term, connect a
generator to the “A1” BNC, and set it for at least a 0V to 3V
swing. Monitoring test points “TP6” (input), and “RB1(RA)”
(output) with a scope allows the Rx prop delays and skews
to be measured. If desired, you can load the Rx output with a
1k
Ω
resistor by adding jumper “J6”
(not numbered)
, located
below test point “RB1 (RA)”. This resistor terminates to the
“VLOAD” banana jack (upper left hand corner), allowing the
resistor to be terminated to GND by shorting “VLOAD” to
GND, or terminated to any voltage by connecting “VLOAD”
to an external supply.
To measure the Rx enable/disable time to/from a high output
state via the active high RXEN pin, start from the previous
jumper configuration, leave the “J6” jumper installed, and
connect the “VLOAD” banana jack to GND. Remove the “A1”
jumper (Rx input is pulled low by its on-chip pull-down),
switch the “(RXEN)”
(11)
jumper to the low position to
engage the 50
Ω
term, set the generator to swing from 0V to
3V, and move the generator to the “NC (RXEN)” BNC.
Monitoring test points “TP12” (input), and “RB1 (RA)”
(output) with a scope allows the Rx enable and disable times
to be measured. To evaluate the Rx enable/disable time
to/from a low output state, install the A1 Rx bias jumper,
“J10”
(2)
.
To evaluate the Rx enable/disable times using the active low
RXEN pin, repeat the previous test but leave the “(RXEN)”
jumper in the “LOW” position, move the “RXEN1”
(10)
jumper to the “LOW” position to connect the 50
Ω
term, and
connect the generator to the “RXEN1” BNC.
Application Note 1248
