2
AN1248.1
May 30, 2006
Functional Diagram
(Default Configuration)
Supply Banana Jacks
There are eight banana jacks at the top of the board for
power supply connections, but only V
CC
and GND are
required connections. The function and use of each jack
(from left to right) is:
VLOAD -
This is a load voltage driving the load resistors
connected to the Rx and Tx outputs (Tx resistors - R5 and
R6 - not populated); used mostly during output enable and
disable time characterizations.
GND -
Common connection for any supplies used.
RXBIAS -
A voltage that can be applied to any or all Rx
inputs via jumpers “J9” and “J10”
(3 & 2)
; the “RXBIAS-VCC”
(15)
jumper shorts this jack to V
CC
, so remove this jumper if
supplying a voltage other than V
CC
.
V+ -
Used to monitor the positive charge pump voltage in
RS-232 mode.
V- -
Used to monitor the negative charge pump voltage in
RS-232 mode.
VCC -
The 5V supply connection.
VHIGH -
Connects to all the “VH” positions on the jumpers to
define the high level voltage for logic and Tx inputs; the
“VL-VHIGH”
(17)
jumper shorts this jack to V
L
, so remove
this jumper if supplying a voltage other than V
L
.
VL -
The logic supply voltage that sets the ISL41387’s Rx
output V
OH
levels, and the logic and Tx input switching
points; the “VCC-VL”
(16)
jumper shorts this jack to V
CC
, so
remove this jumper if supplying a voltage other than V
CC
.
Getting Started
Connect a 5V, 500mA minimum, power supply to the V
CC
and GND banana jacks. It is recommended that an ammeter
be used between the supply and the board, so that I
CC
can
be monitored. Ensure that the “RXBIAS-VCC”
(15)
,
“VCC-VL”
(16)
and “VL-VHIGH”
(17)
jumpers are installed in
the upper right hand corner of the board.
External Loopback Via Jumpers
To evaluate the Rx and Tx performance at the same time, an
external loopback can be implemented simply by installing
jumpers “A1/Y1_LB
(5)
and “B1/Z1_LB
(6)
. In this
configuration, the Tx output lines connect to the
corresponding Rx input lines, so the data driven on the Tx
input(s) appears at the Rx output(s). In RS-485 mode, data
driven on DY loops back through A and B to R
A
. In RS-232
mode, DY loops back to R
A
, and DZ loops back to R
B
.
For RS-485 mode, installing resistors R7 and R9 allows
evaluation of performance with the Tx driving a double
terminated load.
Basic RS-485 DC Evaluation
General Observations
I
CC
should be approximately 1.6mA.
Measure V+ and V- at the banana jacks - V+ = VCC and
V- = GND, indicating that the charge pumps are off for low
power and low noise.
R
A
is high - due to the “full failsafe” Rx - while R
B
is always
tri-stated because it is unused in RS-485 mode.
Note that this board isn’t populated with differential
termination resistors on either the Rx inputs nor the Tx
outputs. If these resistors are desired, they can be added at
positions R9 and R7, respectively.
Receiver Tests
The “full failsafe” nature of the Rx can be evaluated by
manipulating the “A1”
(1)
and “B1”
(4)
input jumpers. In the
default configuration, A1 and B1 float, but RA (measure at
the “RB1(RA)” test point to the right of jumper
(1)
) remains
high due to the failsafe “open” functionality. Installing
jumpers “A1”
(1)
and “B1”
(4)
effectively shorts the two inputs
together (i.e., V
ID
= 0). R
A
still remains high, indicating that
the Rx is also failsafe “shorted”. The combination of failsafe
“open” and “shorted” yields a “full-failsafe” Rx.
To switch the Rx output state leave the “B1”
(4)
jumper
installed, remove the “A1”
(1)
jumper, and install the A1 Rx
bias jumper, “J10”
(2)
. The “RXBIAS-VCC”
(15)
jumper now
drives the A1 input voltage to V
CC
, which switches RA low.
34
V
CC
Y1
Z1
D
Y
SLEW
0.1
μ
F
+
0.1
μ
F
+
0.1
μ
F
28
27
4
5
37
38
1
19
V+
V-
C1+
C1-
C2+
C2-
+
0.1
μ
F
36
35
R
A
A1
2
R
B
B1
3
29
C
1
C
2
+ C
3
C
4
RXEN 20
GND
+5V
+
0.1
μ
F
15, 16
485 / 232
ON
11
21
V
H
V
H
DEN
12
V
H
30
D
R
V
H
31
V
L
SPB
V
H
V
H
14
RXEN 17
V
H
VHIGH (V
H
)
(17)
(16)
Application Note 1248
