7
FN6001.4
January 16, 2005
High Data Rates
The ISL83385E maintains the RS-232 ±5V minimum
transmitter output voltages even at high data rates. Figure 4
details a transmitter loopback test circuit, and Figure 5
illustrates the loopback test result at 120kbps. For this test,
all transmitters were simultaneously driving RS-232 loads in
parallel with 1000pF, at 120kbps. Figure 6 shows the
loopback results for a single transmitter driving 1000pF and
an RS-232 load at 250kbps. The static transmitter was also
loaded with an RS-232 receiver.
Interconnection with 3V and 5V Logic
The ISL83385E directly interfaces with 5V CMOS and TTL
logic families. Nevertheless, with the device at 3.3V, and the
logic supply at 5V, AC, HC, and CD4000 outputs can drive
ISL83385E inputs, but ISL83385E outputs do not reach the
minimum VIH for these logic families. See Table 4 for more
information.
FIGURE 4. TRANSMITTER LOOPBACK TEST CIRCUIT
TIME (20
s/DIV)
T1
T2
2V/DIV
5V/DIV
VCC = +3.3V
SHDN
FIGURE 3. TRANSMITTER OUTPUTS WHEN EXITING
POWERDOWN
C1 - C4 = 0.1
F
ISL83385E
VCC
C1
C2
C4
C3
+
1000pF
V+
V-
5K
TIN
ROUT
C1+
C1-
C2+
C2-
RIN
TOUT
+
VCC
0.1
F
VCC
SHDN
FIGURE 5. LOOPBACK TEST AT 120kbps
FIGURE 6. LOOPBACK TEST AT 250kbps
T1IN
T1OUT
R1OUT
5
s/DIV.
VCC = +3.3V
5V/DIV.
C1 - C4 = 0.1
F
T1IN
T1OUT
R1OUT
2
s/DIV.
5V/DIV.
VCC = +3.3V
C1 - C4 = 0.1
F
ISL83385E