sp331 Exar Corporation, sp331 Datasheet - Page 8

• Manufacturer Part Number: sp331
• Description: Programmable Dual Rs-232/rs-485 Transceiver
• Manufacturer: Exar Corporation

FUNCTION TABLE FOR SELECT PINS
THEORY OF OPERATION
The SP331 is made up of four separate circuit
blocks — the charge pump, drivers, receivers,
and decoder. Each of these circuit blocks is
described in more detail below.
Charge–Pump
The charge pump is a Sipex–patented design
(U.S. 5,306,954) and uses a unique approach
compared to older less efficient designs. The
charge pump still requires four external
capacitors, but uses a four–phase voltage
shifting technique to attain symmetrical 10V
power supplies. Figure 15(a) shows the
waveform found on the positive side of capcitor
C2, and Figure 15(b) shows the negative side of
capcitor C2. There is a free–running oscillator
that controls the four phases of the voltage
shifting. A description of each phase follows.
Phase 1
— V
the clock cycle, the positive side of capacitors
C
then switched to ground and charge on C
transferred to C
Date: 03-04-05
Table 1. Mode Function Table. (Refer to Control Logic Confirmations for Block Diagrams)
1
A
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
and C
SS
charge storage —During this phase of
2
B
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
are initially charged to +5V. C
C
2
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
–
. Since C
D
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
2
+
RS-232/RS-485
RS-232/RS-485
RS-232/RS-485
RS-232/RS-485
RS-485/RS-232
RS-485/RS-232
RS-485/RS-232
RS-485/RS-232
is connected to
Programmable Dual RS-232/RS-485 Transceiver
MODE
RS-232
RS-232
RS-232
RS-232
RS-485
RS-485
RS-485
RS-485
1
l
+
–
is
is
All four RS-232 drivers active
All four RS-232 drivers tri-state
All four RS-232 drivers tri-state
RS-232 (4ch) Loopback
T1 and T2 active RS-232; T3 tri-state RS-485
T1 and T2 tri-state RS-232; T3 active RS-485
T1 and T2 active RS-232; T3 tri-state RS-485
RS-232 (2ch) / RS-485 (1ch) Loopback
T1 active RS-485; T3 and T4 active RS-232
T1 tr-state RS-485; T3 active RS-232; T4 active RS232
All RS-485 and RS-232 drivers tri-state
RS-485 (1ch) / RS-232 (2ch) Loopback
T1 and T3 active RS-485
T1 tri-state RS-485; T3 active RS-485
T1 active RS-485; T3 tri-state RS-485
RS-485 (2ch) Loopback
8
+5V, the voltage potential across capacitor C
is now 10V.
Phase 2
— V
nects the negative terminal of C
storage capacitor and the positive terminal of C
to ground, and transfers the generated –l0V to
C
tor C
is connected to ground.
Phase 3
clock is identical to the first phase — the charge
transferred in C
terminal of C
side of capacitor C
voltage potential across C
Phase 4
clock connects the negative terminal of C
ground and transfers the generated l0V across
C
— V
— V
3
2
. Simultaneously, the positive side of capaci-
to C
SS
DD
1
DD
is switched to +5V and the negative side
transfer — Phase two of the clock con-
charge storage — The third phase of the
4
, the V
transfer — The fourth phase of the
FUNCTION
1
, which is applied to the negative
1
DD
produces –5V in the negative
2
. Since C
storage capacitor. Again,
© Copyright 2005 Sipex Corporation
2
is l0V.
2
+
is at +5V, the
2
to the V
2
SS
to
2
2