Specifications

BASIC Stamp II
Page 210 • BASIC Stamp Programming Manual 1.8 • Parallax, Inc.
When the supply voltage is above 4V, U3 allows its output to be pulled
high by a 4.7k resistor to +5V, which also puts a high on U1’s MCLR
input. U1 starts its internal program at the beginning, which in turn
starts your PBASIC2 program from the beginning.
Power Supply (U4)
The previous discussion of the reset circuit should give you some idea
of how important a stable power supply is to correct operation of the
BS2. The first line of defense against power-supply problems is U4, the
S-81350HG 5-volt regulator. This device accepts a range of slightly over
5V up to 15V and regulates it to a steady 5V. This regulator draws mini-
mal current for its own use, so when your program tells the BS2 to go
into low-power Sleep, End or Nap modes, the total current draw aver-
ages out to approximately 100 microamperes (µA). (That figure assumes
no loads are being driven and that all I/O pins are at ground or +5V.)
When the BS2 is active, it draws approximately 8mA. Since U4 can
provide up to 50mA, the majority of its capacity is available for power-
ing your custom circuitry.
Circuits requiring more current than U4 can provide may incorporate
their own 5V supply. Connect this supply to VDD and leave U4’s input
(VIN) open.
Note that figure H-1 uses CMOS terms for the power supply rails, VDD
for the positive supply and VSS for ground or 0V reference. These terms
are correct because the main components are CMOS. Don’t be con-
cerned that other circuits you may come across use different nomen-
clature; for our purposes, the terms VDD, VCC, and +5V are interchange-
able, as are VSS, earth (British usage) and ground.
Serial Host Interface (Q1, Q2, and Q3)
The BS2 has no keyboard or monitor, so it relies on PC-based host soft-
ware to allow you to write, edit, download and debug PBASIC2 pro-
grams. The PC communicates with the BS2 through an RS-232 (COM
port) interface consisting of pins SIN, SOUT, and ATN (serial in, serial
out, and attention, respectively).
RS-232 uses two signaling voltages to represent the logic states 0 and 1;
+12V is 0 and –12V is 1. When an RS-232 sender has nothing to say, it