Biphasic voltage using basic stamp

Archiver

I was reading this paper from:

http://www.parallax.com/dl/docs/article/Roborat.pdf

and on page 3, 2nd column, 1st paragraph; my question is how did the
authors generate that biphasic voltage. Here is what they wrote:


The microprocessor was pre-loaded with a PBASIC program
that controlled stimulation. The program waits for
ASCII command from the backpack receiver. As soon as a
valid message is received, it branches to the stimulation routine.
When not in use all I/O pins are left in input mode to
prevent cross talk between electrodes. During stimulation,
the requested pair of pins is opened for output and 5V is
alternately applied first to one and then the other, according
to the regimen for that channel as specified in the PC command.
Namely, 5V is applied to each I/O pin for the speci-
fied pulse duration, and then the program pauses to complete
the specified inter-pulse interval. This process repeats for the
specified number of pulses. Thus, the microprocessor's interpretation
of ASCII commands results in a train of pulses
of specified duration, frequency and number. Since the backpack
is electrically floating, applying a voltage to the first
pin and then to the other results in a biphasic pulse. After
stimulation, the pins are reset to input mode, and the program
goes back to wait for the next ASCII command.