IBM 5150 Hardware Reference Manual page 24

The memory is dynamic 16K x 1 chips with an access time of 250 ns
and a cycle time of 410 ns. All R/W memory is parity checked.
The System Board contains circuits for attaching an audio cassette,
the serial keyboard, and the speaker. The cassette adapter allows the
attachment ofany good quality audio cassette via either the microphone
1"""""\ or auxiliary inputs. The board has a jumper for either input. This
interface also provides a cassette motor control line for transport
starting and stopping under program control. This interface reads and
writes the audio cassette at a data rate of between 1,000 and 2,000
baud. The baud rate is variable and dependent on data content since a
different bit-cell time is used for O's and 1 'so For diagnostic purposes,
the tape interface can loop read to write to test the board's circuits.
The system software blocks cassette data, generates and checks data
with a Cyclic Redundancy Check (CRC).
The processor also contains the adapter circuits for attaching the
serial interface from the keyboard. This generates an interrupt to the
processor when a complete scan code is received. This interface can
request execution of a diagnostic in the keyboard.
Both the keyboard and cassette interfaces are provided via 5-pin
DIN connectors, which are right angle mounts on the System Board
and extend through the rear panel of the System Unit.
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The system is provided with a 2-1/4-inch audio speaker mounted inside
the System Unit. The System Board contains the control circuits and
driver for the speaker. The speaker connects through a 2-wire interface
which attaches to a 4-pin header on the System Board.
The speaker drive circuit is capable of approximately a 1/2 watt of
power. The control circuits allow the speaker to be driven several
different ways. First, a direct program control register bit may be
toggled to generate a pulse train; second, the output of Channel 2 ofthe
timer counter may be programmed to generate a waveform to the
speaker. Third, the clock input to the timer/counter can be modulated
with a program controlled
I/O
Register Bit. All three forms of control
may be performed simultaneously.
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