A Qs6/7/8 Pc Serial; B Qs6/7/8 Mac Serial; C Qsr Variant; H8 Uart Clocking - Alesis QS Series Service Manual

2.53A QS6/7/8 PC Serial

In these units the Enable handshake signal is provided by simply closing the loop
provided by the host computer (Enable Return) via the DPDT PC/MAC switch (SW1 SW1). The
switch also is used to inform the H8 that the PC type is currently in use and should adjust the
software and/or hardware accordingly. The BAUD rate is determined by the FREQ input to the
H8 (See Section 2.54). In the QS6, handshake hardware is fixed at 614.4KHz, so no
adjustment can be made by the H8, hence it's BAUD rate limitations.

2.53B QS6/7/8 MAC Serial

In the MAC interface, BAUD rate information must be passed along to the host
computer along with the data itself. This is provided via comparator U2B U31D U24D (used
here as a line driver). In the MAC position, SW1 sends the bit clock out through pin 1 of the QS
Serial port. The switch also is used to inform the H8 that the MAC type is currently in use and
should adjust the software and/or hardware BAUD rate accordingly. The bit clock signal is also
needed by the FREQ input to the H8 (See Section 2.54). In the QS6, handshake hardware is
fixed at 614.4KHz,so no adjustment can be made by the H8, hence it's BAUD rate limitations.

2.53C QSR Variant

Note that the QSR varies slightly in that it's PC/MAC switch is a software function rather
than a physical switch. Q7 is used here provide the necessary switching action to correctly bias
the handshake signal driver U24D.

2.54 H8 UART Clocking

There are 2 internal UARTs (Universal Asynchronous Receiver Transmitter) in the H8 (1
for MIDI and 1 for computer serial I/O). It should be noted here that the MIDI UARTs all operate
at the same fixed rate so further discussion of UART clocking will be limited here to the UART
used for computer serial I/O.
In order for serial data to be transmitted and received at the correct speed, the UART
must be clocked at the correct speed via the FREQ input (pin 92). In the QS6 it is simply
derived from the system clock via U17 and U20A limiting it's BAUD rate. In later models
however, the BAUD rate is variable to match the speed of the host computer, so a little extra
hardware is needed. U26 U27 is used as a software controlled variable frequency divider.
Control input to the 74HC161 is provided by the H8 in the form of a "count" value. The Carry
output is used to force the device to load the new "count" input via U25A U26A. The Carry
output is also passed along to a divide by 2 waveshaping flip-flop (U29A U29A) before being
sent to the H8 FREQ input. There are also 2 clock sources available (again selectable by the
H8 via gate logic comprised of U28 and U25C U28 and U26C). The first is the 20MHz system
clock (M2 M2), and the second is a 14MHz clock (M3 M3) intended strictly for this purpose.
The ultimate purpose of all of this is to determine how fast the H8's TXD and RXD lines send
and receive data.
Alesis QS Series Keyboards Service Manual V1.00
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11/19/06