Tandy 1000 HX Technical Reference Manual page 40

• TANDY COMPUTER PRODUCTS
Joystick Interface
The joystick interface converts positional information from
hand-held joysticks (1 or 2) into CPU data. Each joystick
provides 1 or 2 push-buttons and X, Y position for a total
of 4 bits each. Two joysticks can be used. The joystick
handle is connected to two potentiometers mounted perpendic-
ular to each other; one for X position, one for Y position.
Through the cable, the main logic board applies +5 VDC to
one side and ground to the other of the pots. The pot wiper
is the position signal: a voltage between 0 and +5 VDC.
This signal is applied to one input of a comparator U24.
The other comparator input is the reference signal (a ramp
between 0.0 to +5.0 volts.) When the position signal is
equal to or greater than the reference signal, the
comparator output goes true. This comparator output is the
X or Y position data bit. The ramp is reset to 0.0 VDC
whenever a "write" is made at Port 200/201 Hex. The IOW*
signal turns on Q2, which discharges C129 to 0.0 volts.
When Q2 is turned off, Ql, R22, R28, R35, and CR3 create a
constant-current source that linearly charges C129 to +5.0
VDC in 1.12 milliseconds. The joystick information is
"read" by the CPU at Port 200/201 Hex through U26. See
Figure 12.
Printer Interface
The printer interface is totally contained in a custom Gate
Array U37 and is shown in Figure 13. Functionally, the
printer interface consists of an output data latch (write
port 378) and accompanying input data buffer. The data
written to the output port latch may be read at port 37A.
The input data from the printer connector may be read back
at port 378. The input buffer is for reading printer input
signals (read port 379), I/O address decoding, data
transceiver, and interrupt logic. The interrupt is
logically connected to ACKNOWLEDGE* if interrupts are
enabled (37A Bit 4 ) .
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