Tandy 1000 SL Technical Reference Manual page 25

Clock Generation. The FDC Support IC (u18) generates all clocks
required by the Floppy Disk circuit. These clocks are derived
from a 16 MHz input signal. FDCCLK, required by the FDC
Controller (U12), is derived by dividing the 16 MHz clock by 4.
The resulting 4 MHz clock is also used as a delay counter for the
DMA request signal DRQ as well as a reference clock for the write
precompensation circuit. The 4 MHz clock also generates a 250
nanosecond pulse at a frequency of 500 KHZ. The 500 KHz signal is
used as a write clock for the FDC Controller.
Precompensation. The precompensation circuit is implemented
internally to the FDC Support IC (U18). The write data bit can be
shifted either early or late in the serial bit stream, depending
on the requirements of the Floppy Disk Drive. This function is
programmable and controlled by the FDC IC signals PSO and PS1.
Data Separator. The FDC Support IC (u18) also contains the data
separator circuit. The data separator recovers the clock and data
signals from the serial bit stream of the Floppy Disk Drive. The
FDC Support IC supports only MFM or Double-density mode.
Disk Drive Interface. All FDC outputs to the FDD are driven by
high current 7414 SCHMITT trigger buffers or 7416 open collector
inverters. All FDC inputs from the FDD are buffered by 74HCT14
SCHMITT triggered inverters. The inputs are pulled up on-board by
1K terminating resistors. All outputs should be terminated on the
last FDD by 1K resistors.
Interrupt Con tr ol ler
The Interrupt Controller is contained in the KFIT custom IC (U30)
and supplies the maskable interrupt input to the CPU. The KFIT
custom IC has eight interrupt inputs controlled through software
commands. It can mask (disable) and prioritize (arrange priority)
to generate the interrupt input to the CPU. The eight interrupts
are assigned as follows:
# O
#1
# 2
#3
# 4
#5
#6
#7
Timer Channel 0
Keyboard
Interrupt on the Bus
Interrupt on the Bus
Interrupt on the Bus
Vertical Sync/mC
Floppy Disk Controller
Printer/Sound/DMA
Software Timer
Keyboard Code Received
Optional Bus Interrupt
Modem (COM2)
Hard Disk Controller/
Video Vertical sync
Optional
BUS
Interrupt
Optional Bus Interrupt
RS-232 (COM1)
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