The following image shows the pin diagram of a 8257 DMA controller
These are the
four individual channel DMA request inputs, which are used by the peripheral
devices for using DMA services. When the fixed priority mode is selected, then
DRQ0 has the highest priority and DRQ3 has the
lowest priority among them.
These are the
active-low DMA acknowledge lines, which updates the requesting peripheral about
the status of their request by the CPU. These lines can also act as strobe
lines for the requesting devices.
These are
bidirectional, data lines which are used to interface the system bus with the
internal data bus of DMA controller. In the Slave mode, it carries command
words to 8257 and status word from 8257. In the master mode, these lines are
used to send higher byte of the generated address to the latch. This address is
further latched using ADSTB signal.
It is an
active-low bidirectional tri-state input line, which is used by the CPU to read
internal registers of 8257 in the Slave mode. In the master mode, it is used to
read data from the peripheral devices during a memory write cycle.
It is an
active low bi-direction tri-state line, which is used to load the contents of
the data bus to the 8-bit mode register or upper/lower byte of a 16-bit DMA
address register or terminal count register. In the master mode, it is used to
load the data to the peripheral devices during DMA memory read cycle.
It is a clock
frequency signal which is required for the internal operation of 8257.
This signal
is used to RESET the DMA controller by disabling all the DMA channels.
These are the
four least significant address lines. In the slave mode, they act as an input,
which selects one of the registers to be read or written. In the master mode,
they are the four least significant memory address output lines generated by
8257.
It is an
active-low chip select line. In the Slave mode, it enables the read/write
operations to/from 8257. In the master mode, it disables the read/write
operations to/from 8257.
These are the
higher nibble of the lower byte address generated by DMA in the master mode.
It is an active-high asynchronous input signal, which makes DMA
ready by inserting wait states.
This signal is used to receive the hold request signal from the
output device. In the slave mode, it is connected with a DRQ input line 8257.
In Master mode, it is connected with HOLD input of the CPU.
It is the hold acknowledgement signal which indicates the DMA
controller that the bus has been granted to the requesting peripheral by the
CPU when it is set to 1.
It is the low memory read signal, which is used to read the data
from the addressed memory locations during DMA read cycles.
It is the active-low three state signal which is used to write the
data to the addressed memory location during DMA write operation.
This signal is used to convert the higher byte of the memory address
generated by the DMA controller into the latches.
This signal is used to disable the address bus/data bus.
It stands for ‘Terminal Count’, which indicates the present DMA
cycle to the present peripheral devices.
The mark will be activated after each 128 cycles or integral
multiples of it from the beginning. It indicates the current DMA cycle is the
128th cycle since the previous MARK output to the selected peripheral device.
It is the power signal which is required for the operation of the
circuit.
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