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What are control and status signals? Explain them


What are control and status signals? Explain them.

       Various type of control and status signals are as follows :

(i). Address latch enable (ALE) :-

  • This is an output signal, used to give information of AD0 -AD7 contents.
  • It is a positive going pulse generated during the first clock cycle of a machine cycle.
  • When pulse is high it indicates that the contents of AD0 - AD7 are address. When it is low it indicates that the contents are data.
  • The ALE signal is used to separate AD0 -AD7 (i.e demultiplex) to A0 - A7 and D0 - D7. To do this separation an external latch is connected to AD0 - AD7 lines and this latch is controlled by ALE signals.
(ii) Input output/memory (IO/M) : 
  • This is an output status signals, used to give information of operation to be performed with memory or I/O device.
  • If IO/M = 0, the microprocessor is performing a memory related operation.
  • If IO/M = 1, the microprocessor is performing an I/O device related operation.
(iii) Read (RD) : 
  • This is an active low signal.
  • It is an output control signal that is used to read data from the selected memory location or an I/O location via data bus.
(iv)  Write (WR) :
  • This an active low signal.
  • It is an output control signal used to write data to selected memory location or an I/O location via data bus.
  • A low on this pin indicated that a operation performed is write operation.
(v) Status signal (S1 and S2) :
  • These are output status signals used to give information of operation performed by microprocessor.
  • When S0 and S1 is combined with IO/M we get status of all the machine cycles (operations) performed by 8085 as shown in table below :
Table
Status signals
Operation
Control signal used
IO/M
S1
S2


0
0
0
-
-
0
0
1
Memory write
WR
0
1
0
Memory read
RD
0
1
1
Opcode fetch
RD
1
0
0
-
-
1
0
1
I/O write
WR
1
1
0
I/O read
RD
1
1
1
Interrupt acknowledge
INTA
Z
0
0
Halt

Z
X
X
Hold

Z
X
X
Reset

Where : Z – Tristate ( High impedance condition ) X- Unspecified condition

(vi) Ready : 
  • This is an active high input control signal.
  • It is used by microprocessor to detect whether a peripheral is ready for the data transfer or not. If not the processor waits till the signal goes high.
  • The main function of this pin is to synchronization the microprocessor 8085 with slower peripherals i.e. the microprocessor waits till the peripherals is not ready to accept/send the data.

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