Peripheral Devices Questions and Answers

Which of the ports in 8255, in which part of the port pins can be used as input lines and part of the port pines can be used as output lines ?

A.PA
B.PB
C.PC
D.PD

Exam Year(s)

UGC NET - 2012,

Answer: C

Port C of the 8255 can be split into two halves of four lines each, called upper C and lower C, and the two halves can be set independently, one as input and the other as output. The 8255 has three 8-bit ports, A, B and C, and no port D, so that option does not exist. Ports A and B are each set as a whole port for input or output in the modes of operation, so part of the pins of those ports cannot be input while the rest are output. The answer is PC.

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In the mode word format of 8251 during initialization, \(D_{6}\) and \(D_{7}\) bits as 0 and 1 signify

A.16 X Baud Rate
B.Character length as 7 bits
C.Parity enable and even parity
D.No. of stop bits as \(1\frac{1}{2}\)

Exam Year(s)

UGC NET - 2012,

Answer: D

The bits D6 = 0 and D7 = 1 in the mode word of the 8251 signify 1.5 stop bits, option (D). In the mode instruction of the 8251 USART, bits D7 and D6 select the number of stop bits: 01 means 1 stop bit, 10 means 1.5 stop bits, 11 means 2 stop bits, and 00 is invalid. With D7 = 1 and D6 = 0, the code is 10, which is 1.5. The baud rate factor, option (A), is set by bits D1 and D0. The character length, option (B), is set by D3 and D2. The parity enable and the even parity, option (C), are set by D4 and D5. So only (D) is fixed by D6 and D7.

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The control word of ‘0B5H’ of 8253 indicates that the following counter is selected

A.Counter 0
B.Counter 1
C.Counter 2
D.Counter 3

Exam Year(s)

UGC NET - 2012,

Answer: C

The control word 0B5H of the 8253 selects counter 2, option (C). In binary 0B5H is 1011 0101. The two most significant bits, D7 D6, select the counter: 00 for counter 0, 01 for counter 1 and 10 for counter 2, and here they are 10. The next two bits, D5 D4 = 11, say that the low byte and then the high byte are read or loaded, the bits D3 D2 D1 = 010 select mode 2, and D0 = 1 selects BCD counting. The code 11 in D7 D6 is not a counter selection in the 8253, so counter 3, option (D), does not exist, and counters 0 and 1 in options (A) and (B) would need 00 and 01.

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Identify the peripheral devices for their applications.


(a) 8155	(i) Serial Communication
(b) 8279	(ii) Timers and Counters
(c) 8253	(iii) Keyboard and display interface
(d) 8251	(iv) Additional input/output lines to processor

Codes :
(a) (b) (c) (d)

A.(ii) (iii) (iv) (i)
B.(iii) (iv) (i) (ii)
C.(iv) (i) (ii) (iii)
D.(iv) (iii) (ii) (i)

Exam Year(s)

UGC NET - 2013,

Answer: D

The correct match is a-iv, b-iii, c-ii, d-i, the fourth option. The 8155 gives additional input and output lines, along with RAM and a timer, the 8279 is the keyboard and display interface, the 8253 is the programmable interval timer or counter, and the 8251 is the serial communication interface.

The other options give the keyboard interface to the 8155 or the serial interface to the 8253, which is not so.

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Which mode of 8253 acts as a square wave generator ?

A.Mode-1
B.Mode-2
C.Mode-4
D.Mode-3

Exam Year(s)

UGC NET - 2013,

Answer: D

Mode 3 of the 8253 programmable interval timer is the square wave generator, option (D). In this mode the output stays high for half the count and low for the other half, and then repeats, which gives a square wave.

Mode 1 is a programmable one-shot and mode 2 is a rate generator that gives a short low pulse once per count. Mode 4 is a software-triggered strobe. None of these produces a symmetrical square wave, so options A, B and C are wrong.

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