CSSE2010/CSSE7201 – Introduction to Computer Systems - Semester One, 2016 – Version 2.3 Page 1
School of Information Technology and Electrical Engineering, The University of Queensland
CSSE2010/CSSE7201 - Guide to Drawing Circuit Schematics
Circuit schematic diagrams are a systematic way of describing how a circuit is to be constructed.
They show all of the devices used and the pin numbers which must be connected to the pins of
other devices – in our case 74 series logic chips and the CSSE2010/CSSE7201 IO Board for inputs
and outputs. An example circuit schematic is shown in Figure 1. All circuit schematics must obey
the following:
• All inputs and outputs must be named, using sensible names where appropriate. In the
circuit below, the inputs are A, B, C, D and E and the output is X0. Note the symbol
difference between inputs and outputs, e.g. which way the symbols point. It is
conventional for inputs to be shown on the left and outputs on the right.
• Individual physical devices (chips, IO board etc.) must be uniquely identified using one or
more letters to indicate the type of device, followed by a number (sequential, starting from
1). In an electronic circuit schematic, you might have R1, R2, R3 etc. for each resistor; C1,
C2, C3 etc for capacitors. In a logic circuit schematic we use U1, U2 etc. for each chip and
IO1 for the IO board (IO2 if there was a second IO board). When inputs or outputs are
grouped together there is no need to show “IO1” etc. for each input/output; you can just
show it once for the group as a whole – as shown in Figure 1.
• Where a chip contains multiple gates, each gate within the chip should be labelled (A, B, C
etc) after the chip identifier (U1, U2, etc) with a colon separator. Where a chip has only one
gate/device, there is no need to identify the gate – you can just label the chip (e.g. U3). You
can choose to assign the labels (A, B, C etc.) in any way to the gates within a chip.
• The type of each device must be identified on the line below the device ID. In Figure 1,
these types are 74HCT00, 74HCT04, IOBOARD
• All pins must be numbered as per the device pinout. For example a 74HCT00 has four 2-
input NAND gates with inputs on pins 1 and 2, 4 and 5, 9 and 10, 12 and 13 and outputs on
pins 3, 6, 8 and 11 respectively.
• Power supply connections for each type of device should be shown separately. There is no
need to show this per individual chip, just per type. Where two or more types of chips have
the same power supply connections, they can be grouped together as shown in Figure 1.
There is no need to show power supply connections for the IO Board.
U1:D
74HCT00
1112
13
U1:C
74HCT00
8
9
10
3
U1:A
74HCT001
2
6
U1:B
74HCT004
5
A
B
C
D
12
13
14
15
IO1
IOBOARD
E
25
IO1
IOBOARD
3
1
2
U2:A
74HCT00
U3:A
74HCT04
1 2
X0
17
IO1
IOBOARD
Power Supply Connections
74HCT00
74HCT04 } Vcc: Pin 14GND: Pin 7

Figure 1: Example circuit schematic
CSSE2010/CSSE7201 – Introduction to Computer Systems - Semester One, 2016 – Version 2.3 Page 2
School of Information Technology and Electrical Engineering, The University of Queensland
Common Errors

Figure 2: Circuit schematic showing some common errors
Figure 2 shows a number of common student errors in circuit schematics. Left to right, these are:
1. Two inputs have the same name (A). Each input should have a unique name.
2. Where wires join or split, a dot should be used. (Lines which cross each other are not
electrically connected.)
3. U1:E is incorrectly labelled. There are only four 2-input NAND gates in a 74HCT00 (labelled
A to D in any way you choose), so this gate should be labelled U2:A or similar.
4. U1:F is incorrectly labelled. This is a different type of gate (NOT gate) so should use a
different chip – which must therefore have a different identifier, e.g. U3 (and the gate
might therefore be U3:A).
5. U1:C uses the same pin numbers as U1:A – you can’t use the same gate twice
6. U1:D is labelled as a 74HCT47, but this is a gate from a 74HCT00. All gates labelled
U1:something must show the same chip type
7. There are two errors with the X0 output:
a. The symbol used is for an input – the symbol for outputs is different – see Figure 1.
b. An input should not be connected to the output of a gate. This may destroy the
device. Gate outputs should only be connected to output devices (e.g. LEDs) or used
as the input to other gates.