7485 IC 4-bit Magnitude Comparator
74LS85 or SN54/7485 is a 4-bit magnitude comparator of two binary format inputs. In electronic devices besides addition and subtraction comparison of binary numbers is also important to perform the operations. To solve this issue, we use TTL-based comparators. The basic function of the comparator is to compare the two input bits and then give the three outputs. The output will always be in 3-bits which will describe the input relation with each other. The input can be greater or equal to each other. By using logic gates the comparators can be designed but to avoid large circuit sizes we can use the premade comparator IC.
74LS85 4-Bit Comparator Introduction
The IC 74LS85 is a magnitude comparator integrated circuit that can be used to compare binary data. It is smaller in size and it comes in multiple packages to fulfill the requirements of different circuits. The 74LS85 can compare two 4-bit binary data and output can be received from the output pins in the form of HIGH and LOW state. IC 74LS85 receives 8 parallel inputs for two 4-bit binary data and gives the output in 3-bit in parallel form. The IC has low power consumption and it gives the output in TTL form, which makes it compatible with other TTL-based devices and microcontrollers.
74LS85 Pin Configuration
The pinout diagram of the 7485 binary magnitude comparator is shown below:
74LS85 Pin Description
| Pin No | Pin Name | Description |
| 1 | B3 | Data Input B3 |
| 2 | 1A<B | Cascading Output Pin if A<B |
| 3 | 1A=B | Cascading Output Pin if A=B |
| 4 | 1A>B | Cascading Output Pin if A>B |
| 5 | 0A>B | Output Pin if A>B |
| 6 | 0A=B | Output Pin if A=B |
| 7 | 0A<B | Output Pin if A<B |
| 8 | GND | Ground |
| 9 | B0 | Data Input B0 |
| 10 | A0 | Data Input A0 |
| 11 | B1 | Data Input B1 |
| 12 | A1 | Data Input A1 |
| 13 | A2 | Data Input A2 |
| 14 | B2 | Data Input B2 |
| 15 | A3 | Data Input A3 |
| 16 | VCC | Positive Supply |
Specifications:
| Product Attribute | Attribute Value |
| Product Category: | Logic Comparators |
| Comparator Type: | Magnitude |
| Number of Bits: | 4 bit |
| Logic Function: | A < B, A = B, A > B |
| Output Type: | Active High |
| Package/Case: | PDIP-16 |
| Propagation Delay Time: | 24 ns, 27 ns |
| Supply Voltage – Min: | 4.75 V |
| Supply Voltage – Max: | 5.25 V |
| Supply Current – Max: | 20 mA |
| Minimum Operating Temperature: | 0 C |
| Maximum Operating Temperature: | + 70 C |
| Mounting Style: | Through Hole |
| High Level Output Current: | – 400 uA |
| Low Level Output Current: | 8 mA |
| Operating Supply Voltage: | 4.75 V to 5.25 V |
For more electrical and mechanical dimensions check the datasheet
How 74LS85 Comparator Works?
The make the IC workable we will need to connect the IC first with the power supply and further with a TTL device or microcontroller to use it. In other ICs like an adder, the IC starts working from LSB but in the comparator, the IC will start comparing from the MSB. The data will start comparing from MSB and it will go to the LSB. In this case, at any point, if data A is bigger than the data B or vice versa then the output will be decided. In IC the input data will be of two parts with 4-bit each and output data will be of 3 bits. In input, the data can be any binary numbers but in output data, only one bit will be HIGH other will be LOW and all three bits will describe the output on the base of input. Here’s the circuit of IC as a 4-bit comparator.
Figure 1: IC working as the 4-bit comparator
The IC can be used to compare the data of more than 4 bits by using the input expander pins. In IC those pins are Pin 2,3 and 5.
Truth Table 7485
