In the article “Excess-3 Gray to Decimal Decoder” we will discuss about its key parameters, application and other details.
As illustrated in figure 1(a), this type of decoder has a 4-bit input and a straight decimal output. The 4-bit input, however, is not encoded according to the BCD system but rather according to the excess-3 (gray) code. The difference becomes apparent in the truth table of table 1. This type of code is usually used in special military or industrial applications and is not frequently found in digital computers. In general, this type of decoder operates in the same manner as the BCD-to-decimal version, but the internal connection of the gates must be different.
One important feature of this decoder is that all output terminals will be “1” whenever an invalid code appears at the input. As apparent from the truth table, it is possible to have as many as six invalid codes since up to 16 possible combinations exist for a 4-bit signal.
Key Parameters of Excess-3 Gray to Decimal Decoder
The electrical characteristics are essentially the same as those of the particular digital IC family.
- Maximum power dissipation: 20mW is typically for low-power TTL.
- Quiescent current: The current drawn when no operations are being performed. 400 mA is typically for low-power TTL.
- Propagation time delay: the time required from input to output. In some combinations a two-gate delay occurs and this is typically 18ns, while a three-gate delay is typically 22 ns for low-power TTL devices.
Applications of Excess-3 Gray to Decimal Decoder
This type of code conversion often takes place in long-distance communications equipment and in specialized industrial and military equipment where rotating components, such as synchro and servo devices, are used.
Representative Part Number: Signetics 5443.
Table 1: Truth Table of Excess-3 Gray to Decimal Decoder
| A | B | C | D | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
| 0 | 0 | 1 | 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 0 | 1 | 1 | 0 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 0 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 0 | 1 | 0 | 1 | 1 | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 |
| 0 | 1 | 0 | 0 | 1 | 1 | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 |
| 1 | 1 | 0 | 0 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | 1 | 1 | 1 |
| 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | 1 | 1 |
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | 1 |
| 1 | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 1 |
| 1 | 0 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 |