CD40116 Bi-Directional CMOS to TTL Level Converter

The CD40116 is an 8-bit high-speed bi-directional CMOS to TTL level converter for use in computer buses where CMOS and TTL hardware interface. This component provides a single-chip solution in a PDIP package. The Vdd (pin 1) and Vcc (pin 22) leads set the threshold voltages for the TTL and CMOS logic levels. Hence, logic levels converted from Vcc to Vdd, and Vdd to Vcc, provide TTL to CMOS and CMOS to TTL translation.

Pins A1 to A8 provide a bi-directional CMOS interface, whilst pins B1 to B8 provide a bi-directional TTL interface. This chip is not auto-sensing, and control signals, "Enable" (pin 10), and "Disable" (pin 13), are required to be set, because these pins set the direction of the data flow. There are modern equivalents to this chip that provide bi-directional data flow with an auto-sensing bus, however none in a PDIP package that I am aware. Manufactured by Harris, and currently discontinued, there may be some old stock lying around in eBay. It is very easy to use a pair of tri-state octal buffer chips to provide the same functionality as this chip. Octal buffer chips are very common, for example, the SN74HCT244N is a very low cost chip that is readily available from most electronics shops for a few pence. A pair of such chips provides the necessary logic level conversion required.

Control Signals

When a control signal is applied, one buffer becomes active, whilst the other remains inactive. When the buffer is inactive, its output pin is in a high impedance state to prevent any signals sinking through it. For TTL to CMOS conversion both the Enable and Disable pins need to be set to logic 1 and therefore connected to the Vdd rail. For CMOS to TTL level conversion only the Disable pin has to be set to Logic 0, whilst the Enable pin setting is irrelevant. If the Enable pin is set to Logic 0 and Disable Pin set to Logic 1 all the pins go into a high impedance state for both CMOS and TTL sides.

The CD40116 is a great chip worth having in your toolbox but you can make something similar using a pair of low-cost octal buffer chips and it would work just as well. I have some buffer projects in the Raspberry Pi Projects section of this site worth seeing. The great thing about the CD40116 Bi-Directional CMOS to TTL Level Converter is that it is essentially a pair of octal buffer chips within a single PDIP package thereby saving PCB space.