I've got an early 90's display unit that consists of 8 LED light bars, a 3-digit LED PSI display, a dot matrix LED message display (HDSP-2112) and a LED counter that ranges between -19999 and 19999. The light bars are controlled by a ULN2803A and the Numic displays by three-ICM7218AIJI's. Use of these devices with the Arduino is well covered on the interwebs, and I'm grateful for that.

My problem lies in what the ULN/ICM's are connected to - MC74HC273AN's.

There is little-no information regarding their use with an Arduino, which worries me. I'd like to try to use as much of the original hardware as possible. I've done some light experimentation with the 74HC273AN-ULN2803A combination as connected on the PCB and had mixed, mostly poor, results, via a modified "Blink" sketch wherein I connected D2-D9 to the Data Input pins on the MC74HC273AN. Sometimes the LED bars would illuminate, sometimes not.

Many of the four MC74HC273AN's on the board seem to have common Data Input pins, which is baffling to me, but all the Clock pins are separate.

Has anyone had any success in using an Arduino with these chips - or do I need to devise a different way of interfacing with the LED controllers?


And data inpits of the 273s are connected to what? It should not be hard to use them. But I guess if you are using Arduino you can remove them and drive directly what they were driving.

The 74HC273AN chips have common data pins because they use the same data outputs to control them. (this saves pins required on the chip(s) driving them and saves wiring) By using the clock pin on one of the chips it tells the chip to use the current data being input, otherwise it ignores it. This allows the data inputs to be the connected to the same output. (See the datasheet)