Multiple Relay Control with Microcontroller

Use at least 3 banks of TTL latches on a 8 bit bus with chip select wired to other ports.

You can multiplex the outputs. In this way you can control 64 switches in any combination with a 8 line (+3) Centronix printer port.

As Hendrik says..you can latch data out.
It's always a trade off...big chip with plenty of I/O gives simplicity, or smaller cheaper chip with more stuff hanging on it to control the IO and thus more complex software & circuitry. Alternative is a 2 chip solution, one handling the user interface and one handling the IO... of course you then have the comms probs of getting the two talking to eachother.
I've designed all 3 option in my time.
Big chip with few peripherals, keeping it simple is my personal preference.
Del
(BTW... GA Hendrik, hows that ol' Lion of yours doing?)

I agree with this approach. I used to design ASICs and had to keep the transistor count as low as possible to fit on a hybrid.

The ULN series has serial input drivers. You can stream out the parallel data then latch the output of multiple chips simultaneously.

I think I will go serial all the way. Parallel data is slightly outdated and involves too much soldering. I will probably build my first design sweet and simple as Del stated. Then if I need more features I will multiplex the latches or find a serial or DAC chip as Hendrik stated. You always hope for that miracle IC you never knew about that does exactly what you want.

Thanks for the input guys.

Just use a 4 to 16 Decoder chip so you output a Hex (0000-1111) code to the 4 inputs of the 4 to 16 Decoder chip. The 16 output could drive a latch and be used to drive a relay on/off. Be careful to de-glitch your relay latches with a output strobe delayed slightly after outputing the 4-bit code since only one of the 16 lines can be on at a time. Therefore the latches are needed. Say if you want a certain sequence of relays on in S/W you address each relay one at a time through software, pulse the latch strobe after the Decoder is at a steady hold decoded state. The same for turning a relay off. There is very little latency between addressing the relays in this manner as opposed to 16 dedicated RISC lines driving directly.

This sounds like a lot more external circuity as a hardware method or just use more I/O. There is also serial to parallel registers. I would use RISC's that had lots of I/O lines and drive the relays directly.

Just use a 4 to 16 Decoder chip.

That's OK, but of course it means you are only driving relays one at a time.
Del
BTW...relays, ok for mains, but for DC switching FETs are good these days.

Yes but if you read carefully I said you must latch the 1 of 16 outputs so now you can have multiple outputs driving the relays. A 4-16 Decoder is not buy itself a latch. If you address another line that output is inactive unless latched by another chip to do this. But I think it is to much hardware and a microcontroller is supposed to reduce the amount of hardware and is designed to do that.

Have a look at TLK9201 from TI, it`s an SPI-controled 8-channel latch with brownout detect, others are available from Maxim with I2C-control.