device for converting binary strings into states for switches

Not difficult, unless I have misunderstood you.

The serial data from the computer is clocked into an N-bit shift register. The parallel outputs from the shift register are connected to an N-bit latch. Once the shift register is filled, the latch transfers all the N bits from the shift register to the latch output, simultaneously. There are ICs which combine the shift register and latch, for fixed values of N. These chips can normally be cascaded for larger values of N. I have one electronics board, out of a pinball machine,, which has 4 cascaded serial in, parallel out, chips each with N=48.

A standard PC has a parallel port with N=8. For bigger values of N, it is very simple to use 3 of these output lines from the parallel port of the PC, one for serial data, one for the clock and one for operating the latch. The computer is programmed to output a series of bytes to the port, the first and second byte will contain the first data bit on one line and with alternate values on the clock line, whilst holding the latch line constant. This is repeated for all the N bits, with the final output byte clocking the latch line.

Dear Sue,

Thanks for your answer. Your idea is very good.

I have some small questions (because I'm a computer scientist and I'm not very good at electronics):

I didn't understand why the data from my computer is sent in a serial way to the shift register. I don't want to send bit by bit. I want all the operations to be performed in a parallel way.

I can generate those binary strings into some registers (in a 32 bit register I can have 32 bits corresponding to 32 switches). In this case should I send them one by one to the shift register? Can I do this operation in parallel.

Could you also explain me again (or give me some links) on how could I extend the method for more switches? I didn't understand that part very well.

Thanks. Laura

Pal> > Hi,

The standard data ports on a computer are 8 bits wide. To send more than 8 bits, you have to either sequentially program a number of ports, or sequentially send data to the same port.

The former can be achieved by adding more parallel ports to the computer

- either cards with printer ports or (expensive) I/O cards with 8,

However, it is possible to use a single 8 bit port (eg a printer port) to send as much data as you want. By using it with a shift register and latch.

If you do a Google search on serial in, parallel out shift registers, you can read up on how they work. You can make a shift register as long as you want, simply by adding extra stages.

If you aren't at all into electonics, a card like this may be a better bet for you:

ebay item 7619325709

The card above is only 24 bits wide. You can get them in anything from 8 bits to 128 bits wide. However, you can use several cards together - although they won't work quite simultaneously - there can be a delay of a few mSec between the outputs of one card changing and the outputs of another.

Hi all,

I have received several posts and I've got confused. I mean I'm not very sure if I can do this operation purely in parallel.

I mean: I have that binary string generated into a variable of type int (max 32 bits from C language). Or, imagine that I have it into the processor registry AX. Or, in a smaller registry/variable (that has only 8 bits).

Can I map this string into states of the switches doing only operations in parallel? I need NO serial operations (I mean I DON'T want to have an operation which traverse that string bit by bit - for shift purposes, etc). The serial operations means O(N) complexity, and I want to avoid that. I want to make only O(1) operations. This means that everything should be performed in parallel - from that binary string which is stored in the bits of AX registry .... to the states of the switches that I want to turn ON/OFF.

Is this possible?

Thanks, Laura

Hi all,

I have received several posts and I've got confused. I mean I'm not very sure if I can do this operation purely in parallel.

I mean: I have that binary string generated into a variable of type int (max 32 bits from C language). Or, imagine that I have it into the processor registry AX. Or, in a smaller registry/variable (that has only 8 bits).

Can I map this string into states of the switches doing only operations in parallel? I need NO serial operations (I mean I DON'T want to have an operation which traverse that string bit by bit - for shift purposes, etc). The serial operations means O(N) complexity, and I want to avoid that. I want to make only O(1) operations. This means that everything should be performed in parallel - from that binary string which is stored in the bits of AX registry .... to the states of the switches that I want to turn ON/OFF.

Is this possible?

Thanks, Laura

Sure, you need a 32bit parallel port of some sort though.

If you want the computer to turn the switches on/off then the switches have to have a control input - these sorts of switches are called relays. A control signal determines whether the relay is on or off. If you have 8 relays, you need 8 control signals, one per relay.

Your starting point is a register which can have the value set by a computer program. Once it is there, you have to get it to appear as control signals coming out of the computer.

A card, such as the ebay one, has a number of library routines for taking a register content and making it appear as control signals. eg Write_port_A(val). You set the variable val to the register value and then call the library routine. These typically are 8 bit. So, to output a 16 bit variable, your code may be:

.. ... .. Write_Port_A(High_byte(val)); Write_Port_B(Low_byte(val)); .. .. .. This assumes that val is a 16 bit unsigned variable.

Note that, as the second library call happens after the first one, there will be a delay of a few mSec between the two sets of 8 relays (computer controlled switches) changing. In practice, this tiny delay is less than the variation in operating times of the relays themselves.

If the tiny delay matters, or if you want large values of N, then you have to allow for the computer outputing data byte by byte (or bit by bit) and waiting until all the outputs have been set before passing their values to the relays.

That is when shift registers and latches come into their own. The example I gave was a pinball machine with N = 132. There is no way a computer can change 132 bits all in one go - it doesn't have a register big enough to do it. So it has to do it in chunks and a shift register plus latch is a very, very effective way of handling this.