Digital Techniques

What is meant by Digital?

Digital Signals are the fundamental language of modern computing. Digital devices are based on the concept of binary states or logic being based on two values, One or Zero. We can assign a Logic One or Logic Zero to the status of a device as being either On or Off. Logic One and Logic Zero are represented in the real world as two voltage levels, High or Low. We can use this concept in Data Acquisition and Logging systems to monitor and control various devices by, for example, reading the state of switches and writing values to devices to turn devices on or off as required. We can also use digital signals in counting applications for Totalizing, Speed calculation and Positioning.

What voltages are used to represent Logic levels?

There are a number of standards. Transistor to Transistor Logic (TTL) is very common and used extensively in MCC products. TTL uses 0 volts (Low) to represent Logic Zero and 5 volts (High) to represent Logic One.

What is DIO?

DIO is the Industry abbreviation for Digital Input and Output and is based on reading or writing digital values to and from various devices.

What does the bit number represent?

The Bit Number describes the number of DIO lines a device. For instance our PCI-DIO24 is a 24-bit DIO card for the PCI bus based on the industry standard 82C55 integrated circuit and features 24 separate digital IO lines.

What can I control or monitor with my DIO board or module?

You can use DIO to monitor and control Lighting and Heating or in fact any on/off device. TTL devices are able to handle low values of current only and as mentioned above, operate on either 5v or 0v. They therefore need to be connected to TTL compatible devices or to an interface known as a Digital Signal Conditioning circuit.


What is Digital Signal Conditioning?

Digital Signal Conditioning can be achieved through the use of Solid State Relays (SSR) and Electromechanical (EM) Relays or via a user designed interface specific to the controlled device. Optical and Voltage Isolation, SSRs and EM Relays can also be built into DIO devices to create an integrated DIO solution for user convenience. We recommend the use of SSRs and can supply industry standard SSRs for mounting in External Racks, Din Rail and Integrated DIO modules.

Can I use a Voltage Divider?

Yes if your input voltage is a little higher than the 5V input of a TTL DIO line then a simple voltage divider may be used. We do not recommend using a Voltage divider for high voltages.

What is Sourcing and Sinking?

Current Sourcing is the amount of current a digital line can supply when it is the Logic One or High state. Current Sinking is the amount of current in a digital line when it is in the Low state. These values are typically in the low mA range although higher current options (Hi Drive) are available.

What else do I need to consider?

It is good practice in digital circuit design to hold all inputs and outputs to a particular value whether they are to be used or not so the states are known during start up, reset and fault conditions. This prevents unintended operation of devices which could lead to unsafe situations. We use Pull Up or Pull Down resistors for this purpose. In a Pull Up circuit the inputs or outputs are connected via resistors to the Logic High supply and in a Pull Down circuit the digital lines are connected via resistors to 0 volts (DigGND). The resistors are usually in the form of a Single Inline Package (SIP). Some of our products feature programmable PU or PD circuits, some have switch selectable circuits, while others have places for the user to install SIPs in either Pull Up or Down mode.

What is Switch Debouncing?

When a mechanical switch or relay operates the contacts tend to bounce momentarily creating a pulsating noise signal which can lead to false readings. The solution is using a low pass filter circuit on the input.

Measurement Computing Digital IO devices come in standard TTL and Hi Output devices supplied as 24, 48 or 96 “bits” normally configured in 8 bit ports. We also supply integrated modules, External Relay Bases and SSR Racks to enable high voltage monitoring and switching (IO).

Our selection of DIO Devices

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