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24-channel digital I/O module integrated in rugged enclosure with 50-pin straight up connector

Acces I/O



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Product Datasheet

Product Manual

This product is RoHS-compliant

Please contact us if you require a non-RoHS version.

Please note, Acces I/O products are often available in an array of different options, of which only a selection will be displayed here. Please contact us for pricing if you require a product option that is not shown on our website.

The USB-DIO24-CTR6 is an ideal solution for adding portable, easy-to-install digital I/O and counter capabilities to any computer with a USB port. The USB-DIO24-CTR6 is a high-speed USB 2.0 device, offering the highest speed available on the USB 2.0 bus. It is fully compatible with both USB 1.1 and USB 3.0 ports. The unit is plug-and-play allowing a quick connection whenever you need additional I/O on the convenience of a USB port.

The board features 24 bits of CMOS/LVTTL-compatible digital I/O with high-current capabilities and two 82C54 counter/timers. Each digital port can be programmed to accept inputs or to drive outputs. Groups of two 8-bit ports and two 4-bit ports are designated as port A, B, CHi and CLo. Power is supplied to the card via the USB cable. For higher current sourcing capabilities, external power may be used. The I/O wiring connections are via an industry standard 50-pin male connector any of our optional terminal block cards. For external circuits, fused +5V (or 3.3V) DC power is available on pin 49 of the connector. The resettable fuse is rated at 0.5A.

All 24 I/O lines are buffered by a type 74LVC8T245 tristate buffer transceiver capable of operating in CMOS (+5V) or LVTTL (+3.3V) modes. The mode is set for all pins simultaneously via an onboard jumper. When using CMOS mode the outputs can sink or source 32 mA. In LVTTL mode the outputs can sink or source 24mA. The 24 bits are divided into four ports: two 8-bit ports (Port A, Port B), and two 4-bit ports (Port CLo, Port CHi). Each port can be configured independently via onboard jumpers for pull-up, pull-down, or no bias. Each port can also be configured independently, via software command, for use as inputs or outputs.

The board is available in two models, one with two 8254 counter/timer chips, and one without. The unit without counters is optimally suited for use with industry standard solid state module mounting racks. Because this version does not place any counter signals on pins normally reserved for Ground, it provides a perfectly compatible 50-pin IDC connection.

The USB-DIO24-CTR6 is designed to be used in rugged industrial environments but is small enough to fit nicely onto any desk or testing station. The board itself is PC/104 sized (3.550 x 3.775 inches, 90mm x 96mm) and ships inside a steel powder-coated enclosure with an anti-skid bottom. A DIN rail mounting provision and extended temperature is available for installation in industrial environments.

Unlike most USB digital I/O products which primarily use a human interface device (HID) driver, ACCES offers an easy to use, Windows-based, custom function driver optimized for maximum data throughput. This approach exposes the full functionality of the hardware along with maximizing the advantage of using the high-speed USB 2.0 bus.

Firmware 2.0 Features

Onboard Watchdog Timer. If your application fails to communicate with the device within a specified timeout period (up to hundreds of seconds) the onboard intelligence will change all the output ports and values to the configured "safe" state.

Need a DAC to power up at 1 Volt? Want the digital output port to generate a square wave as soon as the firmware is loaded? Now you can configure the power-on default states for all possible outputs.

Command the onboard intelligence to perform digital input debouncing and latching. Debouncing time constants from hundreds of microseconds to hundreds of milliseconds allows a wide variety of low-pass filtering options. Latching inputs record low- and high-going pulses that your application may otherwise have missed.

PWM allows programmable duty-cycle square wave output on any digital output. MAP is our new Matrix Mapping feature, wherein you can command the onboard intelligence to automagically copy the state of one bit onto another bit, with no computer interaction required.


The OEM (board only) version is perfect for a variety of embedded applications. What makes the OEM option unique is that its PCB size and pre-drilled mounting holes match the PC/104 form factor (without the bus connections). This ensures easy installation using standard standoffs inside most enclosures or systems. The board can be added to any PC/104, PCI-104, or PCI/104-Express stack by connecting it to a USB 2.0 port usually included on-board with embedded CPU form factors. This is especially important since many newer CPU chipsets do not support ISA and have plenty of USB ports. The USB-DIO24-CTR6 OEM board can also be installed using standoffs inside other enclosures or systems. For embedded OEM type applications, an additional miniature USB input header is provided in parallel with the type B connector.

High Retention USB Connector

The ever-growing presence of USB in the industrial/military marketplace has driven the need for USB connections to be reliable, dependable, and unfailing. Gone are the days of loose USB connections. A type B USB connector is used on all ACCES USB/104 products which features a high retention design that complies with the class 1, Div II minimum withdrawal requirement of over 3 pounds of force (15 Newtons). This connector has an orange color-coded insulator to quickly differentiate it from standard USB connectors. Using these USB connectors increases reliability in your system and ensures a tight connection. For embedded OEM type applications, an additional miniature USB input header is provided in parallel with the type B connector.


  • Home
  • Portable
  • Laptop
  • Education
  • Laboratory
  • Industrial Automation
  • Embedded OEM


Two 82C54 chips each include three 16-bit counter/timers factory configured in an optimal module for use as event counters, frequency output, pulse width, and frequency measurement (see the Block Diagram).


The USB-DIO24-CTR6 is available with optional cable assemblies, DIN-rail mountable screw terminal boards, and an external AC/DC power supply. The pin connections are also compatible with industry standard I/O racks such as ACCES A24A, Grayhill, Opto 22, Western Reserve Controls, etc. with optional cable. Also available is our low cost IIB-24 which will add optical isolation to any standard 24-channel digital I/O port on a 50 pin connector. To make use of the miniature embedded USB header connector, we offer a variety of micro-fit USB input header cables.


The USB-DIO24-CTR6 utilizes a high-speed custom function driver optimized for maximum continuous data throughput of 16 MB/s that is hundreds to thousands of times faster than the USB human interface device (HID) driver used by many competing products. This approach maximizes the full functionality of the hardware along with capitalizing on the advantage of high-speed USB 2.0. The boards are supported for use in most USB supported operating systems and includes a free Windows and Linux compatible software package including OSX. This package contains sample programs and source code in C# (.NET), Delphi, and Visual C++, for Windows. Third party support includes a Windows standard DLL interface usable from the most popular application programs and includes example LabVIEW VIs. Embedded OS support includes XP Embedded and Windows Embedded Standard.