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Digital input and output

The Micro1401-3 has full, separate 16 bit digital input and output available on the back panel Cannon 25 pole plug and socket, marked Digital Inputs and Digital Outputs.

Micro1401 Digital input and output

Bits may be read or set individually, or bytes, or the whole word may be read/set.

For the top byte, applications can program the input port so that changes of state are detected and can be timed. The output bits can be associated with clock 2 so that they can change on clock 2 ticks. Top byte output bits 0 and 1 are available also as BNCs on the front panel, and if enabled by software, inputs 0 and 1 are fed from the front panel BNC Event Inputs 0 and 1.

Micro1401 Digital input and output

Technical details

The outputs are buffered through devices which will source or sink 24 mA. Outputs 8 - 15 may be made sensitive to a controlling input, such that they can be made high impedance (tristate). They are normally permanently enabled by closing pole 3 on the Options switch pack. If that switch is opened, these outputs are enabled by holding output socket pin 11 to ground. Disabled outputs become high impedance.

The inputs read 1 if not connected, as they are terminated by a resistor to +5 Volts internally; rear panel value 4k7, front panel 47k. Input voltages of more than 2.0 Volts will always read as a logic one. To appear as logic 0, the input must be pulled down to below 0.8 Volts for at least 1 microsec, which will require a current of approximately 1 mA (rear), 0.1 mA (front).

When data is read by the Micro1401, the DATA TRANSMITTED output line pulses low for 1 microsec. When data is written to the outputs, the NEW DATA READY line will give a similar low going pulse. These outputs are intended to drive a standard TTL load, pulling 1.6 mA at the low level.

There is a 5 Volt output available on pin 25 of both the digital input and output ports. This output is internally fused (250 mA). It is intended to power one or two chips for interfacing purposes. Please do not replace the fuse with a larger value to run external equipment!

We have occasionally had problems with users who blow this fuse very regularly. This is usually caused by plugging a 25 way connector with a metal shroud into the digital input crookedly, touching pin 25 with the shroud, which blows the fuse. If you have this problem, the simple solution is to make this connection with Micro1401 power off, or use a connector with a plastic shroud.

Cambridge Electronic Design Limited

Registered in England: 00972132

Registered office:

  • Cambridge Electronic Design Limited,
  • Technical Centre,
  • 139 Cambridge Road,
  • Milton,
  • Cambridge CB24 6AZ

VAT: GB 214 2617 96

Producer registration number: WEE/BD0050TZ

Terms and Conditions of Sale

For our US customers, we can provide tax form W-8BEN, that identifies us as a UK company.

DUNS: 219151016
NAICS: 423490
Commodity codes
Hardware: 84716070
Software: 85235190

By email:


By post:
  • Cambridge Electronic Design Limited,
  • Technical Centre,
  • 139 Cambridge Road,
  • Milton,
  • Cambridge CB24 6AZ
By telephone:

(Int.+44) (0)1223 420186

From North America (Toll Free):

1 800 345 7794