BCD Thumbwheel Switch is used to input-set data in digital form, this can be read by digital circuits, uC and uP systems and PLC-SCADA Interfaces.
Temperature Measurement and Control
In the early transition of analog to digital, before uP became acceptable, Digital systems without uP were made, it even had printers, RAM and displays. The uP systems were coming in, uC had not yet come and uP systems had to still win the confidence of the Prudent Industrial Design Engineer.
The drawbacks of uP based systems used in Computers, in those days were.
- Power Consumption was very high, needed SMPS.
- Many chips, a CPU had a Retinue of many chips.
- Large Board, Double or Multi Sided due to Bus.
- Fussy, Hangs on minor Power Glitches or Resets.
- Needs Firmware Development and Tight Testing.
- Investment in all these areas, Tools and Manpower.
These made Industrial Automation with uP a challenge. CMOS digital and mixed devices and custom application devices were more easy to implement and affordable.
The coming of Low power CMOS uC changed everything and embedded systems became smaller and robust. These were packable in DIN standard and DIN Rail Mounting enclosures.
Coming back to inputting digital data. CMOS uC and Ni-Cd Battery backed up RAM with keyboards made thumb-wheels and other methods less attractive for digital data inputs. Then the Li-Ion Battery, Flash Memory in Combination with Application Specific uC and SOC have made inputting, retaining digital data very easy and affordable.
Printer Port Circuit to see the inputs logic state and drive a few outputs. The preset control program helps test out the circuit operation. Use safe 5V Levels.
Simple Printer Port Interface, Input-Output – del20005
So i made some example circuits and VB programs for students. A demo Milli Volt source, Digital Voltmeter and Logic Analyzer examples for learning, are on these pages. I have tried to derive power from PC itself in this circuit, Care should be taken not to short circuit or apply any voltages above 5V to PC Port Pins.
74HCT373 is the only chip used to interface the real world with the PC Parallel port. The Inputs to this chip are protected with Resistors and Zeners. 3 outputs of PC are also monitored using LEDs. This gives you a IO interface for experimenting. Below is a Preset counter simulation that you can use with this circuit for testing. Schmitt Triggers and Signal Conditioning may be required for Noisy and Non-Sandard Inputs.
Some outputs are via Dual Differential Line Driver type DS8830. This device will interface with standard TTL systems.
The differential outputs are balanced and are designed to drive long lengths of coaxial cable, strip line, or twisted pair transmission lines with characteristic impedances of 50 ohms to 500 ohms. Differential transmission is superior to single wire transmission in that it nullifies the effects of ground shifts and noise signals which appear as common mode voltages on the transmission line.
Mixed and Interface Circuits
If the signal voltage at the end of the line is found to be of insufficient magnitude then the following circuit may be used (at the recipient equipment end) to boost the levels.
This circuit is a small representation of a very low cost printer sharer. It has no Active Devices only diodes.
Pot the product in epoxy with a black dye, they serve the purpose. Output impedance of this circuit is high, sink is 220K source is 3.9K+ so use some buffers or drivers at Output. If Buffers are omitted, then fix this unit on the printer port connector of Computer.
when Enable A is at float-high impedance or low the output O1-O4 is not influenced by A1-A4 inputs. If Enable A is made logic high or 5V then A1-A4 is available at O1-O4.
By turning Enable A or Enable B high, you can route the data A1-A4 or B1-B4 to the output O1-O4, you can also mix data and you can expand to any number of input sets or data width. 1N4148 is fast, 4nS, that makes this data switch quite fast. This circuit cannot drive long printer cables without drivers. They will load the output.