Paper tape.

Since the dawn of telegraph until the early mid 1990’s Paper Tape was used as a code storage solution for computers. Other companies had different types of setups and specs. IBM and HP both used Punch Cards as their variant of paper based storage.

How does Paper Tape work?
Simple, a typical tape reader uses Photo transistors to sense IR light. A punched hole repents a ONE and a non-punched hole is a ZERO. It reads the tape data horizontally. So while the tape is running through the reader and a punched hole lets the IR light hit the Photo-Transistor at a certain Address then it can put the pieces together and get a Binary number for what the Letter or Digit would be.

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Also there are smaller holes in the middle of the tape that a sprocket will drive the tape. There are different variants such as 5-bit up to 8-bit.

Some people have built DIY readers and will use the smaller sprocket holes as a timing clock pulse. I imagine the electrical mechanical ones had the motor with the drive sprocket timed to work as a clock pulse.

There are many ways to integrate a Paper tape reader to a computer such as injecting the code directly into the Address  BUS and Data BUS that was typically Parallel. Also it could be done via Serial.

The Punch machine came in assortment of flavors. The most common was a Teletype system.

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Typically a programmer would just hit print and the machine would punch out the tape, or the programmer could make it punch the tape in realtime. Then the programmer could use it on a reader and even ran again into a machine to copy the tape.

If you think about it this was a true means of a hard copy however for today’s world for modern software it would take many rolls of tape for a single program. For an example Microsoft Windows is around 50 Million lines of code and Altair BASIC 8K can be stored on a duct tape size roll of paper tape. This is in a span of 40 years. Who knows what will be next in 40 years for data storage. I remember when people said nothing will top the CD. Yet we have Jump Drives and SD cards that are faster and can hold more than a Blu-ray disc.

Reason why I brought all of this up is because I was thinking to make a Tape Reader for my Retro z80 Computer. It’s very simple for the reader but the tape punching would be a major pain to do unless I dished out a few hundred for a punching machine. I could use a non-standard format but I would be reinventing the wheel.

Some z80 Progress

Waiting for parts for my Homebrew Z80 Computer is like watching paint dry or grass grow. I got my Zilog Z804C0020PEC chips in a week ago and finally bread boarded a test circuit that puts the CPU into NOP (No Operation) mode. It just does an endless count,  with a 555 Timer and some LEDs on the Address pins you can see the count.

I found the test circuit from this site.

I also have on my Bread Board a typical Crystal oscillator running at 4MHz but you can’t see the LEDs count away since four million cycles a second can’t be seen with a human eye.

20170624_221100 It’s kinda hard to see the three LEDs on the right are on. I’m using some super old LEDs. The far right is the most significant byte and the left is the least significant byte.

So I have a good working CPU. Now to find a way to test other parts and then start planning on a PCB.

News 6/10/17

It’s been a while. Been busy at work and this is the first weekend in a while that I have some free time.

Last month I finally got my junk together by buying a bookshelf from a thrift store so I could properly store everything instead of piling it up in the corner.

I also bought early last month a Eico 950 Resistance, Capacitance, Comparator Bridge. It’s so old it uses Vacuum Tubes. I bought it from Ebay for around $40.00. The Ebay seller said he restored it but when I opened it up the unit still has a lot of paper caps that are probably ticking time bombs. Some time I plan on recapping it and replacing the AC cord with a polarized one.

To make some extra room on the work bench I put my Analog scope in storage. Before doing so I wanted to checkout inside the case since I never opened it up. Besides some dust bunnies I found something that explaned why the triggering function was a PITA to use. I fixed it and it triggers much better.

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I was visiting my mother and found an old V-Tech learning computer that my brother and I use to play with when we were kids. I took it home and took it apart to clean it up since it had a good layer of nicotine stains on it and to my surprise it has a z80 and  static RAM inside of it.

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I’m thinking to hack it and toss on a Zilog SIO or PIO chip and a EEPROM and see if I can do something with it. Although I’m already building a z80 from scratch. Speaking of that I got some needed parts on order but will take a while to show up. I ordered some z80 20MHz CPUs from China. They claim they’re new old stock but we shall see.

Last week I got some ATMega328P-PU ICs for two bucks that I ordered months ago and forgot about. I tested them out and they didn’t want to work. After looking in my email for the invoice it said the chips were preprogrammed with the Arduino bootloader. After spending about two hours trying some stuff I finally got them working. I had to reset the “fuses”. I plan to use a couple of them for a Serial Terminal project for my z80 build. I would like to finish my TerminalScope clone but Grant Searle’s is cheaper and more customizable. I might go find myself one of those portable B&W TVs and make a mini old school looking dumb terminal.

 

My HP TouchSmart TX2 laptop bit the dust. When you power it on the Cap Lock and Number Lock lights blink and no picture comes up on the monitor. I found out the GPU had loosen up on the mainboard. My Acer Laptop has a few issues as well and I think it is starting to fall victim to GPU failure aswell so I pulled out the old Family desktop my wife has had for eight years. It’s a Gatyeway DX4200-09 with 4GB of RAM and a 500GB Hard drive. I tossed in a nVidia GeForce 220 GT and a USB 2.0 PCI card for more USB ports. It’s not a bad system, I installed Windows 7 on it for now and works quite well. I might toss in a second hard drive and dual boot Debian Linux on it.

Elenco XP-720 Modifications

A while back I bought a Elenco XP-720 off of CraigsList. It was $40.00 and seemed like a deal at the time.

I brought it home and started to play around with it and realised it is missing some major features such as Meters and an Output ON/OFF switch. So I quickly bought the meters and added an Output ON/OFF switch. The switch has a lot of bounce but im not a NASA engineer so it works for my needs.

When the meters arrived and I got around to putting them in I had to back trace the wiring to what went where. I noticed a few things such as the constant 5V+ section had a 16v rated cap when it outputs around 20v from the transformer and rectifier. So I switched it out to a 50v 105*C rated cap. I think I even used double the capacitance as well since after all it’s only a filter cap. The meters I bought were not what I expected. The voltage sense and power are tied together so after I get around 2.6V the meters kick on. I need to figure out how to add a voltage sense lead or get better meters.

I currently have the constant 5V+ disconnected. I was thinking of a way to get a 3amp variable voltage output but after looking at the schematic the input of the regulator is in a voltage divider path. I would have to rebuild the whole circuit for that to work right.

20170610_193011The left Meter is for the positive voltage and the right Meter is for the Negative voltage. Between the pots is the Output ON/OFF switch. The smaller switch between the meter and positive pot was going to be a current limit LED but I think I’ll use it for the constant 5V+ Output ON/OFF.

I goofed on cutting the holes for the meters. I used a Drill for the four corners for each hole and a Coping saw with a metal-cutting blade. Kinda made myself mad since I measured everything five times and still made them a bit big.

I’m planning to repaint the case black and use white lettering for the front panel. This way I can use some black silicone to fill the gaps of the meters. Unless I have to change out the meters for the proper voltage sense and power.

I also wanted to add some 10 turn pots but the pair I ordered from Ebay were not the correct value. The Ebay seller sent me 100K instead of 2K that was advertised.