EEVblog #1021 - Retro Radio Shack TRS-80 Color Computer - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: September 09, 2017, 10:09:15 pm
content: Dave tears down and takes a look at the classic 1980 Tandy / Radio Shack TRS-80 Colour Computer.Includes hacking a composite video output circuit to get it working on a modern HDMI computer monitor.Service Manual With Schematics:http://www.colorcomputerarchive.com/coco/Documents/Manuals/Hardware/Color%20Computer%20Technical%20Reference%20Manual%20(Tandy).pdfManuals:http://www.colorcomputerarchive.com/coco/Documents/Manuals/Hardware/

author: rsjsouza
author_rank: Super Contributor
timestamp: September 09, 2017, 10:37:39 pm
content: These were famous in Brazil as clones named CP400 from a company called Prológica. This will be a video that will bring good memories... Thank you! (Haven't watched it yet)

author: joeqsmith
author_rank: Super Contributor
timestamp: September 10, 2017, 12:20:17 am
content: I had one of these. Did you get the technical reference manual for it?

author: Lightages
author_rank: Unknown
timestamp: September 10, 2017, 01:53:46 am
content: I had a CoCo 1. I purchased it with 4k memory and then spent something like $100 Canadian dollars in 1981 to buy 16k to upgrade it. I then spent then next couple of months designing the logic circuitry to make 64k work in it. Just when I finished that, Radio Shack released the 64k model and the firmware needed to make it work came out too. Oh well, it was a good exercise. All that was needed was something like a 74SL138 and some bodge wires.The CoCo was actually a relatively powerful machine for its time. The 6809 was actually a mix of 8 bit and 16 bit instructions. I started to design some test equipment cartridges for it but got they never got done. Until the time that the CoCo 3 came out, it was probably more powerful than any IBM, but was marketed as a gaming toy instead of a real computer. That is why it died.

author: jonovid
author_rank: Super Contributor
timestamp: September 10, 2017, 02:57:29 am
content: update-I had a Tandy TRS-80 Coco 2 as the Coco 3 had a RGB output.TRS-80 video color saturation was always a bit too high. the sharpness is poor because of the TV rf modulator.to get good video out of a TRS-80 color computer you need to tap video,like Dave did in the video, or mod it for a RGB output.the color palette is truly trouble. its only4 colors at a time, the color combinations are also trouble.its the horrible NTSC color palette. this is no C64.I have used a white model 64k TRS-80 its the same but better keyboard too. but the same trouble color palette.had a TRS-80 assembler cartridge but moved on to other, so never made a game with it. only stick figure animations.TRS-80 uses special character blocks to make its not so high definition graphics.in B&W two color mode, you can bleed the colors for a rainbow FX by checkerboarding the ALT key character blocks.also to note that the floppy drives in the 80s was so expensivesame gos for the C64 floppy drives. color palette matters in a game.the TRS-80 colors truly suck in PAL video. I still own the old white model TRS-80 cassette deck from the 80'simages- \/a gamecolor palettea white model TRS-80 cassette data/audio deckthe TRS-80 color palette

author: n3vti
author_rank: Contributor
timestamp: September 10, 2017, 03:25:53 am
content: I have a Coco 2, including one still new in the box (don't know what I'm going to do with it), both with 64K. And yes, I still used it up until I moved from my old house back in 2014. I wrote timekeeping programs, and I wrote a program that output morse code whenever you press a key. Perfect for when I was practising for my ham license.The Coco 3 had 128K, I believe.I also have a 300 baud and 1200 baud modem as well as a thermal printer for it.And yes, I still have a couple games, the joysticks, the floppy expansion (Disk Color BASIC, anyone?) and lots more. I'll have to find them and post a picture.I recognize the RF modulator. It appears it's the same kind that RS used to sell separately for DIY stuff. I had a schematic for it with the pinouts, but I have no idea where it went, but it was I think it was 6 pins, 1 for video, 1 for audio, and a common ground.Picture quality is horrible for LCDs, but kinda okay for old CRTs.The reason why you still get the green line on top is because it's programmed to always show black text on a green background. I forget the exact reason, though.

author: bitseeker
author_rank: Super Contributor
timestamp: September 10, 2017, 06:11:56 am
content: The CoCo was my first home computer. I still have it, but alas it's got a memory problem. Powering it up yields garbage characters on the screen. Perhaps I'll dive into it as a winter project.

author: blacksheeplogic
author_rank: Frequent Contributor
timestamp: September 10, 2017, 07:44:47 am
content: Thank you for taking the time to hook it up to the monitor. Love the old computers, sadly I had to thin my collection years ago and now only have the Atari's and a couple of commodore's.

author: rsjsouza
author_rank: Super Contributor
timestamp: September 10, 2017, 10:27:35 am
content: "Hello world"?!? C'mon, Dave, everybody knew the first BASIC code you ever did was:10 PRINT "Dave"20 GOTO 10RUN

author: Fungus
author_rank: Super Contributor
timestamp: September 11, 2017, 10:35:57 am
content: Ninecolors? How does that work? If they're using four bits then why not sixteen colors?Edit: It's all here:https://en.wikipedia.org/wiki/TRS-80_Color_Computer#Graphics_displayPS: Ewww! Calling it a "color computer" is a bit of a stretch....

author: VK3DRB
author_rank: Super Contributor
timestamp: September 11, 2017, 10:48:17 am
content: Excellent video. Great history there. I was a member of the TRS-80 Users Group in Melbourne at the time.Ah remember when...10 X=X+120 PRINT X30 GOTO 10Life was so simple then.

author: Fungus
author_rank: Super Contributor
timestamp: September 11, 2017, 11:22:53 am
content: 800kHz on a 6809 CPU would probably have been faster than the 1MHz 6502s and 3.5MHz Z80s that were the norm back then.

author: free_electron
author_rank: Super Contributor
timestamp: September 11, 2017, 04:10:00 pm
content: i never really understood why these early microcomputers wanted to use DRAM.was it price ? (dram was cheaper than sram maybe ? )it surely wasn't because of ease-of use. you had to waste cpu cycles on refresh , unless you had a dedicated dram controller, which almost none of the home computers had ....

author: rsjsouza
author_rank: Super Contributor
timestamp: September 11, 2017, 04:22:28 pm
content: Quote from: free_electron on September 11, 2017, 04:10:00 pmIIRC from the Commodore book I read a few years ago, that was the reason: cost versus density (pretty much the same paradigm as today).

author: Fungus
author_rank: Super Contributor
timestamp: September 11, 2017, 05:41:11 pm
content: Quote from: free_electron on September 11, 2017, 04:10:00 pmYes.

author: WN1X
author_rank: Unknown
timestamp: September 11, 2017, 10:52:51 pm
content: And let's not forget the 5 1/4 inch floppy drive option that connected via the cartridge slot. It was a nice upgrade to do away with the slow cassette tapes

author: bitseeker
author_rank: Super Contributor
timestamp: September 11, 2017, 11:10:28 pm
content: For me, cassette tapes were an upgrade. Prior to that, it was type everything into the computer and leave it running until you're sure you don't want to run the program anymore. If you want to save the program, retype it on the typewriter before powering off.I only got the benefit of floppies when I upgraded to an Atari 8-bit. Yes, what an improvement in speed.

author: Andrew McNamara
author_rank: Regular Contributor
timestamp: September 12, 2017, 02:13:33 am
content: Quote from: free_electron on September 11, 2017, 04:10:00 pmA dynamic RAM bit is essentially just a single transistor and a capacitor, whereas a static RAM bit is a flip-flop, and requires six transistors - so for a given area of silicon, you got roughly 6x as many DRAM cells as SRAM cells... or you paid roughly 6x as much for the same amount of memory. If I remember correctly, DRAM also tended to be a bit faster (as long as your application was forgiving of the refresh cycles).

author: Red Squirrel
author_rank: Super Contributor
timestamp: September 12, 2017, 03:22:03 am
content: It's interesting how relatively simple these old school computers seemed to be. I mean I'm sure a lot of work went into programming all those chips and the OS etc but it's still simpler than a computer of today. Could make a pretty cool project to try to build an old school style computer using microcontrollers and stuff. I'm sure it's been done.

author: EEVblog
author_rank: Unknown
timestamp: September 12, 2017, 03:57:06 am
content: Quote from: free_electron on September 11, 2017, 04:10:00 pmYes, SRAM was expensive, at least in any usable size like 4-8KB you needed for a half decent BASIC.

author: EEVblog
author_rank: Unknown
timestamp: September 12, 2017, 09:50:35 am
content: Quote from: Red Squirrel on September 12, 2017, 03:22:03 amThere are oodles of old school computer processor projects available, and you can buy many on ebay.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 12, 2017, 11:33:47 am
content: It looks like the graphics shares the DRAM with the CPU. This could slow down the computer a lot - maybe not as much as with the ZX80. With the basic interpreter common in these days, speed can also depend a lot on it implementation.For a small memory, like 1-2 kB SRAM was feasible at that time. A common SRAM chip of that time was 1 K x 4 , thus 0.5 kByte. The 4-8 bKytes Memory needed for a Basic is ROM. AFAIK the Commodore ViC20 had just 2 KByte of RAM. The memory for the screen would be close to 1 kByte in text mode already.

author: Fungus
author_rank: Super Contributor
timestamp: September 12, 2017, 03:14:04 pm
content: Quote from: Kleinstein on September 12, 2017, 11:33:47 amWikipedia says: "... This process is called "Interleaved Direct Memory Access" (IDMA) by Motorola and ensures that the processor and VDG always have full access to this shared memory resource with no wait states or contention."https://en.wikipedia.org/wiki/TRS-80_Color_Computer#SAMEven if it wasn't, the BASIC interpreter runs in ROM so there wouldn't be any wait states for program fetch, only data.

author: GreggD
author_rank: Regular Contributor
timestamp: September 13, 2017, 03:11:06 pm
content: Way back around 1978/9 I made a interface for an orginal B&W Trash-80 parallel port to a ASR33 Teletype.I used a AY51013 uart that required 5V, 12V, and -5 Volts and a total of 35 other ICs. All connections were done wire wrap.The tricky part was to detect a carage return, place a BUSY on the parallel port so I could send a Line Feed out the uart, then unset BUSY.The teletype used a 60mA current loop.Worked great.

author: edavid
author_rank: Super Contributor
timestamp: September 13, 2017, 03:53:47 pm
content: Quote from: free_electron on September 11, 2017, 04:10:00 pmPrice, density, power. SRAM uses a 4 or 6 transistor cell, so SRAM chips have at best 1/4 the capacity of DRAM in each generation. 64K of SRAM would have been too expensive and too hot.Refresh was not a big issue. The Color Computer described in this thread does have a dedicated DRAM controller (part of the MC6883).Quote from: Kleinstein on September 12, 2017, 11:33:47 amThe 6809 CPU is inefficient in using memory bandwidth, so it was easy to design a system where the memory runs twice as fast as the CPU, with alternate memory cycles used for video refresh.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 13, 2017, 04:14:45 pm
content: There was a portable TRS-80 (https://en.wikipedia.org/wiki/TRS-80_Model_100) that used SRAM, however this looks more like based on the original B&W TRS80.SRAM had a significant price tag, and only got low power in the CMOS versions. The CMOS versions were really low power, but AFAIK only available later.With all the interfacing / refresh integrated in the 6883 (= 74LS385) it is easy to use DRAM. So this simple looking 74LS... is a really important part for the CoCo - it's the chip that holds things together. The whole circuit looks a lot like the typical circuit in the 6883 data-sheet.

author: edavid
author_rank: Super Contributor
timestamp: September 13, 2017, 04:27:26 pm
content: Quote from: Kleinstein on September 13, 2017, 04:14:45 pmActually 74LS783. I wonder why they bothered with a 74 number, since there was no second source.74x385 is a quad serial adder/subtractor