Jan-80
Posts: 6
Joined: Mon Aug 07, 2017 9:03 am

Using the RasPi as a Parallel Terminal

Mon Aug 07, 2017 9:40 am

I am an old computer user. I am 53, and I use old computers, as a collector of Tandy and TRS-80 computers. Tandy had the TRS-80 Model 100, one of the first laptops. For that machine, there existed a Disk/Video Interface that gave the '100 disk and big screen capabilities. The DVI were expensive, bulky and scarse, and I would like to rebuild their functionality with a Raspberry Pi. So I'm looking for assistance here.

The DVI appeared to be a (Z-80) computer on its own, receiving its commands via the parallel/system bus of the '100. In order to be able to handle the disk I/O, it had to load (BASIC) extensions to its ROM, in some kind of bootstrap fashion. But the screen management portion of the '100 was already build into the ROM-BASIC, so I assume it didn't need externally loaded extra software. So I would like to get that working, using the RasPi at the terminal end. I can get disk functionality in other ways.

The problem seems to me that the DVI is addressed in a parallel way as a terminal. Sadly, there is no mention of a protocol anywhere, but I have to assume it was something that was used before. The Model 100 could be used as a terminal, using VT-52 and maybe VT100 emulations. Tandy hardly ever invented something new... Maybe the chips used could give you some clues. (see below)

Assuming the above, it might boil down to:
1. making the proper connections and construction a cable
2. setting up the RasPi to correctly interprete the parallel signals and display the information on a sceen.

But of course, it's never that simple. :twisted:

Included:
1. DVI Connector Pin Assignments. Since the cable was a straight-thru, we can assume that the '100 bus had the same signals.
2. DVI System Block Diagram. Maybe, from this schematic, someone can figure out how this thing worked. Only consider the video portion.

I have a Raspberry Pi (1) B, with 256 MB RAM. AFAIK, this is a very early/old version (suits me!) It has a 26-pin header.
Attachments
DVI-SystemBlockDiagram.jpg
DVI-SystemBlockDiagram.jpg (51.63 KiB) Viewed 880 times
DVI-ConnectorPinAssignments.jpg
DVI-ConnectorPinAssignments.jpg (57.14 KiB) Viewed 880 times

Jan-80
Posts: 6
Joined: Mon Aug 07, 2017 9:03 am

Re: Using the RasPi as a Parallel Terminal

Wed Feb 07, 2018 10:03 pm

OK.
For starters. Both the the DVI as the Model 100 were 5V. But the R.Pi isn't. I'm trying the R.Pi to emulate a DVI.
How do I convert the 5V data from the Model 100's data bus to the 3.3V for the R.Pi GPIO pins ?

asandford
Posts: 1999
Joined: Mon Dec 31, 2012 12:54 pm
Location: Waterlooville

Re: Using the RasPi as a Parallel Terminal

Thu Feb 08, 2018 12:33 am

Jan-80 wrote:
Wed Feb 07, 2018 10:03 pm
OK.
For starters. Both the the DVI as the Model 100 were 5V. But the R.Pi isn't. I'm trying the R.Pi to emulate a DVI.
How do I convert the 5V data from the Model 100's data bus to the 3.3V for the R.Pi GPIO pins ?
Use level shifters, both uni and bidirectional are available.

Jan-80
Posts: 6
Joined: Mon Aug 07, 2017 9:03 am

Re: Using the RasPi as a Parallel Terminal

Thu Feb 08, 2018 9:38 pm

I heard of those.
Any pointers for this specific project?
(or do I just Google "Level shifters" ?)

asandford
Posts: 1999
Joined: Mon Dec 31, 2012 12:54 pm
Location: Waterlooville

Re: Using the RasPi as a Parallel Terminal

Fri Feb 09, 2018 12:01 am

Jan-80 wrote:
Thu Feb 08, 2018 9:38 pm
I heard of those.
Any pointers for this specific project?
(or do I just Google "Level shifters" ?)
Here's one I've used in the past. It's a bit lightweight on current side, so if driving things is important to you, read the info on that page for alternatives.

Jan-80
Posts: 6
Joined: Mon Aug 07, 2017 9:03 am

Re: Using the RasPi as a Parallel Terminal

Fri Feb 09, 2018 9:58 pm

Thanks. It's cheaper than I thought.

PhatFil
Posts: 448
Joined: Thu Apr 13, 2017 3:55 pm

Re: Using the RasPi as a Parallel Terminal

Fri Feb 09, 2018 10:46 pm

Quite an expensive option that imho?? look to the far east and wait 3 weeks+ for delivery and you will probably pick up 50 for that much..

asandford
Posts: 1999
Joined: Mon Dec 31, 2012 12:54 pm
Location: Waterlooville

Re: Using the RasPi as a Parallel Terminal

Fri Feb 09, 2018 11:31 pm

PhatFil wrote:
Fri Feb 09, 2018 10:46 pm
Quite an expensive option that imho?? look to the far east and wait 3 weeks+ for delivery and you will probably pick up 50 for that much..
I've found them here for ~$3, so you'd get almost 3 for that amount.

Whilst the majority of stuff I've got from dx.com has been fit for purpose and very reasonably priced, I would put more faith in the likes of Adafruit for an ($8) interface between the ($35) Pi and a potentially destructive voltage source.

hippy
Posts: 3781
Joined: Fri Sep 09, 2011 10:34 pm
Location: UK

Re: Using the RasPi as a Parallel Terminal

Sat Feb 10, 2018 11:44 am

Jan-80 wrote:
Mon Aug 07, 2017 9:40 am
The DVI appeared to be a (Z-80) computer on its own, receiving its commands via the parallel/system bus of the '100.

Sadly, there is no mention of a protocol anywhere, but I have to assume it was something that was used before.
This sounds like a typical 'memory mapped peripheral'; the TRS writing bytes to a specific address or set of addresses to instruct the DVI to do something, passing out bytes for disk and reading bytes back.

What protocol is used in doing that is anyone's guess and I wouldn't even start on this project unless you can get some inkling of what that is, whether simplistic or complicated. You may only be interested in video but you might have to cater for much more in order to get the TRS to recognise that DVI is there and to handle whatever it might do once it has detected it is.

But only you can decide if your 'wouldn't it be great if...' project is worth the potential effort or whether there may be something else you could be better investing your time and effort in.

Jan-80
Posts: 6
Joined: Mon Aug 07, 2017 9:03 am

Re: Using the RasPi as a Parallel Terminal

Sat Feb 10, 2018 3:24 pm

hippy wrote:
Sat Feb 10, 2018 11:44 am
Jan-80 wrote:
Mon Aug 07, 2017 9:40 am
Sadly, there is no mention of a protocol anywhere, but I have to assume it was something that was used before.
This sounds like a typical 'memory mapped peripheral'; ..
What protocol is used in doing that is anyone's guess .. whether simplistic or complicated. You may only be interested in video but you might have to cater for much more in order to get the TRS to recognise that DVI is there and to handle whatever it might do once it has detected it is.
I agree. I might have biten off more than I can chew.
The advantage of focussing on video-only is, that video doesn't need 2 way communication. If I can get the M100 to output anything towards a device that understands it, does it matter that the M100 knows it's there?
hippy wrote:
Sat Feb 10, 2018 11:44 am
But only you can decide if your 'wouldn't it be great if...' project is worth the potential effort or whether there may be something else you could be better investing your time and effort in.
You are right. But a video-out device has been discussed in the community for years now. I just want to start it. Even though it's way above of my technical abilities. Maybe, smarter people will take over. At least, will have done something, even if it turnes out to be useless.
https://en.wikipedia.org/wiki/Charge_of ... ht_Brigade

hippy
Posts: 3781
Joined: Fri Sep 09, 2011 10:34 pm
Location: UK

Re: Using the RasPi as a Parallel Terminal

Sat Feb 10, 2018 8:49 pm

Jan-80 wrote:
Sat Feb 10, 2018 3:24 pm
The advantage of focussing on video-only is, that video doesn't need 2 way communication. If I can get the M100 to output anything towards a device that understands it, does it matter that the M100 knows it's there?
I suppose it depends on whether you are trying to clone or emulate that DVI device or simply using the Pi as a means to get data out of the TRS under user program control.

The first may be hard but the second should be fairly easy, no different to taking data in via UART, putting that up on the screen. Just that in this case it's coming in as parallel and the Pi needs to grab whatever data is written.

drgeoff
Posts: 8497
Joined: Wed Jan 25, 2012 6:39 pm

Re: Using the RasPi as a Parallel Terminal

Sat Feb 10, 2018 10:32 pm

The 46505 was a clone of the Motorola 6845 CRT controller (https://en.wikipedia.org/wiki/Motorola_6845). Get the info on that. Typically used with a simple mapping of memory address to raster scan.

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