test-drb April 1999

test-drb@ccmp.vtda.org

170 participants
1190 discussions

PDP-8/E PSU Debugging (was Q-bus pinout)

by allisonp＠world.std.com

< <Check on the mesh cover and the slots in the right hand side. The PSU <though has no indication that it ever had any catches on it (no adhesive <residue at all) Perhaps it was mounted in a desk or something. the cover was part of the air flow system and required or a case that supplied a close cowling was needed. Allison

25 years, 9 months

stepping machanism of Apple Disk ][ drive (was Re: Heatkit 51/4 floppies)

by edick＠idcomm.com

At the risk of becoming the resident infidel . . . The 6502, particularly in its later incarnation by Rockwell embodied the cleverness fostered by its earlier versions and the non-Intel family of processors. How the elegance of their instruction set became lost is a mystery to me. The most primitive analysis of the microcontroller's task is (1) fetch input signals, (2) take appropriate action. Now this is oversimplified to the max, but if you consider a comfortable and quick way to do this on most any processor, you'll quickly get a feel for what I mean to say, even though I can't easily articulate it. On inspection, the "general" way to handle this is with a "computed go-to." That doesn't say it's the ONLY way, but it's a way which does the job ALWAYS. Just to get my hands around the problem, let's assume we're making a simple communication device which process ASCII characters, mostly, by way of two serial ports, one in, one out. We can then hook up the UARTs such that the lsb is always grounded and the 7-bit ASCII appears left justified in the UART's data register. On a 65C02 from Rockwell (making the distinction because there were several CMOS 6502's, all slightly different) you load the input value into an index register and then jump, indexed indirect, to the routine which is appropriate for that pattern of inputs. This requires, then, that you have a table with 256 bytes, more correctly 128 words, with each word the address of the routine which is used to process the left-justified ASCII data. This is tremendously fast! It requires no STACK, and it requires only two instructions. Another way of doing this involves building a stack frame and loading the return address with a value looked up in a table, then executing a return. This can be done with any number of processors. On a Z-80 you can jmp HL, and I'm sure there are other neat ways of doing this simple thing. I've never seen anything more elegant than that simplistic sequence on the 65C02. How the MOTOROLA people let this go by the wayside in the design of their 6809, 6801, 68K family, and countless others puzzles me. I've not made an extensive study of other processors, but I have looked at a few. The only processor I've used which has a similar mechanism at its disposal is the 8051 core. It has a data register which can be used as an offset for a jump instruction. Now, I doubt that anything that simple can be used to discriminate between what's "best" and what's not, but it's for certain that it's a nice feature not available on the 6809. I used the 6809 extensively while I was in the aerospace industry, and found it fairly friendly. BUT, it still is relatively slow, as compared with processors of the same generation from sources clever enough to arrange the bytes the other way around in memory so you didn't have to fetch and the discard a high byte when there wasn't one. Fortunately, many tasks don't require a really fast processor. Dick -----Original Message----- From: Tony Duell <ard(a)p850ug1.demon.co.uk> To: Discussion re-collecting of classic computers <classiccmp(a)u.washington.edu> Date: Friday, April 09, 1999 4:46 PM Subject: Re: stepping machanism of Apple Disk ][ drive (was Re: Heatkit 51/4 floppies) >> >It proved software was more important than hardware. The best cpu with no >> >software was still nothing. The z80 was maybe the best at the moment but >> >the 6502 was as versitile and plenty fast enough to make up for it's >> >limitations. >> >> >> There were many people who disagreed with that performance comparison. The >> two processors (I used them both extensively) were different enough that >> comparison and contrast was not easy. The Z-80 had many registers and a >> rich instruction set of which much was awkward and difficult to use. The >> 6502 had fewer registers and fewer instructions but they were amplified by >> addressing modes not readily available to the Z-80 user, particularly if his > >IMHO, the best of the 8-bitters was the 6809 (very clean instruction set >and quite fast), but that came out rather too late to be a serious >competitor to the 6502/Z80. > >I grew up on the Z80, and for a time I thought that the 6502 was a >horrible little chip. But then I got to use a 6502 in the BBC micro and I >totally revised my opinion of it. The 6502 instruction set was simple and >fairly clean. For high-level languages it was a fine processor (BBC basic >on a 2MHz 6502 was faster than similar BASICs on 4MHz Z80 machines). > >> >> The TRS-80 could have been put out with (1) an 80x24 display rather than the >> 16x64, it could have solidly supported double-density FD's (in the model 3) > >I've never had any problems with the M3 disk controller, and I've used it >pretty extensively (well, I did have a 1793 die once, but that's not a >design problem). > >> and it could have operated at about 4MHz rather than the 2.-something it >> used, and it could have switched in and out the ROM so it could run CP/M but >> for the greed of Tandy Corp. It would have cost them an additional $5 and > >Err.. That sounds very like the Model 4 to me. Lets see, 80*24 screen, 4MHz >CPU, ROM can be switched out... Yes, a model 4. Of course it used the >same disk controller as the M3, so if you had problems with that you'd >still have problems... > >> on the Apple II. If Tandy had gone with the better design, which was on the >> table, there probably would be no IBM PC today. > >No. Never forget the 3 magic letters 'IBM'. That's why the PC was >succesful. Technically there were plenty of better machines around at the >time, but they didn't have the appeal of coming from a company that >_every_ computer centre and DP manager had heard of. > >-tony >

25 years, 9 months

Variac

by ddameron＠earthlink.net

Hi Sam, At 03:04 PM 4/10/99 -0700, you wrote: > >I have a Variac here that I need help connecting up. > >The panel also shows the voltages as depicted. I'm assuming the output is >2 and 4. But what is the input? > >The 20v sections seemingly say that you can tap off of them to get some >combination of 20V, 120V or 140V? > Normally the input would be across a 100+20=120 V section (one wire at the CCW end = common, neutral). The output would be across the arm and common so you would get 0 to 140 volts as you turn the dial CW. -Dave

25 years, 9 months

Heathkit H-8 questions

by fpp＠concentric.net

> I saw a H-8 on eBay tonight. What year were these made? > The year was about 1976-77. Saw it at the Heathkit store in LA. That > and the 16 convinced them to go full steam into computers and rest is > out of business history! >Don't you mean the H-11? Ah, yes you are right it was the 11. I remember looking at a DEC terminal , chain drive printer and keyboard for about $1500. It was cheaper at Heath than any other dealer. Had to be ordered though as the warranty started when it left the DEC loading dock.

25 years, 9 months

Identify this vector Graphics machine

by rigdonj＠intellistar.net

At 11:27 PM 4/9/99 -0700, George wrote: >Somebody liked it.... It is gone! George, That's becuase you ignored the first rule of hamfest, surplus store and trift store shopping: If it looks interesting, buy it! Joe

25 years, 9 months

[OT] <10 yo DG Aviion

by geoffrob＠stmarks.pp.catholic.edu.au

---------- > From: David Williams <dlw(a)trailingedge.com> > To: Discussion re-collecting of classic computers <classiccmp(a)u.washington.edu> > Subject: [OT] <10 yo DG Aviion > Date: Saturday, April 10, 1999 6:20 > > I know this system isn't 10 years old yet, but a company I use to > work for gave me a 88000 based Data General Aviion. I believe it is > an AV 4625 model if I'm reading the back right. DG's site only > talks about P-II and P-III based Aviions with nothing about the > earlier 88000 based ones. Anyone have any web site pointers or > info on these for me? I'm currently hitting each of the main search > sites without much luck. I got one of those things. 33mhz 88k with 32mb ram and a couple of 500mb scsi drives. Not sure if it's the same model, but came loaded with DG unix, which I eventually managed to get in to. I did find a couple of sites with a fair bit of gear for the 88k. I'll dig up the url and mail it to you. Is your system complete? I lack the mouse and a monitor. Cheers Geoff Roberts Computer Systems Manager Saint Marks College Port Pirie, South Australia geoffrob(a)stmarks.pp.catholic.edu.au

25 years, 9 months

FM, MFM, and GCR channel codes (was Re: stepping machanism of Apple Disk ][ drive)

by edick＠idcomm.com

YES! and that's exactly why the pulse overlap was correctable with write-precompensation at least in hard disks at 10x the data rate. In general the amplitude of the pulses was sufficient to be detected by the usual circuitry, but because the timing was quite far off due to the peak shift introduced by the summing effect of the head/media combination. Precompensation, which was not needed for FM, was between 188 and 125 nsec, depending on the drives in use. On the older drives, 188 was pretty common. The "bit-shift" was mitigated somewhat by the reduction of write-current on the inner tracks. Dick -----Original Message----- From: Eric Smith <eric(a)brouhaha.com> To: Discussion re-collecting of classic computers <classiccmp(a)u.washington.edu> Date: Saturday, April 10, 1999 4:36 AM Subject: Re: FM, MFM, and GCR channel codes (was Re: stepping machanism of Apple Disk ][ drive) >I wrote: > >> I thought about this for a few minutes. Ignoring rise and fall times, >> for 250 kHz FM, I expect to see spectral peaks at 250 kHz and 500 kHz. >> >> For 500 kHz MFM, I expect to see peaks at 250 kHz, 375 kHz, and 500 kHz. > >Oops, I wasn't thinking clearly enough about how the write channel works, so I >was off by a factor of two. That should have been 125 and 250 kHz for the FM >case, and 125, 187.5, and 250 kHz for the MFM case. > >> Therefore, it seems to me that a channel with reasonably flat response from >> 250 kHz to 500 kHz should be able to handle either 250 kHz FM or 500 kHz > ^^^^^^^^^^^^^^^^^^ >Make that > 125 kHz to 250 kHz >

25 years, 9 months

What if,... early PCs (was: stepping machanism

by edick＠idcomm.com

Anything's possible, of course, and when you haven't got access to what you're working with in terms of the organization, it doesn't help you to understand what their goals are. The unit we delivered, was rock solid, though. It would handle diskettes like nothing Tandy ever delivered, even much later, though much of the credit goes to the programmer who wrote the low-level code. I avoided coding whenever I could, and, I guess, still do. By means of a slide-switch, it changed the memory map such that hardware appeared as hardware should, in one position for TRSDOS, and the other for CP/M. We got paid in full, and promptly, so I guess the RS people weren't disappointed. I don't know the history of the Model-1, I know they had a much better prototype at their disposal than the production units they chose to ship. It always "hurt" just a bit to see them ship that piece of dirt when they had such a decent option. The sad thing is, the marketplace proved we were on the right track. People wanted to use the box for CP/M, so they bought a "mapper" which, as an option, remapped the addresses to make the low-end ROM go away and replace it with RAM. Somebody sold a video enhancement which attempted to fudge the video around so it displayed 24x80, but I'd like to have seen that at least once, and there were several "enhanced" data separators on the market for the model-3. As for the model 2, I never saw one of these that wasn't for sale. Dick -----Original Message----- From: Sellam Ismail <dastar(a)ncal.verio.com> To: Discussion re-collecting of classic computers <classiccmp(a)u.washington.edu> Date: Saturday, April 10, 1999 3:47 AM Subject: Re: What if,... early PCs (was: stepping machanism >On Sat, 10 Apr 1999, Richard Erlacher wrote: > >> The RS people had their heads wedged, probably due to politics. They used a >> strange mix of parts, seemingly cobbled together from various vendors' app >> notes. Their FDC used a TI TTL VCO, a Motorola phase detector, and a >> Western Digital controller chip. Additionally they used some wierd TI clock >> generator and some other stuff I couldn't justify. Their clock recovery >> circuit was pretty poor, i.e. poorer than average, and cost about 6x what I >> was used to seeing. Their dynamic memory handling wasn't any sort of slick, >> nor was their video circuit. > >Rumor has it the Model 1 design was stolen from a third party consultant >by a less than talented head engineer at Radio Shack, and purportedly made >into the production model, bugs and all, so its not surprising that other >RS hardware was designed shoddily. > >Sellam Alternate e-mail: dastar(a)siconic.com >--------------------------------------------------------------------------- --- >Don't rub the lamp if you don't want the genie to come out. > > Coming in 1999: Vintage Computer Festival 3.0 > See http://www.vintage.org/vcf for details! > [Last web site update: 04/03/99] >

25 years, 9 months

Speaking of Tandy & TRS-80 (was: Re: What if,... early PCs

by steverob＠hotoffice.com

On Saturday, April 10, 1999 10:16 AM, Charles P. Hobbs [SMTP:transit@primenet.com] wrote: > > > Does anyone remember a similar electronics store called "Lafayette"? They > were a big chain, probably not nearly as big as Tandy Radio Shack though; > I think they went under in 1980 or 1981 I used to visit the Lafayette store in Ft. Lauderdale on a regular basis. IIRC this was in the Early 70's. The store primarily had audio gear, HAM equipment, and electronics components. Similar to the Radio Shacks of that time. As a matter of fact, I still have a Lafayette Stereo Amplifier at home. I used it for about 10 years then it quit working on one channel. Probably a bad output transister. I just never got around to fixing it... I don't recall ever seeing any digital stuff in the stores. Of course this was VERY early in the micro-processor development stage. Steve Robertson - <steverob(a)hotoffice.com>

25 years, 9 months

What if,... early PCs (was: stepping machanism

by edick＠idcomm.com

In late 1979-and much of 1980, I worked for a consultant here in the Colorado front-range, who happened to have a contract with Tandy, as did several others, to produce a prototype of the next generation personal computer for Radio Shack. There were sobstantial guidelines and some direction, e.g. the packaging was more or less determined already, and the noise and power supply characteristics were handed to us, but we had quite liberal discretion as to what would go into our prototype. I was tasked with the memory subsystems, including rotating memories, as I had specifically applicable skills which they desired. ( I had built several really solid all-digital clock recovery circuits which worked with both FM and MFM, among other modulation schemes, and made them work with the then new winchester disks in 5.25" form factor. I also had extensive experience with DRAMs. ) We ended up with the creature I described a couple of posts ago. Tandy happily paid us but later told us that the $5 or so which it cost over and above the cheapest prototype presented them by one of the other firms similar to ours, and since they anticipated sales of about 1M units, they wanted the 5 million for themselves. That's why I put forth the detailed lamentation. It's true, they eventually came around, but not until after it was too late. Dick -----Original Message----- From: Fred Cisin (XenoSoft) <cisin(a)xenosoft.com> To: Discussion re-collecting of classic computers <classiccmp(a)u.washington.edu> Date: Friday, April 09, 1999 2:54 PM Subject: What if,... early PCs (was: stepping machanism >In other words, if RS were to have released their Model FOUR at about the >time that they came out with their model ONE, then they might have had >more of a competitive advantage. Hmmm. > >The RCA TV set design that RS used for a monitor for the model 1 was NOT >really adequate for 80x24 display. (YES, I've done it.) >Double density was NOT readily achievable in 1978. And the poor quality >double density of the model 3 was comparable to the rest of the industry. >And RADIO SHACK was NOT capable of being THAT much of a technology leader! > > >OTOH, when RS came out with the model 3 a few years later, that WOULD have >been an appropriate time to make ALL of the model 4 changes, including >revising the memory map (to permit CP/M), 80x24 display, Ctrl key, etc. > > >Now, if intel were to have come out with the 233 Pentium in the 80s... >If Apple were to have come out with the Mac in the 70s... >If IBM were to have come out with the PC in the 60s,... >If Windoze were reliable,... > > >On Fri, 9 Apr 1999, Richard Erlacher wrote: > >> In the late '70's, I think the microcomputer market was highly simplistic >> with respect to what it is today. Take a look at the comments I've imbedded >> in your text below. >> >> >> The TRS-80 could have been put out with (1) an 80x24 display rather than >> the >> >> <snip> >> >> >> If Tandy had gone with the better design, which was on the >> >> table, there probably would be no IBM PC today. >> >> Yes, that's true, but, the 4MHz+ (4.9152 MHz, actually) Z-80B with 64K of >> RAM, a 24x80-charcter display, double-density diskette interface that >> actually worked, built-in capability to install a hard disk, AND the ability >> to run CP/M right out of the box, in 1978-1979 e.g. at the fall '78 COMDEX, >> which was BEFORE there were "Over 10,000 programs written for the Apple" >> would have been hard to beat . . . particularly under the aegis of a >> nation-wide company with these facilities under one management already in >> place. Remember APPLE had to rely on small-time stores like Computerland >> for distribution, and their service, mostly indirect, was slow and costly. >> >> The things which seemed to make the Apple fit the business model the best >> (before Visicalc) was the 24x80-character display and the 8" diskette drives >> sitting next to it. With the aid of the Videx video display adapter and the >> Sorrento Valley Associates' 8" disk drive interface, the machine suddenly >> began to look like what people had come to expect when they learned about >> computers and how to use them. >I wasn't aware that the SVA drive ever had significant market share; >certainly not enough for IT to have been what made the ][ popular. > >> It's true that "He with the biggest dick didn't always go home with the >> babe" but you mustn't forget that in this case, the dick was overtly >> measured and advertised. Whereas the above described TRS80-III wouldn't >> have been the fastest on the market, it had the packaging and the ability to >> turn into much more computer for much less money than the Apple, though with >> the gradually and later not so gradual increase in Apple's market share, >> they were able to become somewhat more competitive in spite of the high cost >> of distribution and service. The way it turned out, Tandy Corp ended up >> with precisely the smallest, didn't it? A barely-over 2MHz processor which >> stroked memory more at about 1.5 microsecond per memory cycle??? It was >> obvious to everyone who used the Radio Shack model III that their computer >> was SLOW. The Z-80-card in the Apple was significantly (and noticeably) >> faster. The two machines otherwise occupied about the same desk space, and, >> aside from the stupid, Stupid, STUPID choice to leave the Tandy machine's >> display at 16 lines of 64 characters (about half of what was on a 24x80, and >> about what was on an Apple with the standard display), they were quite >> similar. Of course the Radio Shack machine was SLOW . . . >

25 years, 9 months

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test-drb April 1999