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Thursday, 29 June 2017

System Profile: Acer AcerMate 386SX/20N [Part One]

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Part One of this article profiles the hardware itself and its history.
Part Two features cleaning it up and using it for gaming.

A few years ago I acquired a 386 system for 99p from eBay. It was an Acer AcerMate 386SX/20N. I don't know why I got so excited about it - I think it was because it very closely matched the spec of my first PC, which was an Ambra 386SX/25 with 2MB RAM and a 40MB hard drive. I played A LOT of games on that system so this one would make an excellent replacement. This system was a very similar spec and even came with Windows 3.11 installed, along with DOS 5.0.

The seller's pics intrigued me because there was an expansion card in one of the 2 ISA slots, but I couldn't quite tell what it was. It looked like a sound card of some vintage, as it had a DB-9 port and what looked like two 3.5mm jacks. I was wrong. It was an 8-bit network card with an AUI port and two holes in the shield (for activity LEDs, presumably). Nevermind. (I have since replaced this with a 3Com Ethernet card.) Nonetheless, this turned out to be quite an intriguing system.

Its design is completely modular and you can remove all serviceable parts without the need of a single tool. There are two catches under the front panel that allow it to be rotated up and removed.

Once this is gone, the top case needs to be unlocked via the two blue sliding catches on either side. I would worry about these in the long-term, as they are a bit stiff and could be broken if the plastic becomes brittle. With a bit of a shove, the top case slides backwards and lifts off.

This allows easy access to the RAM slots. The expansion slots are located on a riser in-keeping with the small form factor.

Note the metal bar passing alongside the speaker, by the floppy disk - this is the mechanical power switch for the system.

The drive cage and proprietary power supply can also be easily removed from here, revealing the motherboard, which appears to be LPX, but is referred to as 'proprietary' in The PC Engineer's Reference Book [source:].

Note the use of what appears to be an MCA slot for the ISA riser. There were a lot of these floating about when non-IBM MCA systems didn't materialise in any great number but slot-makers had already manufactured a butt-load. They were also used for VESA Local Bus. Also note the empty sockets next to the BIOS ROM chips. More on those later. Also, it needs a good clean.

All the elements of the system are integrated into the board:
  • Intel 386SX 20MHz CPU with socket for co-pro
  • Dallas RTC (which I replaced as it had a flat battery)
  • 1MB RAM on-board (I think) plus 4 slots for 30 pin SIMMS
  • Acumos AVGA1A video chipset with 256KB memory
  • Floppy drive controller (3.5" floppy drive included, 2nd drive optional)
  • Fixed disk controller (3.5" Connor drive included, optional upgrade)
  • 2x ISA slots
  • PS/2 mouse and keyboard
  • DE-9 serial
  • DB-25 serial
  • DB-25 parallel
  • DE-15 VGA
  • Fan header (for the single fan blowing over the RAM)
Even power for the drives is distributed, via a Molex connector, from the motherboard itself rather than the PSU, something I've never seen elsewhere. The PSU is a 43W Delta unit with passthrough for monitor (which I opted not to take from the seller - I think it was faulty and I don't need anymore CRTs).

Lots of people complain about the Dallas real time clock, but they're not that much of a pain. Of course, compared to a coin cell, they're less convenient and more expensive, but they're a hell of a lot better than a barrel battery as there's zero risk of the board being corroded to death.

There are some extra features on the motherboard that are hard-wired:

J15 - CPU Speed Selector
I'm guessing that there was a 16MHz variant of this system and that this jumper was set at the factory depending on which model was installed. The CPU crystal dictates the speed and the jumper controls some kind of divider. Given that it's located on the opposite side of the board that would make sense. This makes me wonder whether I could replace the 40MHz crystal for a 50MHz one to overclock to 25MHz (although I believe AMD CPUs of this class behave better when overclocked).

These ones are interesting. Aside from the typo (it should say 'precharge'), J13 is for another factory-set option. I know that CAS and RAS are to do with how frequently the RAM is refreshed, and a bit of digging pointed me in the direction of this patent, detailing the 'invention' of half-wait states. The explanation for this is that integer wait states were previously adequate for CPU speeds but, with the advent of faster 286 models and the 386, performance gains could be had by using N+0.5 wait states, where N is a whole value between 0 and 4. Given that this board is set to the quicker '1/2T' setting, I would guess this was set according to the CPU speed being 16 or 20MHz.

Having written the above, I rediscovered this post by a Chilean member of the Vogons forum. He found a Unisys PC apparently with the exact same Acer board within it, complete with typo and everything. His system, however, came with the 16MHz CPU, 32MHz crystal and, sure enough, it's set to 1T:

[source: user 133MHz via Vogons forum]
Interestingly these systems appear to use the old method of using two chips for the BIOS ROM, one for odd bits and one for even. I understand that this was IBM's solution to providing 16-bit access to 8-bit chips when the 286-based AT was introduced. Why they didn't just use a 16-bit EPROM is unusual.

There are also sockets for 'Shell ROM' and 'DOS ROM' to the right of the BIOS. I can't find any info whatsoever on what a shell ROM would do but, given that it's just the *nix term for 'command interpreter', I'm guessing this system could support an embedded version of either Linux or DOS. Without documentation it's hard to tell.

And that brings me onto my next point. The strangest thing about this system is the apparent complete lack of documentation available. In fact this is page is probably the most comprehensive source of info on AcerMate systems anywhere on the Internet. Considering it's from a major manufacturer, who are still going today, you would think that a manual would have survived somewhere in some form. My search will continue but, for now, the only documented record of a manual is in the State Library of Queensland in Brisbane. The lack of an electronic copy suggests a lack of surviving units and, therefore, a lack of end-user enthusiasm for these machines. This general lack of information, the modular design, form factor, and budget pricing suggest that this system was picked up by businesses who wanted to equip a workforce cheaply. Given that governments, businesses and corporations tend to write off and recycle most of their old kit, that would explain the lack of these systems in the wild. The only references I've found to this class of system so far in the press are the following:

[source: Computerworld, 11th May 1992]
[source: InfoWorld 27th July 1992]

So it appears that the network card it came with was a stock option on the N model and I have subsequently upgraded the hard drive to a larger model, but the BIOS restricts the size to 500MB.

At least I managed to find some technical documentation detailing the jumpers on the board:
[source: The PC Engineer's Reference Book Volume 2: Motherboards]

This means I have the option to upgrade the Acumos chipset (later acquired by Cirrus Logic) by disabling the internal graphics but no other options to speak of. And I don't know whether I would call any ISA alternative an 'upgrade'.

The BIOS itself is nice and simple. It's by Acer themselves, rather than AMI or Phoenix, and provides not only the usual configuration options but also a low level format option for the hard drive, should one be installed. Interestingly, PCem has the 25MHz model of the AcerMate on the list of machines it can emulate, although it looks like the graphics chipset differs [source:].

Later models based on the 486 and beyond had a slightly more standard case, which could accommodate a CD drive plus another 5.25" device (or a 3.5" device in a bracket). Note the unusual placement of the hard drive in the final pic.

Acer AcerMate 600 [source:]

Internal view of RAM, CPU and ISA slots [source:]

Top-down view illustrating placement of components, including what appears to be a Sound Blaster 1.5 [source:]
Despite the lack of official information, it seems there are a lot of AcerMates floating about the Internets, particularly in Russian museums it seems. I found another article here, written by a Russian enthusiast, which sheds a little light on why these systems seem to be so prevalent in Russia. While retelling the ramblings of the friend he acquired his AcerMate 450s from, he says

then followed the story that this Acer was at a time in any financial institution (maybe even the ministry?), the bank and the FSB, and even in all these places at once.

Google translate not doing the Russian language any justice whatsoever, but giving my theory of who bought these systems some credence.

In terms of the games I would play on this system, this would be based on what I played on my first computer in 1993 and in 1994 prior to upgrading. Most are early VGA games, plus some EGA examples, that ran quite comfortably. Given that the 486 had been around for about 4 years by this point, and the Pentium was not far off, games were already starting to push the limits of the hardware and some were appearing on CD-ROM. As a result, I played quite a few games that didn't perform brilliantly on my lowly 25MHz 386, so they would perform even less well on this Acer system. Plus I only had a 40MB hard drive so was limited in what I could install. Anyway, for a list of games I would play on this system, go to the Game Gallery: 386 page.

Monday, 12 June 2017

CGA, EGA, Serial & Modem Cables: The Differences & How To Tell

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A bunch of serial cables? Maybe not... [source: author]

So I recently had a situation where I needed a cable but I couldn't work out which was the right one. I knew at least one of them was a null modem cable and my hope was that one of them was a CGA cable. Maybe one of them was a serial cable. How do I tell? None of them worked with my monitor. Part of the problem was that I needed a cable that was male at one end and female at the other and I could only achieve this by linking two together. Now I initially had a vague idea that the different types of cable are wired differently, while some have fewer wires in them than others. CGA is 'straight through' i.e. pin 1 at one end corresponds to pin 1 at the other end. This is not the case with a null modem cable and I have no idea about a serial cable. Being aware of this is all well and good but it doesn't tell me which cable is which if I can't see the wires inside the cable and where they connect. So here's how to tell.

Method 1: Visual Inspection

DE-9 cables are usually either male-male or female-female at either end. If they're male-female then it's likely (but not definitely) an extension and, therefore, straight-through. All serial ports on devices are male, so it can be safely assumed that a female-female cable is some kind of serial or modem cable (there's more than one). It thus follows that, because CGA/EGA ports on PCs are female, it could be assumed that the corresponding cables are male-male. This is where it gets tricky though, so visual inspection is of limited use.

Method 2: Dismantlement

Yeah I probably made that word up but you know what I mean. One of the cables had removable covers at each end, so I took 'em off and had a butchers:

A dismantled serial cable [source: author]

You can see quite clearly that the pin assignments at each end differ. Also note how the shielding of the cable is soldered to the shell at each end. Just by visually inspecting the cable, I was able to map out the pin assignments on a handy bit of paper. The pin numbers are indicated on the plastic next to each pin in the socket:

Wiring diagram of null modem cable [source: author]

As I said, clearly not straight through, so not a CGA cable. It's also not the cable in the photo above, just an example of the process. So what kind of cable is it? Well apparently the one I've mapped out here is a null modem with full handshaking [source:]. I'm sure this will be useful someday but, right now, it's bloody useless.

Method 3: Measurement

The next two cables could not be dismantled without destroying them, as they were factory-moulded. Time to get the multimeter out.

Finding out which pin connects to which [source: author]

By setting the multimeter to measure resistance, we can work out which pin is connected to which pin. Because the probes aren't slim enough, I broke a paper clip in half and shoved them into pin one on both ends. I could then connect the probes to the metal and measure resistance between each end. 1 means infinite resistance in other words no electricity is flowing. In this case, that means no connection. On the left I kept the positive probe on pin 1 and moved the negative probe to pin 2. I did this with every pin combination. Why didn't I stop when I find a connection? Because some leads are connected to more than 1 pin, that's why. When you find a connection, the resistance will reduce from infinity to negligible (or zero, depending on the sensitivity of your equipment). You'll get a rough idea of how many wires there are. Some serial cables only have 3 so watch out:

We have a match, but the pins are different numbers [source: author]

As you can see here, we have a connection between pin 6 and pin 4. If I were to swap them over we would also find a connection between pin 4 and pin 6. After testing every pin, I used a neat, free, open source program called TinyCad [source:] to draw the diagrams below:
Wiring diagram of null modem cable with partial handshaking [source: author]

As you can see, this differs slightly from the first cable (and is the first cable I photographed). Apparently this is a null modem with partial handshaking. Again, bloody useless. The third cable, however, was quite a different result. For a start, both ends are male, which is a clue all by itself. It didn't take me long to establish that this cable is indeed straight through and exactly what I'm looking for!

Now the only issue I have is that one end of my cable is the wrong gender. I'm not buying some adaptor from a shop because I'm brassic. Instead, I'll take the dismantleable cable I've got and resolder it so it's also straight through. The only issue with that is that there are only 7 wires and I've got 9 pins to worry about. So now I need the CGA pinout to find out what's what:

Pinout for CGA and EGA graphics standards [credit:]

As you can see, I've included the EGA pinout as well for comparison. The original MDA (monochrome graphics adaptor) only used 5 of the 9 pins available, with pins 2-5 having no connection. CGA stopped using pin 7 for video and instead assigned RGB signal to 3, 4 and 5 respectively. This leaves 2 and 7 unconnected and that happens to be 7 pins in total. Bingo!

There is one drawback. Although EGA uses DE-9 too (yes, DE-9), it needs those two extra pins (and hijacks the 6th pin, formerly used for intensity in MDA) in order to provide support for 2 bits per colour (hence two pins per colour). There are certain circumstances where the two can be interchanged but that would depend on compatibility between the monitor and the graphics card. If I wanted to maintain compatibility with MDA I would also need pin 7 but this will be exclusively CGA so I don't care.

Right, where's my soldering iron...

Rewired null modem cable for 'straight through' operation [source: author]

So here is the result of my meddling (no such thing as 'meddlings' apparently, but I think it sounds better). Pins 7 and 8 were already linked by a blob of solder, which is no skin off my nose considering 7 is unused. I snapped the covers back on the cable, plugged it in and...


Took me a good 15 minutes to remember that there's a switch on the front panel of the 1084 where you can choose which input to display (like every other bloody monitor - what an idiot!). Anyway it's not like I rewired the cable for nothing - it still wouldn't have worked even with the monitor on the right input selection. So now it works like a charm. I think I'll do another article soon on the differences between CGA composite and RGB.