Tag Archives: A500

Microbotics RAM Expander

An old school review of a RAM expander for an Amiga 500. Specifics about this part and lots of good general tech knowledge. The company also put out an M501. The M501 did not have a metal cover.

MicroBotics, Inc.
Amiga 500
Price: $159.00

Your new Amiga 500 came stocked with half a megabyte of RAM. That’s twice as much as the 1000 originally had. Of course everyone I know dropped a RAM expander into their 1000 to boost it from 256K to 512K. And a bunch of other RAM expanders are available to take the 1000 into the megabyte and beyond range.

Just as the 1000’s 256K was not enough for many users yesterday, the Amiga 500’s 512K will not be enough today. And, like the 1000, the 500 is designed so that doubling its RAM is child’s play. Your 500 manual describes Commodore’s A501 memory and clock expansion cartridge. For my money, there’s a better alternative.

MicroBotics formally calls its Amiga 500 RAM expander the “M5501 Memory and Clock/Calendar Expansion Unit” They describe their expander as “identically compatible in every feature with the Commodore unit designed for the same purpose.”

An external look at Commodore’s expander and MicroBotics’ shows two metal cases, MicroBotics’ being an inch shorter. Not much to see.

Inside the two units is where we find the similarities – and the differences. Each board has 16 256K dynamic RAM chips, an even larger number of capacitors, a ni-cad battery, a clock chip, and a handful of resistors and other components. The smaller size of the MicroBotics board is simply a function of placing the various components closer to one another.

(Remember that the size of dynamic RAM chips is measured in bits, not in 8-bit bytes. A 256K chip stores 256K bits, so it takes 8 of these chips to add 256K bytes to your computer.)

After the density of the components, the next thing that jumps out at you is the speed of the RAM chips. The Commodore unit uses 150 nanosecond (ns) chips, while MicroBotics uses 120ns. The 120ns chips are capable of operating about 20 percent faster.

Does this mean that the MicroBotics unit will run faster? Not really. The speed of your computer is controlled by other components. RAM just has to be fast enough to keep up. But I certainly wouldn’t complain about having the faster, more expensive RAM on the MicroBotics board. At today’s mail order prices, that’s $55 to $60 worth of RAM chips.

Why all the capacitors on this (or any other) board? Technically, they are decoupling or bypass capacitors. Two principles of board design demand them. First, it is considered good practice to use a decoupling capacitor between the power and ground pins of a chip, no more than an inch or two from the chip’s power supply pin. Too few decoupling capacitors and a board may work erratically. A second, related principle requires that a capacitor be placed between the power supply and ground close to every second TTL (transistor-transistor logic) chip. Whenever one of the logic gates in a TTL switches, it will draw a burst of current capable of disrupting neighboring chips.

If you disassembled the two units from Commodore and MicroBotics, you’d also notice that the metal cases were slightly different. The MicroBotics case is heavier gauge metal and snaps together. The thinner Commodore case is soldered top to bottom. That’s a mild pain for a reviewer. It is potentially more of a pain for you.

A Commodore A501, the bottom view. The top and bottom and soldered together.

The clock in both these units relies on an internal power source, a ni-cad battery to keep the time and date when your Amiga is turned off or even unplugged. A ni-cad battery has a long life. but can’t last forever. Eventually you will have to change it. With MicroBotics’ snap-open case, you’ll find opening the case to be no more difficult than popping out the battery once you’re inside.

The thicker case of the MicroBotics did not fit into my 500 as closely as the Commodore unit. So when I snapped the plastic access cover back on the bottom of the Amiga, the MicroBotics unit bowed it out slightly. The cover fastened securely, however.

I bought one of the first Amiga 500’s I could get my hands on. Long ago I gave up any delusion that I had enough will power to resist buying as much RAM as a computer would hold. Knowing that Commodore had a RAM expander w take the 500 to a meg, I just assumed I’d buy it. I hadn’t even heard of the MicroBotics unit.

But when I bought my 500, I couldn’t find a Commodore RAM expander. One dealer offered me the MicroBotics for less than the Commodore would cost when it became available. Was I suspicious. Not yet having ripped both units apart, all I knew was that MicroBotics somehow managed to be cheaper and quicker to market.

MicroBotics’ explanation is simple and humble. As developers, they had received a 500 early on. They knew that you can’t have too much RAM and chose their first product for the 500 accordingly. And they did not experience the “delays” that Commodore must have.

Installing a 512K expander (whether it’s MicroBotics’ or Commodore’s) in your Amiga 500 is pure simplicity; no tools required. Just disconnect the cables, turn the 500 over, pop open the plastic access panel, set the RAM expander into the slot, and slide it toward and onto the pins protruding from one end of the slot. Pop the plastic cover back on, reconnect the cables, and 1 meg is yours. After the installation, you may want to leave your Amiga powered up for several hours to put a good charge on the ni-cad battery.

With the new Workbench 1.2, your Amiga will automatically recognize the new RAM. Startup-Sequence grabs a few K here and there so you’ll have about 905K free memory with your new MicroBotics RAM expander. Without it, you’d have only 393K free. At the top of the Workbench screen is the Amiga’s “memory meter” which will show free memory unless you’re working in a window. (Since 1K equals 1024 bytes, 905K will show up as over 926,000 bytes of free memory on the meter.)

My 500’s Workbench 1.2 disk automatically creates a 10K buffer for the disk drive (a little RAM area to speed up disk access) and opens a RAM disk complete with Workbench disk icon. Actually, it’s the command “Dir RAM: in the file Startup-Sequence in the S subdirectory that opens the RAM disk. The Amiga’s RAM disk gives you the best of both worlds – speed and flexibility. A RAM disk, by definition, uses some of your precious RAM to simulate a disk drive for quick access. On most computers, you must define the size of your RAM disk and reboot to increase or decrease that size. Amiga’s RAM disk is dynamic. It starts out using only 1K, then grows as you copy programs or files to it and shrinks whenever you delete one of those files. (Try the INFO command to check this out.)

RAM disks are a good reason to drop a MicroBotics unit into your 500, especially if you’re running with a single floppy drive. If you want access to all the Amiga DOS goodies, like CD Copy, Dir, List, Rename, Why, Delete, EndCLl, and Type, you can edit Startup-Sequence to copy all those files to the C subdirectory of the RAM disk. Then use an “Assign C: RAM:C:” to tell the Amiga to find them there and quit bugging you to “Please replace Workbench 1.2 in any drive” every time you need a DOS command.

If you try this trick with a stock 512K Amiga (and copy all the C subdirectory files), you will have less than 215K free. With a MicroBotics unit, you’ll still have over 720K left.

One of the nicest things about MicroBotics’ unit isn’t the RAM at all, but how well it works with the new Workbench 1.2 that comes with your 500. Amiga has always had a very conservative approach to memory management. Under older Workbenches a memory management error, like an application program using a portion of RAM that it shouldn’t, was enough to cause the Amiga to reset after a “software failure” Or, if the system ran out of memory, it was liable to crash.

But the new Workbench 1.2 seems to be pretty solid with the MicroBotics unit. Not only is all your extra RAM automatically recognized, but even running out of RAM does not seem to be a problem. All I could get was a mild-mannered “Ran out of memory. Please free some and try again” message.

Another nice feature is the clock/calendar (which works just like the Commodore expander’s). To set the clock, you issue the “SetClock opt save” command from the CLI after having entered the date and time. Then put the “SetClock opt load” command in Startup-Sequence and your Amiga will boot up with the correct time and date. Time stamping your files, so you can tell one version from another, is nothing to sneeze at either.

Of course the best thing about 1 meg of RAM is…1 meg of RAM. Unless you limit yourself to playing 512K games and producing small- to medium size documents with a word processor, that RAM will be a real benefit. VIP Professional, for instance, is a great spreadsheet, but it leaves only 36K for data on a SI2K Amiga. With the MicroBotics expander, you will have over 560K, enough room to build a proper spreadsheet.

A Commodore A501 opened up.

Like spreadsheets, many database programs can make good use of additional RAM. And, naturally, graphics programs love the stuff. Hi-res graphics use enough RAM to make you wish your 500 would hold two MicroBotics boards.

Finally, if you’re a bit adventuresome, all that RAM will come in handy for multiprocessing. That’s running two or more programs (executing two or more processes) at the same time. The Amiga can assign each of several processes its own priority and, since the 68000 CPU can handle only one at a time, juggle the various processes with each one running according to its priority.

An easy way to see this is to open three CLI windows (with the NewCLI command) and arrange them so they do not overlap. Then run an application in each one. Or select several of the graphics demos from the Workbench 1.2 demo drawer at once. As you add each one, you’ll see the speed of the individual demos degrade.

Your Amiga does a lot of multiprocessing behind the scenes. Change a disk while a program is running and the Amiga will log in the new disk without your ever being aware, unless you notice the green light or disk sound. But this well-mannered computer will wait until your application is “resting”; it won’t interrupt.

(Exec is the set of Amiga routines that is responsible for managing lists of tasks, switching tasks, posting messages between tasks, allocating memory, etc. For a good discussion of multiprocessing, try Robert Peck’s Programmer’ Guide To the Amiga from Sybex.)

So, you’ve got your new Amiga 500, a nice monitor, and a fistful of software. What’s your next purchase? I recommend the MicroBotics M5501. The extra RAM is great whether for programs or as a RAM disk, the clock is handy, the warranty is 120 days (1/3 longer than Commodores), and the price (about 20 percent less than Commodore’s) is definitely right.

MicroBotics, Inc., 811 Alpha Drive,
Suite 335, Richardson, TX 75081

Richard Herring