Garrett's Workshop

06/28/2024

Garrett’s Workshop model GW4402B 8 MB ROM SIMM released!

Hello from Garrett’s Workshop! We are mainly known for our Apple II RAM products but we also offer Macintosh products including 68-pin VRAM SIMMs. Some of you may be familiar with our old model GW4402A 8 MB ROM SIMM for the Mac II-series and SE/30. We sent this to quite a few friends and testers in 2020 and 2021 but due to a few technical and business issues we were not able to formally release it. This is despite there being over 100 people on the waitlist for the item on the Tindie Maker Marketplace. So we’re happy to finally be releasing this product in the form of the (revised) model GW4402B ROM SIMM!

Our ROM SIMM is compatible with the Mac IIx, IIcx, IIci, IIfx, IIsi, and SE/30. It has a total of 16 MB of flash ROM onboard but due to addressing limitations of the Mac II machines, only 8 MB is accessible at once. The SIMM can store any combination of 2 MB, 4 MB, and 8 MB ROM images totaling 16 MB and you can select between the images using a DIP switch on the board. As with other ROM SIMMs, ours is pre-programmed with a customized ROM based on the Mac IIsi ROM which enables the use of more than 8 MB of RAM, disables RAM test for faster booting, and has a ROM disk function. Unlike other currently available ROM SIMMs, our SIMM is pre-programmed with two different ROM disk images. There is a 7.5 MB System 7.1 image and a 3.5 MB System 6 image. Both images have various useful utilities such as ResEdit, HexEdit, TeachText, Lido, Disk First Aid, Stuffit Expander, and more. The SIMM is also pre-flashed with the original Mac IIx (same as IIcx and SE/30) and Mac IIsi ROMs in case you don’t want to use the ROM disk. Any of these four ROMs (System 7 ROM disk, System 6 ROM disk, IIcx ROM, and IIsi ROM) can be selected with the switch on the right side of the SIMM board.

One unique thing about our ROM disk driver is that it’s not a derivative of the Big Mess O’ Wires or bbraun ROM disk drivers. The driver is our own and is available on GitHub under the GPL license. Our ROM disk driver has some interesting features compared to the other ones out there. Using our control panel, you can configure the ROM disk to mount when booting from another partition. The ROM disk has a fast data transfer rate so this gives you fast access to the utilities on the ROM disk such as ResEdit and Lido so you don’t need to keep these on your other partitions. Of course, like other ROM disk drivers, you can mount the ROM disk as either read-only or as a read-write RAM disk so that it can be temporarily modified. Mounting as a RAM disk requires 3.5 MB or 7.5 MB of RAM, for the System 6 and System 7.1 ROM disks respectively, but if you have the RAM, the read-write capability is required for using certain system services like AppleTalk when booting from the ROM disk.

The driver and control panel also solve a particular issue relating to Macsbug and the Apple CD-ROM Extension. Macsbug 6.6.3, the version included on the System 7.1 ROM disk, requires something like 8 MB of RAM. This is a lot even if you have something like 16 or 32 MB in your machine. Usually if you don’t have enough RAM, Macsbug 6.6.3 puts up an error message and refuses to load but booting continues. Sometimes, however, if the amount of available RAM is just right, Macsbug 6.6.3 throws a more serious error and the boot process can’t continue. Therefore we have implemented a PRAM setting which controls whether Macsbug is loaded at boot. By default, Macsbug is disabled, but you can enable it in the ROM SIMM’s control panel. Similarly, the Apple CD-ROM Extension is included on the System 7.1 ROM disk. This is useful for working with CDs but if your SCSI bus isn’t properly set up, the extension can delay booting by 10 seconds when loaded. So we have a setting to disable the CD-ROM extension too.

One big concern with ROM SIMMs is the difficulty getting them to make good electrical contact in the Mac’s motherboard. Many users with pre-1990 Macs that have ROM SIMM sockets with plastic retaining clips have reported a lot of difficulty getting new-manufacture ROM SIMMs to work in their machines. Part of the issue is due to an industry-wide transition from imperial to metric PCB thicknesses. When the Mac II series and the SE/30 were designed, 0.05 inches or 1.27mm was the nominal thickness for a SIMM board. Since then, the entire industry has transitioned to metric and the closest common increment is approximately 1.2mm. The fact that new production SIMMs are often 0.07mm thinner than nominal, combined with degradation of the sockets and warpage of the old Macintosh motherboards, makes for difficulty getting new ROM SIMMs to make good electrical contact when installed.

We have addressed this electrical connectivity issue in several ways. Regarding the SIMM PCB itself, have specified a thicker dielectric inside the board. This new dielectric stackup is about 0.06mm thicker than what we have used previously on SIMMs, although the tolerance range is +/- 0.10mm as is common. This is sufficient for use with new SIMM sockets with metal retaining clips, but may not suffice when installed in SIMM sockets with worn-out plastic clips or when the underlying motherboard is warped. To address this issue, we supply fitment clips and fitment stickers with our ROM SIMMs. The fitment clips are based on Joel “PotatoFi” Crane’s design and licensed under the Creative Commons BY-SA 4.0 license. These fitment clips slide over the sides of the ROM SIMM socket and push the SIMM onto the pins, promoting good electrical contact. The fitment stickers go on the back of the ROM SIMM and thicken the PCB slightly where the SIMM socket’s plastic retaining clips touch the SIMM PCB. The extra thickness helps push the SIMM onto the socket pins similarly to the clips.

As with all of our products everything is open-source even for commercial use, including the SIMM PCB, ROM disk driver, and our modifications to Joel’s ROM clips. Source for everything is available on our GitHub: http://github.com/garrettsworkshop

Altogether we are pretty happy with our ROM SIMM offering and we think we have sufficiently addressed the electrical contact issue. Our SIMMs are working well even in our Mac motherboards with the most worn-out SIMM sockets. For starters, we are releasing ten units on eBay here: https://www.ebay.com/itm/256554602527

Soon we will release more quantity of the SIMM on eBay and then on Tindie. Currently we have about 150 units to sell and we will be making more as long as there is demand. After we get the ROM SIMM shipping in quantity, our next Macintosh product to be released is our MC68HC000-based accelerator for Mac SE, the “WarpSE”. That should be released in the early September timeframe.

03/05/2024

The Infrequently Discussed Apple II “ROM-Sharing” Protocol
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By Zane Kaminski, Engineer @ Garrett’s Workshop ([email protected])

Many Apple II enthusiasts (and hopefully all DMA-capable card developers) are aware of the DMA daisy chain, a feature of the Apple II peripheral card bus which allows for fixed-priority DMA arbitration between cards. Much less discussed, however, is an Apple-sanctioned “abuse” of this feature which enables the coexistence of multiple Language Card-type RAM and ROM cards in a system. At Garrett’s Workshop, we call this protocol “ROM-sharing” and we offer our model GW4208B “RAM128” Saturn 128-compatible RAM card with full ROM-sharing support. This article gives an overview of what this ROM-sharing protocol is, its history, and why it works.

In a computer system with a multidrop, DMA-capable bus like the Apple II, there must be some arbitration scheme to ensure that two cards don’t perform DMA at the same time. Beyond the result of two cards doing DMA simultaneously being functionally undefined, it’s important to prevent them from driving the address bus at the same time. This would result in bus contention, colloquially called a “bus fight.” In addition to usually crashing the computer, the address bus drivers on the cards involved in the bus fight can be damaged if the condition occurs for a long time. To address this issue, the Apple II’s DMA daisy chain provides a simple arbitration function that allows cards to avoid these bus fights. Each Apple II peripheral card has a DMA In input and a DMA Out output. On the motherboard, each slot’s DMA Out pin is wired to the DMA In pin of the next higher numbered slot. Slot 0’s DMA In pin and Slot 7’s DMA Out don’t go anywhere, and all cards using the daisy chain should put a pull-up resistor on their DMA In pin in case no previous card is driving it. When a card sees its DMA In pin is low, this means that a lower-numbered card is inhibiting DMA. In this case, the receiving card is not allowed to drive the /DMA signal low to begin DMA (or to drive the address bus). When DMA In is high, that means no lower-numbered cards are inhibiting DMA. Cards which do not use DMA can simply wire the DMA In pin to the DMA Out pin to maintain the continuity of the daisy chain. On a DMA-capable card, the DMA Out signal must be the logical AND of the DMA In signal and a card-internal active-low DMA inhibition signal. Of course, this card-internal inhibition signal must always go low when the card is doing DMA, although it is possible for a card to inhibit DMA by higher-numbered cards without actually doing DMA itself. This DMA daisy chain arrangement forms a simple priority arbitration scheme and is where a number of rules for using cards in the Apple II come from, namely the requirement that there can be no empty slots between DMA-capable cards.

The DMA daisy chain is well-known, but the DMA In and DMA Out pins have a more obscure alternate function for arbitrating between Language Card-type RAM and ROM cards. As it turns out, arbitration between Language Cards is quite similar to arbitration between DMA devices. Just as with DMA devices driving the address bus, multiple cards driving the data bus will cause a bus fight, so the result of arbitration should be the inhibition of all but one of the Language Cards in the system, same as with DMA. Despite this Language Card arbitration function using the same pins as the DMA daisy chain, this feature is fully-compatible with well-designed DMA-capable cards, even when the cards are installed in contiguous slots. Notably, because a card doing DMA drives the address bus during PHI1 and must finish setting up the address before the next PHI0, the DMA daisy chain does not provide any useful information during PHI0 when the address bus is unchanging. It’s known which card won the DMA arbitration at the PHI0 rising edge, so there’s no need to use the daisy chain during PHI0. Therefore, as long as all DMA-capable cards functionally latch their DMA In pins at the PHI0 rising edge, the daisy chain can be repurposed during PHI0 to arbitrate between Language Cards with no adverse effects. Since Language Card RAM usually goes in Slot 0, the existing priority scheme works well with a ROM card in Slot 1 and a RAM card in Slot 0. That way, the machine boots up with the ROM card in slot 1 replacing onboard ROM with its own, but when the RAM card in Slot 0 is activated, it inhibits the ROM card in the next slot so as to safely drive the bus. To this end, several RAM and ROM cards designed by Apple gate their RAM and ROM chip enable signals with the DMA In signal. Similarly they output the logical AND of their own memory chip enable signals and the DMA In signal on the DMA Out pin in order to inhibit Language Cards in higher-numbered slots. Since these cards’ memory chip enable signals are only active during PHI0, this does not disturb the daisy chain during PHI1, effectively time-multiplexing a “Language Card daisy chain” onto the DMA daisy chain during PHI0 time.

When we at Garrett’s Workshop designed our older-model revision A “RAM128” Saturn 128-compatible language card, we were not aware of this particular use of the DMA daisy chain, so like the original Saturn 128, we did not support the ROM-sharing protocol. It was David Mutimer, creator of the MultiROM ROM card, who showed us this particular use of the DMA daisy chain. Since we found little to no discussion of this feature or even an official name for it, we started calling it the “ROM-sharing” protocol. Of course it might as well be called “RAM sharing” since it allows arbitration between any kind of Language Card. Since the primary reason ROM-sharing is desirable in a Saturn 128 clone today is to use a card like the MultiROM in Slot 1, we called it ROM-sharing.

We shipped the original revision A RAM128 which did not feature ROM-sharing in September 2020. Our customers were satisfied but as always, we had various improvements in mind for revision B. It was an easy decision to add ROM-sharing to the list of proposed changes. We shipped revision B of the RAM128 with ROM-sharing and dramatically reduced power consumption in September 2021. After that, there were not many more obvious improvements to make to the RAM128 design, so revision B remains the latest version of the card. Just in case there exist any DMA-capable cards incompatible with ROM-sharing, the revision B RAM128 has two DIP switches that allow ROM-sharing to be disabled. One switch, marked “in,” controls whether the RAM128 inhibits RAM access when its DMA In pin is low. The other switch, marked “out,” controls whether the RAM128 drives DMA Out low during RAM access to inhibit Language Cards in higher-numbered slots. Setting these switches should allow users to work around any possible incompatibility with ROM-sharing. We’re always looking for more ways to improve our products, so please let us know if there are any other features we should add to a future revision of the RAM128.

RAM128 from Garrett’s Workshop, with full support for ROM-sharing and ultra-low power consumption, is available for sale on eBay and Tindie:
https://www.ebay.com/itm/255145476189
https://www.tindie.com/products/garrettswrkshp/ram128-gw4208b-128kb-ram-for-apple-ii/

128kB RAM for Apple II ][ II+ ][+ -- Saturn 128 compatible

01/28/2024

RAM2E 8 MB IIe aux (RAMWorks) RAM card back in stock!

Hello from Garrett’s Workshop! After two years of unavailability due to the chip shortage, we are pleased to reintroduce our model GW4203B “RAM2E II” auxiliary/RAMWorks RAM card for Apple IIe. RAM2E II is an open-source design which provides the Apple IIe with 8 MB of aux RAM and enables 80-column functionality. The card is built using modern, low-voltage SDRAM for high reliability and low power consumption.

Price is $42 USD including free shipping to the 48 contiguous United States. We are selling the card on eBay: https://www.ebay.com/itm/256358902723
The first units sold will ship approximately a week and a half from now. If we are out of stock, check back later; we are making several hundred cards over the next few months. We also sell our products on Tindie but we prefer doing business on eBay so the card will be released on eBay first.

RAM2E II’s sibling product is our RAM2GS II card, basically the same thing for the Apple IIgs. We had a post on Facebook and published in Call A.P.P.L.E. a few months ago discussing the features of the RAM2GS II. Almost everything in there is applicable to the RAM2E II. RAMGS II is also currently available for sale on eBay: https://www.ebay.com/itm/256237573802

More about RAM2E II—
RAM2E II provides 8 MB of RAM by default but also has an adjustable capacity feature. Using our GWRAM.SYSTEM utility program, the RAM size can be set to one of several sizes: 16 MB, 8 MB, 4 MB, 1 MB, 512 kB, or 64 kB. 16 MB RAM size is not recommended for application compatibility reasons so 8 MB is the default and the capacity we advertise. The RAM size can be set temporarily or saved in nonvolatile memory so it’s restored on power-up.

A new feature since we last shipped RAM2E II in 2021 is the activity LED. By default, the LED is disabled but it can be enabled using the same GWRAM.SYSTEM utility used to set the RAM capacity. The LED is a soft amber color (605 nm) and flashes only during CPU or DMA RAM access, not during RAM refresh. As with the RAM size, the LED can be enabled or disabled temporarily or the setting can be saved in nonvolatile memory.

RAM2E II achieves low power consumption in part by using one modern SDRAM chip. Idle power is 0.15 watts maximum (30 mA @ 5V) and active power is 0.5 watts maximum (100 mA @ 5V), leaving plenty of power available for other peripheral cards. These are maximum figures applicable to the worst-case temperatures and chip process variation. We usually observe about half as much power draw. Power consumption is low enough that the LED represents 5-15% of power usage when the RAM card is in active use. That’s why the LED is off by default.

Using modern SDRAM is not a turnkey solution to low power consumption. It would be easy to fritter away any power saved on things like voltage regulator quiescent current or clocking and I/O power. RAM2E II always uses voltage regulators with minimal quiescent current, and RAM2E II’s SDRAM control command sequence is optimized for low power. The SDRAM is put into clock disable mode as much as possible and the RAM address bus always holds its value until it needs to change, reducing I/O power which would otherwise be spent needlessly charging up and discharging the RAM address bus.

RAM2E II is built with a high-quality, four-layer, ENIG gold-plated PCB and is fully EU RoHS-compliant (lead-free). We are particularly pleased with our environmental achievements on this card. Other Apple IIe RAM card designs using legacy chips cannot be produced in accordance with the latest EU and California regulations restricting the use of leaded solder and other environmentally unfriendly materials. Our design, using modern parts, achieves full RoHS compliance. Modern lead-free solder has been criticized as less reliable than leaded solder so we use copper- and silver-bearing solder to minimize solder joint cracking and tin whiskers.

Great attention has been paid to signal integrity. RAM2E II has internal power and ground planes, minimizing generation of and susceptibility to electromagnetic interference. Wiring lengths are short and all Apple IIe bus signals are buffered at the card’s edge connector. Capacitive loading on the Apple IIe bus has been minimized by careful hatching of the power and ground planes underneath the edge connector contact pads. RAM2E II uses fast, low-voltage 74LVC or 74AHC-series buffers for input signals. For the 6502 data bus and video data bus output, 5 volt 74AHCT-series chips are used for their slow edge rates and full-swing 5 volt output. The slow edge rate prevents ringing on the IIe’s data bus and the 5 volt output swing is compatible with the 5V CMOS input thresholds of recent-production 65C02 CPU chips.

Another point of note is that RAM2E II uses only high-quality Samsung and Murata multilayer ceramic capacitors. There are no electrolytic or tantalum capacitors on the board. All bypass capacitors are X5R or X7R temperature grade and are rated for about triple the nominal working voltage. We also do not use any IC sockets. These can be unreliable over time compared to a soldered connection between a board and a thin, lightweight chip. Because RAM2E II uses modern components and no sockets, the finished board is under 4.5mm thin. This prevents it from touching a long card installed in slot 1.

We produce every unit ourselves in our semi-automated factory using a pick-and-place machine with vision system and a multistage conveyor belt reflow oven. After reflow soldering and post-assembly visual inspection, we ultrasonic clean each board to eliminate any sticky flux residue from the soldering process. Our subsequent testing steps include automated test vectors as well as functional test on a real Apple IIe system.

In order to insulate our product from future shortages, we have developed multiple versions of RAM2E II, each supporting a different CPLD/FPGA chip controlling the RAM on the card. The designs function identically and look almost identical. We have versions supporting six different CPLD/FPGA families from three different chip vendors. This will allow us to continue producing our cards and pricing them favorably in case of future shortages.

We’ve sold nearly 500 cards in the RAM2E and RAM2E II series between 2020 and 2021. For anyone interested, the model GW4203B “RAM2E II” succeeded our previous “RAM2E” series of RAM cards in August 2020. The older RAM2E series had only 2 MB of RAM and was initially released in March 2020. We replaced the 2 MB RAM2E with the 8 MB model GW4203B “RAM2E II” in August 2020. When we added the LED in preparation for the card’s rerelease this year, we considered bumping the revision from -B to -C but decided against it. The addition of the LED isn’t as big of a change as from the old 2 MB A variant, which differed significantly in physical size, power consumption, as well as RAM capacity.

And of course, we are committed to open-source development. RAM2E II’s design is completely open-source even for commercial use. This includes all schematics, board layouts, CPLD firmware, and utility software. Altogether, we believe we have the most sophisticated product in this category and we encourage others to build on top of our work if they want. We’re always very happy every time we see a clone of one of our products for sale online.

Soon we will be rereleasing our other pair of sibling cards, TimeDisk and GR8RAM. These are Slinky/RAMFactor-compatible RAM disk cards with extra features including a ROM-based restore partition (only on TimeDisk), battery backup (only on TimeDisk), and real-time clock (only on TimeDisk).

11/14/2023

RAM2GS 8 MB IIgs RAM back in stock!

After two years of unavailability due to the chip shortage, we are pleased to reintroduce our model GW4201D “RAM2GS II” RAM card for Apple IIgs. RAM2GS II is an open-source design which provides the Apple IIgs with 8 MB of expansion RAM. The card is built using modern, low-voltage SDRAM for high reliability and low power consumption.

Price is $42 USD including free shipping to the 48 contiguous United States. Currently it is only available to domestic US customers. We will begin international sales soon.
We are selling the card on eBay: https://www.ebay.com/itm/256237573802
If we are out of stock, check back later; we have enough parts to make about 275 cards and will be making more if there is demand. We also sell our products on Tindie but we prefer doing business on eBay so the card will be released on eBay first.

More about RAM2GS II—

RAM2GS II provides 8 MB of RAM by default but also has an adjustable capacity feature. Using our GWRAM.SYSTEM utility program, the RAM size can be set to either 8 MB or 4 MB. 4 MB size is useful because most DMA devices do not function properly with an 8 MB RAM card. The RAM size can be set temporarily or saved in nonvolatile memory so it’s restored on power-up.

A new feature since we last shipped RAM2GS II in 2021 is the activity LED. By default, the LED is disabled but it can be enabled using the same GWRAM.SYSTEM utility used to set the RAM capacity. The LED is a soft amber color (605 nm) and flashes only during CPU or DMA RAM access, not during RAM refresh. As with the RAM size, the LED can be enabled or disabled temporarily or the setting can be saved in nonvolatile memory.

RAM2GS II achieves low power consumption by using one modern SDRAM chip. Idle power is 0.15 watts maximum (30 mA @ 5V) and active power is 0.5 watts maximum (100 mA @ 5V), leaving plenty of power available for other peripheral cards. These are maximum figures applicable to the worst-case temperatures and chip process variation. We usually observe about half as much power draw. Power consumption is low enough that the LED represents 5-15% of power usage when the RAM card is in active use. That’s why the LED is off by default.

RAM2GS II is built with a high-quality, four-layer, ENIG gold-plated PCB and is fully EU RoHS-compliant (lead-free). We are particularly pleased with our environmental achievements on this card. Other Apple IIgs RAM card designs using legacy chips cannot be produced in accordance with the latest EU and California regulations restricting the use of leaded solder and other environmentally unfriendly materials. Our design, using modern parts, achieves full RoHS compliance. Modern lead-free solder has been criticized as less reliable than leaded solder so we use copper- and silver-bearing solder to minimize solder joint cracking and tin whiskers.

Great attention has been paid to signal integrity. This is especially important because RAM2GS II runs its SDRAM from a 60 MHz clock signal generated on the card. RAM2GS II has internal power and ground planes, minimizing generation of and susceptibility to electromagnetic interference. Wiring lengths are short and all Apple IIgs bus signals are buffered at the card’s edge connector. Capacitive loading on the Apple IIgs bus has been minimized by careful hatching of the power and ground planes underneath the edge connector contact pads. RAM2GS II uses fast, low-voltage 74LVC or 74AHC-series buffers for input signals. For the data bus output, 5 volt 74AHCT-series chips are used for their slow edge rates and full-swing 5 volt output. The slow edge rate prevents ringing on the IIgs’s data bus and the 5 volt output swing is compatible with the 5V CMOS input thresholds of recent-production 65C816 CPU chips.

Another point of note is that RAM2GS II uses only high-quality Samsung and Murata multilayer ceramic capacitors. There are no electrolytic or tantalum capacitors on the board. All bypass capacitors are X5R or X7R temperature grade and are rated for about triple the nominal working voltage. We also do not use any IC sockets. These can be unreliable over time compared to a soldered connection between a board and a thin, lightweight chip. Because RAM2GS II uses modern components and no sockets, the finished board is under 4.5mm thin. This prevents it from touching a long, thick card installed in slot 7.

We produce every unit ourselves in our semi-automated factory using a pick-and-place machine with vision system and a multistage conveyor belt reflow oven. After reflow soldering and post-assembly visual inspection, we ultrasonic clean each board to eliminate any sticky flux residue from the soldering process. Our subsequent testing steps include automated test vectors as well as functional test on a real Apple IIgs system.

In order to insulate our product from future shortages, we have developed multiple versions of RAM2GS II, each supporting a different CPLD/FPGA chip controlling the RAM on the card. The designs function identically and look almost identical. We have versions supporting six different CPLD/FPGA families from three different chip vendors. This will allow us to continue producing our cards and pricing them favorably in case of future shortages.

We’ve sold nearly 1000 cards in the RAM2GS and RAM2GS II series between 2019 and 2021. For anyone interested, the model GW4201D “RAM2GS II” succeeded our previous “RAM2GS” series of RAM cards in September 2020. The older RAM2GS series had only 4 MB of RAM and was initially released in April 2019. The old 4 MB RAM2GS had three major revisions—GW4201A, GW4201B, and GW4201C—each improving the appearance, quality, manufacturability, and power consumption but still using legacy chips. We replaced the 4 MB RAM2GS with the 8 MB model GW4201D “RAM2GS II” in September 2020. When we added the LED in preparation for the card’s rerelease this year, we considered bumping the revision from -D to -E but decided against it. The addition of the LED isn’t as big of a change as between the old 4 MB A, B, and C variants, which differed significantly in their physical sizes and power consumption.

And of course, we are committed to open-source development. RAM2GS II’s design is completely open-source even for commercial use. This includes all schematics, board layouts, CPLD firmware, and utility software. Altogether, we believe we have the most sophisticated product in this category and we encourage others to build on top of our work if they want. We’re always very happy every time we see a clone of one of our products for sale online.

In the coming weeks we will be reintroducing RAM2GS’s sister product, the model GW4203B “RAM2E II.” RAM2E II provides the Apple IIe with 8 MB of auxiliary RAM and has all the same features as RAM2GS II, including adjustable RAM capacity and activity LED. Then we will be rereleasing our other pair of sibling cards, TimeDisk and GR8RAM. These are Slinky/RAMFactor-compatible RAM disk cards with extra features including a ROM-based restore partition (only on TimeDisk), battery backup (only on TimeDisk), and real-time clock (only on TimeDisk).