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Replace 2kx8 RAMs With 8kx8 Nonvolatile FRAMs
By Sid of Stone Marmot
Jan. 28, 2019
Many of the old synthesizers and effects from the 1980's used 2kx8 static RAMs, such as the HM6116 or TC5517, for their battery backed memory. Because of technology changes, you can now replace these battery backed static RAMs with nonvolatile FRAMs, which retain all their memory content when no power is applied to the unit without back up batteries, as discussed in this article.
2kx8 FRAMs are not available. But there are vendors who sell the 8kx8 FM16W08 28 pin SOIC nonvolatile FRAM mounted on a 24 pin 0.6 inch wide carrier board already permanently wired in a 2kx8 configuration that is plug in compatible with the common 2kx8 static RAMs that have JEDEC compliant pinouts. You could then plug this FRAM carrier directly in place of each 2kx8 static RAM in your system, allowing you to eliminate the batteries in these synths and effects.
These devices, like most semiconductors, are much cheaper in quantity and I have a lot of devices that I'd like to install these FRAMs in, so I buy the 28 pin 8kx8 versions in quantity and modify them myself as needed to the 2kx8 configuration. I will describe these mods in this article.
Figure 1 shows the pinout of the 28 pin FM16W08 FRAM compared to the pinout of a typical 24 pin JEDEC compliant 2kx8 static RAM. Note that the signals on pins 3 through 26 on the 28 pin FRAM (below the dotted line) are identical to the signals on all the pins 1 through 24 of the 24 pin RAM with only two exceptions:
1) Pin 23 on the FRAM is address line 11 (A11) while corresponding pin 21 on the RAM is write enable low (WEL).
2) Pin 26 on the FRAM has no connection internally (NC) while corresponding pin 24 on the RAM is the positive power supply (VDD).
Figure 1 - Pinouts of the 28 pin FM16W08 FRAM compared to the typical 24 pin 2kx8 static RAM
That means that you could plug the 28 pin FRAM in place of the 24 static RAM so that pins 3 through 26 of the FRAM lined up with pins 1 through 24 of the RAM socket. The only signal conflict you would have is between pin 23 on the FRAM (A11) and pin 21 on the RAM (WEL). This conflict can be eliminated by cutting the printed circuit (PC) trace that runs between pin 23 of the FM16W08 SOIC and pin 23 of the carrier board, which I did with a small drill bit.
Three signals (pin 1 is a no connect, NC) on the 28 pin FRAM carrier board hang beyond the 24 pin socket for the RAM:
1) Pin 28, VDD, which is provided by a jumper between pin 26 and pin 28 on the FRAM carrier board. Remember, pin 26 on the FRAM carrier is plugged into pin 24 of the RAM socket, which has VDD on it.
2) Pin 27, WEL, which is provided by a jumper between pin 27 and pin 23 on the FRAM carrier board. Remember, pin 23 on the FRAM carrier is plugged into pin 21 of the RAM socket, which has WEL on it.
3) Pin 2 is A12. It could be tied permanently either high or low. I chose to tie it low to pin 14, VSS, the power return or ground, on the carrier with a jumper.
That just leaves just one other signal, pin 23 on the FM16W08 SOIC, A11. That also can be tied permanently either high or low. I again chose to jumper it low to pin 14 on the carrier.
Figure 2 shows the 8kx8 FRAM carrier modified to be 2kx8 and installed in a JL Cooper MSB Plus Rev 2. The JL Cooper MSB circuit board already had the original UM6116 2kx8 battery backed RAM installed in a socket. Since there is a small 14 pin IC close to pins 1 and 24 of the RAM socket, I plugged another socket (28 pins, since I had a lot of them in my parts bin) into the RAM socket which I then plugged the FRAM carrier into so that pins 1, 2, 27, and 28, which overhand the RAM socket, would clear this small 14 pin IC. Note that the ICs on either side of the FRAM carrier board are also 24 pin 0.6 inch wide DIPs in sockets, which gives you an idea of the size and position of the original RAM socket under the FRAM carrier. You can cut off pins 1, 2, 27, and 28 on the underside of the carrier board if you need more clearance.
Figure 2 - 8kx8 FRAM SOIC mounted on carrier board modified to replace 2kx8 RAM
Note that all the mods are on the FRAM carrier board. The jumpers are easy to see. The small drill hole in the PC path between pin 23 of the FM16W08 SOIC and pin 23 of the carrier board (between the two orange jumpers) is a bit harder to see. I also drilled out part of the plated through hole that connected pin 26 of the FM16W08 SOIC and pin 26 of the carrier board (the small hole under the "-" in "Rev-2"). Since pin 26 on the FM16W08 is a no connect, this probably wasn't necessary. But it was easy to do and adds a little peace of mind. Note that neither drill hole goes completely through the carrier board but only enough to assure the trace is cut.
Figure 2 also shows that this is rather precision work. You need decent eyesight, steady hands, a fine point soldering iron, and fine solder to successfully make these mods. If you are not confident in your soldering skills, you may want to have an experienced electronics technician do this work for you. Or, you can just buy the FM16W08 FRAM installed on a carrier that is already designed and built to plug in place of a 24 pin 2kx8 static RAM.
Figure 3 shows a schematic of the 8kx8 FRAM carrier board modified to plug in place of the 2kx8 battery backed static RAM. The "X"s indicate where the two existing traces on the carrier board were cut by drilling. The dotted line box indicates the edge of the carrier board. Here is a link to a better resolution version of this schematic.
Figure 3 - Schematic of the 8kx8 FRAM carrier board modified to plug in place of a 2kx8 RAM
You should remove the battery after you install the FRAM carrier board in place of your battery backed RAM. But first be sure that the battery voltage only goes to the static RAMs you replaced and the logic devices that control these RAMs. The regular system +5V should still go to all these devices when you power up with the battery removed.
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