If you don't have access to a ROM board with Sideways RAM socket(s) but, like me, have access to an Eprom programmer, then you could build this little device.
It need not only be restricted to storing Sideways ROM images but may be used with other Eprom based devices that need to be continually re-developed. For example, it can also be used with Maplin's UM5100 speech programmer replay only board, which accepts Eproms only.
Parts
The circuit diagram (which you can call from the Utilities Menu) shows electrically how to assemble the unit. It requires the following parts:
1
x
62256 SRAM (28 pin)
1
x
28 pin DIL socket
1
x
SPDT switch
1
x
Battery box to hold 3 x AA batteries
2
x
IN4001 diodes
In addition, it may be useful to have a ZIF socket mounted on the outside of your case, to reduce the stress on levering the device in and out.
Benefits
Switches automatically between battery backup and system power supply
Can emulate chips 27128/27256
Low power CMOS static RAM. Lasts for weeks on a set of batteries
Cheaper than EEPROM (around £22)
Hardware selection of write enable/write protect. Gets around protected ROM images
Build
As the pin outs on a 62256 and 27128 (or 27256) are virtually identical, all but print 1,27,28 can be direct one to one connections.
Pin 1,27,28 can be bent out of shape carefully with a pair of pliers - being careful not to stress the metal.
Solder a small wire to pin 1 and run this under the chip and plug it into pin 27 of the socket. This becomes address 14.
Not CS and Not OE are handled by the Eprom programmer or microprocessor so can be left alone.
Having bent pin 27 out of the way of the small wire you just soldered on, attach this to the centre pole of the switch. The other two terminals of the switch go to pin 14 (GND) and 28 (VCC).
Pin 28 also must be split. The two diodes prevent the microprocessor power supply from trying to recharge the batteries and vice versa.
The battery clip and switch should be wire ended and insulated to prevent shorts in confined spaces and to allow the device to fit under the keyboard on a BBC B.
Hints
Below around 2.5v the contents of SRAM may become corrupted or lost
I found this very useful to test out the suitability of ROM images before committing them to EPROM
I also fitted a ZIF socket to the ashtray (the punch out section on the left of the B's keyboard where the speech sockets would go) to save having to keep levering the pack in and out
If you do make a ZIF socket, there may be timing problems with piggy back ROMs such as Mega 3. Try connecting a 100nF capacitor from pin 28 to pin 14
The auto power switching only works if there is a difference of around 0.6v between the main power supply and the battery pack - hence the use of 4.5v of cells in the battery
1st April 1996