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32 Kb ROM Circuit Diagram

  1. This versatile, exchangeable, memory module should appeal to programmers developing software for computers other than the one being used for writing, testing and debugging the program.
  2. It is absolutely necessary to first fit all the SMA parts, at both ! sides of the board, then the f fitting this onto the 27 protruding pins at the copper side. Connect the battery supply E wires and the wire to Si (N WDS) to the respective points at the component side. Use a pair of precision pliers to carefully bend pins 28, 27, 22 and 20 of the 43256 or 6264 slightly to the right of the other pins in the row.


  5. The ICs fitted were Types 74HC00 (SMD) and a 43256C-12L (120 ns). The module is configured as a 32 Kbyte RAM block by fitting wire jumper A-C, while jumper B-C selects 2 x16 Kbyte. The latter configuration is required when the socket that receives the module is intended for a maximum memory capacity of 16 Kbyte (ROM or RAM), as on the BBC sideway extension board. A Type 6264 RAM can be used in the IC; position when only 8 Kbytes are required.

  7. The battery back- up function of the module ensures that data is retained, and so makes it possible to use "portable", software that is ROM-based and yet can be altered readily without having to program and erase an EPROM a number of times.
  8. It is recommended to open S1 after turning the computer off to prevent the battery having to supply some 50 nA for prolonged periods: this current flows into the NWDS driver via Rm. N0n—BBC or Electron Plus-l users should note that the NWDS signal is the same as WRITE, not READ/WRITE.
  9. When a miniature battery is available, this can be fitted underneath the RAM chip. For BBC users: wires CE and? are conveniently connected to pins 22 and 20 respectively of a 28-way IC socket for plugging into the adjacent ROM/RAM socket on the BBC’s sideway extension board; the NWDS signal is available at pin 8 of ICn. Switch S1 is mounted at a convenient location on the computer’s rear panel, and when opened inhibits writing into the RAM.

  11. The input voltage dropped from 1.5 to 1 V This effect is normal, however, and is due to the inputs of the HC gates briefly being in an undefined state.

  13. This enables pushing these four IC pins in the previously mentioned, separ- ate, socket pins, while the 24 others are inserted in the usual manner. The battery is conveniently mounted at some distance from the module.

  15. Input 1 of Ns is grounded via Rv, so that E on the RAM is held high, causing the chip to switch to the power-down (standby) mode.
  16. The memory module is based on the use of a Type 43256 i 32 Kbyte static CMOS RAM from NEC—see Fig. li Other 32 K types, such as the 62256, should also work here. A battery (2 button cells, or a 2.4 V NiCd cell when D1 is bypassed 5 with a resistor to enable charging) enables the chip to retain its contents when the computer is off. When the +5V supply from the computer is on, T1 drives pin 1 of Ns high, so that this gate can enable the RAM via the ci input.
  17. The supply set ; up around T3-T2 then feeds all the chips on the board with about 4.8 VY The drop across the C-E junction of Ti is less than 0.2 V here since the transistor is driven into saturation.
  18. When the computer is switched off, the Q circuit is fed from the battery f via germanium diode D1 Voltage divider R5-Rs causes T1 to be turned off when the supply level drops below some 4.5 V .

  20. Neither jumper need then be fitted. Successfully constructing the RAM module requires great care in soldering the SMA parts onto the board shown in Fig. 

32 Kb ROM Circuit Diagram

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