SYMBiFACE II:Realtime clock

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This is the documentation about the realtime clock of the SYMBiFACE II expansion card.

My (arnoldemu's) SYMBiFACE II uses a Dallas DS12887A. Documents claim this has 128 bytes RAM. Century register doesn't exist. It's ram inside the device. The picture from the Symbos website shows a DS1287A. This has 50 bytes RAM. Century register doesn't exist.

Both are PC compatible RTC chip and are programmed in the same way.

Therefore do not rely on a specific amount of RAM inside the RTC chip.


  • #FD15 (write only) register select
  • #FD14 (read/write) read from or write into selected register


Number  Content             Values
#00     actual second       00-59 (bcd or binary)
#01     alarm second        00-59 (bcd or binary)
#02     actual minute       00-59 (bcd or binary)
#03     alarm minute        00-59 (bcd or binary)
#04     actual hour         00-23 (bcd or binary)
#05     alarm hour          00-23 (bcd or binary)
#06     day of the week     ??
#07     day of the month    01-31 (bcd or binary)
#08     month               01-12 (bcd or binary)
#09     year                00-99 (bcd or binary)
#0A     status A            see below
#0B     status B            see below
#0C     status C            see below
#0D     status D            see below
#32     millennium          19 or 20 (bcd or binary)
#??     memory              all other registers can be freely used as persistent memory
  • Note: Register B, Bit 2 allows to select BCD or Binary mode for registers 0..9 (however, this does - of course - not affect RAM location 32h)


  • Register 32h is meant to contain the Century (not Millennium)
  • Register 32h can be only either bcd or binary (not both)
  • Register 32h documentation does not specify whether it shall be BCD or Binary
  • Register 32h source code supports only two centuries (19 and 20) this is probably due to misunderstanding the RTC chip datasheet (which sorts of suggests the 8bit RAM location can contain only two different values, which is total nonsense)
  • Whether other RAM locations are reserved for whatever purpose (like info about connected 40/80 track drives) is unknown. See note 1 please.
  • The RAM capacity is 50 bytes: register 0Eh..3Fh (although SYMBiFACE II is incorrectly advertised to have 128 byte persistent RAM)

Status registers

  • Status A
bit0-3  [not used] interrupt frequency
bit4-6  time frequency (must be 010; if not, set it to this value, otherwise the RTC will not work correctly)
bit7    1 = time is being updated at the moment, so don't read it!
  • Status B
bit0    1 = take summer time into account
bit1    0 = 12 hours time format, 1 = 24 hours time format
bit2    0 = all values in BCD format, 1 = all values in binary format
bit3    1 = [not used] activate rectangle generator
bit4    1 = [not used] generate interrupt after time update
bit5    1 = [not used] generate interrupt if alarm
bit6    1 = [not used] generate periodic interrupt (see status A, bit0-3)
bit7    1 = stop time update; set this, during you update the time and reset it after the update!
  • Status C
bit4    1 = [not used] interrupt has been generated because of time update (see status B, bit4)
bit5    1 = [not used] interrupt has been generated because of alarm (see status B, bit5)
bit6    1 = [not used] interrupt has been generated because of periodic (see status B, bit6)
  • Status D
bit7    0 = battery is nearly empty, please charge or replace


The RTC of the SYMBiFACE II is supported by

See also