284 -- Digital Status and Control

This card has a bunch of 0.75 second momentary pulsed bits. It also has a single word of status.

Pulsing is performed by the card itself, not software. While the card is busy it returns ~Q, so we can simply pause and retry. Pulsing is accomplished with a single F(16)A(0) command containing a mask word, telling which bits to pulse.

There is thus no possibility of conflicts, and no need for arbitration.

SSDN (Reading, Basic Status):

If the "wild card" bit is set (ls bit of the fourth word of the SSDN), then we read the F(2)A(0) status word from all 24 Main Ring non-zero houses. Length and offset slicing are not available.

The "slot" field of the SSDN tells us in which slot to expect the 284 card, but the 24 crate addresses are derived from the 24 Main Ring House Codes.

The data returned is simply all 24 words stacked up, houses A1 to F4. If an individual house could not be read (Camac I/O error), we plug the "eeee" error value from the SSDN into the buffer in place of the missing value.

If the "wild card" bit is zero, we simply read the F(2)A(0) status word from the crate and slot specified in the SSDN.

SSDN (Basic Control):

Basic Control data is 2 or 4 bytes long. There are three formats of basic control data: MAC-A replacement style, masked write style, and single bit control style.

• MAC-A Replacement style reduces application program conversion effort by mimicking the old DO-15 typecode, viz:

  	Word 0:  House Code (A1 = 8, A2 = 9, ... F4 = 31)
  	Word 1:  16 bit mask, where '1' bit means pulse this bit.
  

  This style is signalled by turning on the so-called "wild card" bit (bit 2**15 of the third SSDN word).

  The SSDN for a MAC-A Replacement style device contains the correct Camac slot, but the crate is ignored. We derive the crate address from the "house code" in word 0 of the setting data. In this way, only one device is necessary for controlling all 24 service buildings, one at a time.

• Masked-write style and single-bit style both have the "wild card" bit (ls bit of the fourth SSDN word) set to zero. For these devices, the crate and slot in the SSDN are real.

  Masked-write basic control data looks like this (two words long):

  	Word 0:  Must be zero
  	Word 1:  Mask with a '1' for every bit you want to pulse
  

  Single-bit basic control data is either one or two words long:

  	Word 0:  Bit number (1 - 16)
  	Word 1:  May or may not exist, it is ignored.
  

SSDN (Digital Alarms):

EMC (Digital):

Digital alarms mask and nominal apply to the whole F(2)A(0) status word. If you have several database devices for the same physical card, you can put extra data into the SSDN and EMC to ensure uniqueness.

The "zzzz" fields are user-defined except that the high bit in the third word has to be 0. The general convention is to put a mask word in the first word of the SSDN. It is important that the "zzzz" fields in the SSDN and EMC match exactly for a given device.