The "A" command returns an ASCII time string in the format:

          HH:MM:SS.T xx, where

          xx = 'AM', 'PM', or ' '

The "D" command returns 24 pairs of bytes (48 bytes total) containing the variable divisor value at the end of each of the previous 24 hours. This allows the trimming process to be observed (trimming is described in the reference manual which accompanies the clock). UTC hour 00 is always returned first. The first byte of each pair is the high byte of the divisor * 16; the second byte is the low byte * 16. For example, the byte pair 3C10 would be returned for a divisor of 03C1 hex (961 decimal).

The "I" command returns 8 bytes of information on the clocks operation and configuration. These bytes are defined as follows:

       | TH | TL | ER | DH | DL | U1 | I1 | I2 | , where

       TH = minutes since timebase last trimmed (high byte)
       TL = minutes since timebase last trimmed (low byte)
       ER = last accumulated error in 1.25 ms increments
       DH = high byte of (current variable divisor * 16)
       DL = low byte of (current variable divisor * 16)
       U1 = UT1 offset (/.1 s):  | + | 4 | 2 | 1 | 0 | 0 | 0 | 0 |
       I1 = information byte #1:

               | W | C | D | I | U | T | Z | 1 | , where

               W = set by WWV(H)
               C = CAPTURE LED on
               D = TRIM DN LED on
               I = HI SPEC LED on
               U = TRIM UP LED on
               T = DST switch on
               Z = UTC switch on
               1 = UT1 switch on

       I2 = information byte #2:

               | 8 | 8 | 4 | 2 | 1 | D | d | S | , where

               8, 8, 4, 2, 1 = TIME ZONE switch settings
               D = DST bit (#55) in last correct frame
               d = DST bit (#2) in last correct frame
               S = clock is in simulation mode

The "P" command returns 24 bytes containing the number of WWV BCD frames received without error during UTC hours 00 through 23, providing an indication of hourly propagation on 10MHz. These bytes are updated each hour to reflect the previous 24 hour period. UTC hour 00 is always returned first.

The "T" command returns the UTC time in the following format:

       | HH | MM | SS | T | , where

       HH = tens-of-hours and hours (packed BCD)
       MM = tens-of-minutes and minutes (packed BCD)
       SS = tens-of-seconds and seconds (packed BCD)
        T = tenths-of-seconds (BCD)

The "@" command is undocumented in the programming documentation. This command dumps the contents of the clocks scratchpad memory. This information is 181 bytes long and contains all of the information returned by the other commands as well additional information on the running status of the clock. Most of this information is of little use to the programmer. The following table documents those fields which may be of interest. The byte number offsets are zero based (C style arrays). Under the data column, information in parentheses is the data register name as defined in the GCW-1001 firmware:

The "`" (ASCII 96) command is undocumented in the programming documentation. This command displays an "Easter Egg" with the names of all of the developers involved in creating the clock.

Utilizing the information returned by the "@" command, I have written a 'C' program for MSDOS mode that will display the current status of the clock. An actual screen dump from this program is available here.

Feature information on the Heath Most Accurate Clock II (Model GCW-1001) had been available from Ricky Ponder's Web site at http://www.cheta.net/ricky-ponder/mac.htm. However, it is no longer online. A copy of that web page can be viewed here.

Compiled by Robert G. Schaffrath, N2JTX