HP 5501A Operating And Service Manual page 109

As a listener, the HP-IB card receives a four-bit-wide command and a four-bit-wide address
and combines them into an eight-bit-wide instruction that is sent out over the instruction bus
on the backplane of the coupler. Each time its address buffer is loaded, the HP-IB card sets a
backplane line called Instruction Valid (INSV) to the true state to inform cards in the coupler
to look at this instruction. Whenever INSV is true, the HP-IB card will not allow new data to
be loaded into its command and address buffers. It sets the HP-IB Not Ready for Data (NRFD)
line low whenever INSV is true and it is addressed to listen. This informs the talker on the
HP-IB that the HP-IB card is not ready for data.
There are two algorithmic state machines (ASM) on the HP-IB card. They operate essentially
independently but because they share some common circuits they do not operate simultane-
ously. One ASM controls the binary-to-BCD converter and the other controls the talker
function.
Whenever an output instruction for another card occurs on the coupler's instruction bus, the
HP-IB card takes the binary data, the decimal point position code, and the error flags that
are put on the backplane a s a result of the instruction and loads this information into
i t s
input
data register, its decimal point register, and its error bit register, respectively. Then
if
a con-
version instruction occurs, the conversion flip-flop
is
set and a signal is sent to the coupler
backplane indicating that the instruction need not be held. The output of the conversion flip-
flop is logically ANDed with a signal from the talker ASM signifying that it is not presently
talking. The result is a qualifier input to the converter-control state machine. When this
qualifier goes true, the converter-control state machine starts through its sequence of clocking
the binary-to-BCD converter and loading its output into a random-access memory. Once
started,
it
proceeds on its own until the conversion
is
complete, at which time
it
clears the
conversion flip-flop.
The binary-to-BCD converter takes the 28-bit binary word in the input data register and, each
time it receives a clock pulse, outputs a four-bit BCD character. This conversion takes place
least-significant bit first to most-significant bit last. The 28-bit binary word translates a nine-
character BCD word and therefore it takes nine clock pulses to complete the conversion.
Before clocking the converter each time, the converter-control ASM checks to see if the RAM
address counter output agrees with the decimal point position code stored in the decimal point
register.
If
it does, the converter is not clocked but the RAM address counter is advanced one.
This leaves a space in the data string for the decimal point.
The converter-control ASM is clocked by a two-phase clock with a frequency of 3.33 MHz
+20%. Thus the time required for a complete conversion of the 28-bit parallel binary number to
a nine-character BCD number with decimal point information is between 7 ps and 10.5
ps.
The
typical value
is
8.4 ps. To date, there is no controller that can call for a conversion and issue a
second command before the conversion is finished. But just in case one is built, the converter-
control ASM and the talker ASM are interlocked so they will not try to operate simultaneously.
In addition, the HP-IB card will hold up a data transfer on the bakcplane until it finishes con-
verting the number. To take the converted data in the RAM and transfer it to the controller,
all that is necessary is for the controller to address itself to listen and the HP-IB card to talk,
and put the HP-IB into the data mode (ATN false). When this happens the talker ASM takes
the four-bit-wide BCD data in the RAM, adds to it three additional bits to make it seven-bit
ASCII, and outputs it on the HP-IB. The talker ASM monitors the NRFD line and the NDAC
line on the HP-IB and drives the DAV line in accordance with the rules of the HP-IB. For
additional information and schematics, refer to the 10745A HP-IB Interface Operating and
Service Manual.