Pattern Definition Registers
VL-486-4 Reference Manual
CIO Chip – 89
Table 61: Register Bit Assignments (Continued)
Bit
Mnemonic
Description
D2
(Read)
GCB
Gate Command Bit — GCB is a command bit that can be used to halt a
countdown sequence. By writing GCB with a 0, the countdown sequence is
halted. Returning to GCB to 1 allows the sequence to resume where it left off.
The state of the GCB bit does not affect the operation of the trigger inputs.
GCB is a read/write bit.
D1
(Read)
TCB
Trigger Command Bit — Writing a 1 to the TCB the counter/timer. It causes
the down-counter to be loaded with the time constant value and a countdown
sequence to be initiated. It can also retrigger the counter/timer if the Retrigger
Enable bit (REB) is set to 1. TCB is a write-only bit. When read it always
returns to 0. In this way, erroneous trigger commands are not issued when bit
set or clear operations are performed on the other bits in this register.
D0
(Read/
Write)
CIP
Count In Progress — CIP is a status bit that indicates if a countdown
sequence is on progress. It is automatically set to 1 when the counter/timer is
triggered and the down-counter is loaded with the time constant value. It is
automatically reset to 0 when the down-counter reaches a count of 0. The
state of the gate inputs (internal and external) has no effect on this bit. CIP is
read-only.
C
OUNTER
/T
IMER
T
IME
C
ONSTANT
R
EGISTERS
CIOCT1TCM
(READ/WRITE) 16H
CIOCT1TCL
(READ/WRITE) 17H
CIOCT2TCM
(READ/WRITE) 18H
CIOCT2TCL
(READ/WRITE) 19H
CIOCT3TCM
(READ/WRITE) 1AH
CIOCT3TCL
(READ/WRITE) 1BH
D7
D6
D5
D4
D3
D2
D1
D0
TC15/TC7
TC14/TC6
TC13/TC5
TC12/TC4
TC11/TC3
TC10/TC2
TC9/TC1
TC8/TC0
Each of the Time Constant registers is 16-bits and holds the value loaded into the down-counter
of its counter/timer when a trigger is detected. It is accessed by the CPU as two consecutive 8-bit
registers (Most Significant Byte and Least Significant Byte). These registers can be read and
written at any time. However, care must be taken when writing them so that a trigger does not
occur while the time constant value is changing. A reset does not effect the Time Constant
register.
