General; Programming - Texas Instruments 990 Maintenance Manual

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945407-9701
SECTION III
OPERATING INSTRUCTIONS
3.1 GENERAL
Operation of the 16 Input/ 16 Output TTL Data Module consists entirely of the programming
required to perform the necessary interface with the attached peripheral device. The service
routines implemented to handle the interface use the SBO, SBZ, TB, LDCR, and STCR
instructions from the 990 Computer instruction repertoire (see the 990 Computer/TMS 9900
Microprocessor Assembly Language Programmers Guide, Manual Number 943441-9701 ). All of
these instructions require the use of an effective CRU address that addresses bit 0 of the module
under consideration. Since the module can be inserted into any of the available CRU chassis
locations, and the locations are wired for pre-established addresses, the base address becomes a
variable dependent upon the hardware configuration.
3.2 PROGRAMMING
The interface between the CPU and the module consists of 16 addressable 1/0 bits. The 1/0
routines written to handle communications with the attached devices vary for each device type.
This is due to the differences in the controls accepted and the status returned by the device.
Generally, the I/O routine must first establish communications with the device and then proceed
to read/write on a character-by-character basis. At the same time, the routine must be prepared
for irregular status indicators and, if the 16 I/O TTL board is so wired, be capable of handling
interrupts.
The simplified control system example presented in Section I is a case where the single-bit CRU
instructions are used to control operation of the data module interface. A service routine of the
following type would execute after the computer had been interrupted and the current machine
status saved:
TANKLO EQU
TANKFL EQU
OPVALV EQU
START TB
JNE
TB
JNE
EXIT RTWP
OPEN SBO
0
2
TANKLO
OPEN
TANKFL
CLOSE
OPVALV
Define label.
Define iabei.
Define label.
Is fluid level too low?
If yes, jump to the corrective routine.
Is the tank full?
If yes, jump to the corrective routine.
Interrupt corrected. Return to the interrupted
program and restore status.
Open the input valve to correct the fluid
low condition.
3-1
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