CRT terminal for a programmable controller

Programmable controllers {PCs) have been well established in industry for more than a decade I'2. They are popular in India, especially in cement, iron and steel, power generation industries etc. 3 Control programs are entered into a PC's memory through a programming panel. Three types of programming panel are available • a manual programmer in which data and instructions are entered through thumbwheel and toggle switches, function keys etc. • a CRT terminal in which a segment of the program can be entered and seen at the same time • a teletype from which a hard copy output is obtained Of these, the CRT terminal is the most popular type of programming panel for the PC. The incorporation of a microprocessor into the CRT terminal has made this terminal 'intelligent '4-9. This paper summarizes our development of an intelligent CRT terminal for a PC constructed by our coworkers. System description The system was designed to act as the programming panel for a PC. Consequently it is expected to display the relay ladder diagrams that are commonly used in designing control panels. It displays all the ladder network elements (relay contacts and coils, counters, timers etc.) and assembles them into the rungs that constitute the ladder network. A block diagram of the system 1° is shown in Figure 1. The system is based on the Intel 8085A microprocessor and consists of the following main parts • microcomputer module • display subsystem • keyboard subsystem Microcomputer module An Intel 8085A MPU, a RAM, an EPROM, and a USART {universal synchronous/asynchronous receiver/transmitter) form the basic microcomputer. The EPROM is used to store software, lookup tables and data tables. The RAM consists of display RAM and scratchpad RAM. Display RAM is interfaced to the display subsystem through a direct memory access (DMA) controller. The USART is used for serial synchronous and asynchronous communication with the outside world. Display subsystem The display subsystem comprises a video monitor, character generator, CRT controller and DMA controller. The video monitor used is a noninterlaced standard TV Electrical Engineering, he specializes in control systems. His current research interests are microprocessor applications in control and instrumentation, and the development of stepping motors and controllers. monitor, the Motorola M 3970. It accepts H-sync, V-sync and video signals to generate patterns on the screen. The character generator is a custom EPROM that generates all the relay ladder network elements. Its output is …