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RPBASIC-52 PROGRAMMING GUIDE

1-8

MULTITASKING CONSTRUCTS

COUNT M ultitasking

RPBASIC-52 on the RPC-3XX series of cards can

count pulses while a program is running. Checking

and counting is performed at assembly language

speed during each system tick time (every 5 ms).

This capability effectively speeds up program

performance and simplifies programming.

This section describes only software counters on the

card. Hardware counters are in a separate category

are disc usse d in the hard war e section of t he card's

manual.

Just about any valid digital I/O line can be designated

as a counter input. Exceptions are interrupt inputs,

keypad, and display lines. Even if a digital line is an

output, it can be designated as a counter input. This

is useful in situation where you may want to limit or

keep track of the number of pulses to a motor,

solenoid, or lamp.

Eight software counters are available. They are

numbered 4-11. Counters 0-3 are reserved for any

hardware ones that may or may not be on your board.

Counting is enabled as soon as a line is designated as

a counter using ON C OUN T. The digital line is

sampled every 5 ms. When it goes from a high to low

state, its counter is incremented. A line must be

sampled at a high state before it can be counted again.

A line must be at a high and low state for a minimum

of 5 ms each to ensure detection. In theory the

maximum counting rate is 100 Hz. However, due to

other multitasking events (mainly serial ports),

effective maximum rate is about 95 Hz assuming a

perfect square wave.

There are two commands used in COUNT

multitasking: COUNT and ON COUNT. Notice

there are two COUNT commands. One is a function,

which returns a value. This is the one used by the

software counters. The other COUN T command is a

statement, which writes a value to a hardware

counter. This is not used by the software co unters.

Software counters canno t be preset.

ON COU NT declares or clears a multitasking

process. T here are thre e variations of this command.

Referring to the ON C OUN T command in this

manual, the first syntax defines the digital line to

count, number of pulses to count before executing a

subroutine. When the specified number of pulses is

reached, the counter resets and a count interrupt flag

is set. Should a higher priority interrupt be executing,

the count subroutine is delayed until the higher one is

finished. The COUNT func tion is not usually used in

conjunction with this version.

The second syntax simply declares a line for

counting. Use the COUNT function to return the

number of pulses at the line. When the count reaches

65,535 it rolls over to 0. To reset o r clear a count,

simply re-declare the ON COUNT statement for that

line.

The third syntax shuts off multitasking for that

counter.

The ON COUNT command can be used to expand

the number of lines used as an ON LINE command.

The limitation here is an interrupt is generated only

when a line goes low. Set the count to 1 in the ON

COUNT declaration.

Serial Communication Multitasking

ON COM $ defines a program branch when either a

specific character or number of characters is met.

Criteria are specified in the O N COM$ statement.

When the criteria is met, the incoming data is referred

to as a packet.

This statement is especially useful in a networking

application using the RS-485 serial port. Other

devices, such as modems or scales can be used to

generate an interrupt using RS-232. A ll serial ports

can use ON COM$.

Data packets are retrieved using the COM $ function.

In RS-485 networking applications, the STR (8,...)

function is useful for determining its address.

Two serial application program s are in this manu al.

The first program is a simple RS-485 network

communication handler, shown in Appendix A. The

second uses a modem to auto receive and is in

Appendix B.

The RS-485 network handler is set up as a master-

slave protocol. Slaves "do not talk unless spoken to".

The host transmits to all receivers. All receivers

transmitters go to the hosts receive line. The host

does not transmit until it receives a response from a

node or a timeout is reached.

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