Accu-Sort 10, Руководство по эксплуатации и обслуживанию

Страница: 75 / 102

ASCII C
OMMUNICATIONS
A-5
Solutions with Vision
M
ESSAGE
S
EQUENCING
The master unit initiates all data transfers by either sending data to a slave or
requesting data from a slave. This protocol is strictly half duplex; only one device
may be transmitting at any time. A slave device should not transmit unless it
receives a valid message that requires a response--when it does receive such a
message, it must respond quickly (See Timing). The master unit should respond
in a timely manner, but is not under the same constraints as a slave. The
following is an example of processing a Master/Slave interaction.
Master Slave’s response Master’s response
1. Wake up ACK -none-
2. Poll Data ACK
3. Poll No data -none-
4. Poll -none- -none-
5. Data ACK -none-
T
IMING
If a slave unit is going to respond to a poll from the master, it must start its
response within two character times of the end of the carriage return at the end of
the poll.
This makes the response time dependent upon the baud rate.
The slave must turn on its transmitter within two character times after receiving
the CR of the master’s poll. The slave must place the STX at the beginning of its
response, into its serial port no later than three character times after receipt of the
master’s carriage return.
Once the slave begins transmitting, it must not allow a gap of more than one half
a character time between characters. Most transmissions will take place under
interrupt, so this should not be a problem; however, it means that serial port
interrupts may not be disabled for an extended period of time during data
transmission.
Master/Slave Timing Diagram
“S” is the start bit, “0123456” are the character bits, “P” is the parity bit and “s”
is the stop bit.