Accu-Sort 10, Operation And Maintenance Manual

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A-6 M
ODEL
10 O
PERATIONS AND
M
AINTENANCE
M
ANUAL
Accu-Sort Systems
Typically, the “RTS” line is used to control the transmitter. In this diagram,
“RTS” is high when the transmitter is enabled and low when the transmitter is
disabled (“tri-stated”).
The slave’s “FF” may be replaced with a one character time (10/baud rate) delay
between transmitter turn-on and transmission of the STX.
Time Limits
Maximum 2 character times (20/baud rate)
A
Minimum 0
Maximum
4 character times (40/baud rate)
B
Minimum 2 character times (due to guard cha STX transmission time)
Maximum
1/2 character time (5/baud rate)
C
Minimum 0
Both the master and the slave must disable their transmitter as soon as possible
after transmitting the carriage return at the end of the message. The transmitter
must remain enabled while the carriage return is being sent out, however. This
means that the transmitting device must wait for a “transmitter empty” (as
opposed to a “transmitter ready”) indication from the serial port before disabling
the transmitter.
This protocol has been designed for a "slow" master to communicate with a
"fast" slave. The only time-critical item for the master is for the master to release
control of the line immediately after sending a message to a slave. While the
slave must respond within a very short time window, there are no such
constraints on the master. The master may have any amount of time between
messages or between characters within its message.
Error Recovery
Error The slave does not understand a poll message.
Recovery None. The master will time out, waiting for the slave’s response, then will go on to the next unit.
Error The slave does not understand a data message from the master.
Recovery The master will retransmit the data message again after timing out while waiting for the acknowledgement.
Error The master does not understand the slave’s acknowledgement of a data message.
Recovery The master will retransmit the data message after timing out while waiting for the acknowledgement. The slave
will acknowledge the retransmitted message and discard it, since the message will have the same sequence
number as the last message received.
Error The master does not understand the slave’s data message (response to a poll).
Recovery The master will time out waiting for the slave’s response, then continue on to the next poll. Since the slave did not
receive an acknowledgement for the data message, it will retransmit the same message in response to the next
poll.
Error The slave does not understand the master’s acknowledgement of the slave’s data message.
Recovery The slave will retransmit the same message in response to the next poll. The master will see that it is a duplicate
message, acknowledge it, and discard it.
Error
The slave does not understand a broadcast message.
Recovery None. The message will be lost.
The general rules are as follows:
1. Each data message will be acknowledged by the recipient. If a data
message is not acknowledged, the transmitter should retransmit it again up to
three retries. After the third retry, a communications error message should be
displayed and the message discarded (in some systems the message may be
recorded in a disk file or on a printer to prevent data loss).