14
Accu-Sort Systems
receiving end then counts the 0’s and 1’s and compares the total to the odd or even recorded by the parity bit. If a discrepancy is
noticed by the receiving end, it can flag the error and request a retransmission of the data.
A stop bit is used to signal the end of the character. (Stop bits are typically one or two bits in length. The slower the transmission
speed, the more stop bits required for recognition of the end of the data frame.)
In addition to the direction of data flow and the data framing, there are other considerations to insure uniform transmissions.
Certain operating parameters must be followed to prevent the loss of valuable data.
The first consideration is the speed of transmission, known as baud rate. Serial data transmission is measured in bits per second
(BPS). The baud rate selections typically available are: 110, 300, 1200, 2400, 4800, 9600 and 19200. To enable two devices to
interact, they must both be transmitting/receiving data at the same baud rate. If it is not possible to do this, there must be a buffer
(typically additional storage memory) that accommodates the differences in communications speed.
Many serial communications links also use a flow control system to handle data transmission in addition to memory buffers.
X-ON/X-OFF Protocol
A common type of flow control is the X-ON/X-OFF protocol. When a receive buffer nears its memory capacity, the receiving
device sends an ASCII X-OFF signal to the transmitting device, telling it to stop sending data. When the memory buffer has
enough space to handle more data, the X-ON signal is sent to the transmitting device, telling it to start sending data again.
ACK/NAK Protocol
Another common protocol is ACK/NAK protocol. When the device transmits a message to the host, the host responds with either
an ACK (06H) or a NAK (15H). If the host transmits an ACK to the device, the device deletes its transmit message and the
communication sequence is complete. If the host transmits a NAK, the device will retransmit. The device resends data a
maximum of three times. Optionally this may be changed to 1, 2, 3, or infinite retransmits by the user. If the device receives a
fourth NAK, it will delete the data in its transmit buffer and display “MAX REXMITS”.
A transmitting device ignores ACK and NAK characters received during data transmission. If, for example, a device receives a
NAK during a data transmission, it will not resend the data at the completion of the transmission.
The device also has a retransmit timer. This timer is activated each time the device transmits data to the host. If the timer runs for
two seconds (this is also changeable) and the device does not receive an ACK or NAK from the host, a timeout occurs and the
device retransmits its data. Each time the device retransmits because of a timeout, it treats the timeout the same as receiving a
NAK from the host computer. If the device does not receive an ACK before the end of the fourth timeout, it will delete the data
in its transmit buffer and display “MAX REXMITS”. The device deletes data in its transmit buffer and displays the error message
when any combination of four timeouts and NAKs from the host occurs.
When the device receives a message from the host, it calculates the BCC for the message and compares the calculated BCC to the
received BCC. If the two values match, the device transmits an ACK, ending the communication. If the values do not match, the
device transmits a NAK to the host and waits for the host to retransmit the message. The host, like the device, should retransmit a
maximum of three times.
The sequence number starts at zero (30H) and is incremented each time a device transmits a new message. When the sequence
number reaches nine (39H), it wraps around to one (31H). If the sequence number skips a number, the receiving device knows
that a message was lost. If the same sequence number is received on two sequential messages, the second message is responded
to with an ACK or NAK (as appropriate) and ignored.
Shielding
Protective covering that eliminates electromagnetic and radio frequency interference.
Side Read
The scanner is mounted to read the side of a box as it passes by the head.
Signal
An impulse or fluctuating electrical quantity (i.e.: a voltage or current) the variations of which represent changes in information.
Skew
Rotation about the Y-axis. Rotational deviation from correct horizontal and vertical orientation; may apply to single character,
line or entire encoded item.
Space
The lighter elements of a bar code symbol formed by the background between bars.
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