Data Highway Plus Communications - Allen-Bradley SLC 500 Series Reference Manual
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Table of Contents
13-10
SLC Communication Channels
Table 13.3 SLC 5/03 Channel 1 and SLC 5/03 and Higher Channel 0 DH-485 Channel Status (Continued)
Status field
Bytes
Total Bad Packets
8
Received
Packets with bad type
9
byte
Packets with bad CRC
10
Packets with Parity Error
11
Packets with Framing
12
error
Bad Packets due to
13
Overrun
Unexpected Characters
14
Received
Data Highway Plus
Communications
Publication 1747-RM001G-EN-P - November 2008
Displays the
Number of incorrect data packets the processor has received.
Number of messages that the processor could not receive because they were of an illegal type
that contained a bad control byte.
Number of messages received with a CRC (cyclic redundancy check) transmission integrity error.
Number of messages that could not be processed because of a parity error. Parity is used to detect
errors in data bytes. If the parity of a received character is invalid, this counter is incremented.
Number of messages containing misaligned data. Each data byte has a start bit and a stop bit.
These bits "frame" the actual data byte. If a character is received where these bits are not
correctly placed, the character is invalid and this counter is incremented. These errors normally
mean an electrically noisy environment or poorly terminated cabling.
Number of messages that could not be handled because the processor could not move data fast
enough before new data arrived. In this case data characters are lost and the transmission is bad.
Number of characters the processor received with parity or with errors and discarded. If this is not
zero, it could indicate that an internal hardware problem has occurred. This problem could be that
the communication channel's receiver has been disabled, but the DH-485 driver is still receiving
characters. If the controller is operating properly, this counter should remain at zero.
For processors running OS Series C, FRN 6 and higher, clicking the Clear
button while monitoring Channel Status of either channel 1 or channel 0
online, will reset all of the channel status diagnostic counters for both channels
to zero. Prior to OS Series C, FRN 6, the only channel status diagnostic
counters that are reset when the Clear button is clicked are the ones on the
channel that the programming terminal is connected to. For example, if your
programming terminal is connected online via channel 0 and you are
monitoring the Channel Status of channel 1, when you click on the Clear
button, only channel 0 diagnostic counters will be reset, not channel 1
diagnostic counters.
Data Highway Plus (DH+) implements peer-to-peer communication with a
token-passing scheme to rotate link mastership among a maximum of 64
nodes. Since this method does not require polling, it helps provide
time-efficient reliable data transport. The DH+ network features:
• remote programming of PLC-5 and SLC 500 processors on your
network.
• direct connections to PLC-5 processors and industrial programming
terminals.
• easy re-configuration and expansion if you want to add more nodes
later.
• communication rates of 57.6K baud, 115.2K baud, or 230.4K baud.
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