BusWorks
®
900EN-S005 Ethernet Switch User’s Manual Ethernet I/O
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Acromag, Inc. Tel:248-624-1541 Fax:248-624-9234 Email:
http://www.acromag.com
44
Collision Retry
The switch will normally retry a transmission if a collision occurs and drop
packets after 16 or more collisions. Optionally, you can choose not to drop
packets after 16 or more collisions via DIP switch S1-4 (or Register 4 bit 3).
Note that in order for CSMA/CD to function, all of these devices (stations,
hubs, etc.) must reside within the same collision domain (e.g. connect to the
same switch port).
Illegal Frames
The 900EN-S005 will accept frames up to 1536 bytes (less if VLAN tags are
present—1522 for tagged packets or 1518 for untagged packets), or
optionally up to 1916 bytes (huge packet support is enabled via Register 4
bit 2), and will discard frames less than 64 bytes (minimum valid packet
size). DIP switch S1-6 (or Register 3 bit 3) may be used to disable the
maximum frame length check.
Late Collision
After beginning transmission in CSMA/CD, a device listens for a period of
time in order to detect a collision (this is the slot time). It listens for an
amount of time equivalent to the time it would take to transmit the smallest
valid packet in Ethernet (64 bytes or 512 bits of data). However, if a transmit
packet has a collision after 512 bit times of the transmission, the packet will
be dropped. Late collisions refer to collisions that exceed the slot time.
The occurrence of late collisions is indicative of illegal cable length. Protocol
analyzers are often used to detect late collisions and troubleshoot cable
problems.
Packet Frame Checking & Collision Controls
CONTROL REGISTER
FUNCTION
Global Control 1 bit 0
(Also see Switch S1-5 in
DIP Switch Mode)
Half-Duplex Aggressive Back-Off Enable: Set
to enable the more aggressive half-duplex
back-off (proprietary). Clear for standard IEEE
802.3 binary exponential back-off.
Global Control 1 bit 3
Frame Length Field Check: Set to cause the
switch to check the frame length field of the
packet and if it doesn’t match the measured
length, drop the packet. Clear to not verify
frame length.
Global Control 2 bit 3
(Also see DIP Switch
S1-4 in DIP Switch
Mode)
Disable Packet Drop For Excessive Collisions:
Set to prevent packets from being dropped after
16 or more collisions occur. Clear to drop
packets when 16 or more collisions occur.
Global Control 2 bit 2
Huge Packet Enable: Set to allow huge
packets up to 1916 bytes long (this over-rides
maximum legal packet size set below).
Global Control 2 bit 1
(Also see Switch S1-6 in
DIP Switch Mode)
Maximum Legal Packet Size: Set to allow
packet sizes up to 1536 bytes. Clear to set
maximum to 1522 bytes (tagged packets), or
1518 bytes (untagged packets). Packets larger
than this maximum will be dropped.
CSMA/CD
