ProSoft Technology WRC-CANX - Manual (Page 20)

Western Reserve Controls WRC-CANX-xx Series 4
PUB 14.1 User’s Manual
16
The CANX will verify and accept (consume) on either network side any and all valid CAN
messages. The message will be momentarily stored internally and the CANX will immediately attempt to
re-send (produce) the message on the other side bus. Because the other side of the CANX is a different
bus electrically and from a data link perspective, it will perform a negotiation session on that bus section to
retransmit (produce) the message onto that section. This action has the effect of doubling the overall
number of total attempts to get onto the bus of each message. This is the case because every message
is reproduced independent of where the target node exists; that is, the CANX does not interrogate the
message to determine the target node and does not know physically where the target node is connected.
As the bus negotiation is performed on the second side, the retransmitted message may or may
not get on the next bus section immediately. That will depend solely upon the priority of the message the
CANX is sending with respect to the messages being generated by nodes on that side of the bus and on
the amount of overall bus traffic on that side. In the case where the CANX does not immediately win the
negotiation it stores the messages and attempts to resend it in the same fashion as any other device on
the network. To the extent that it cannot produce immediately the message will be stored in an internal
buffer until it can be sent.
As other messages arrive at the CANX they also are put into an internal buffer and retransmitted
as quickly as possible. The internal buffers (one for each side) are FIFO buffers so that the messages for
each side will be retransmitted in the order that they are received with respect to the other messages
received on the same side. Up to 100 messages can be held in the internal AMT buffers.
5.1.2. Network Throughput Design Considerations
The effect of the operations described above and the internal propagation delay inside the CANX
(approximately 75us) as it performs the store-and-forward is that it may be possible to design a network
with more traffic that the CANX can be expected to handle. Care should be taken to design the network so
that the amount of total traffic does not exceed the capabilities of the total system. Each application and
installation has its own requirements and configurations so a definitive formula would be very difficult to
define. However, a good rule of thumb would be to set up the scan time for a network that includes a
CANX that is no faster that twice the fastest scan time possible to achieve the desired communications
that may be obtained without the CANX.
In the event that the network operation prevents all messages on one side from getting out before
the buffer on that side fills, subsequent messages will be lost. The CANX will continue to process
transmitting all stored messages and will accept new messages once room clears in the buffer. The red
LED labeled NSA or NSB will illuminate and will remain illuminated for 5 seconds after the “buffer full”
condition clears.
5.1.3. Fixed Operation
Fixed baud rate operation has the specific advantage that it can be used in networks where all
devices are autobaud only.
The baud rate is set by setting SW1 switch positions before powering up the A side of the CANX.
See Table 4-1.