What do I have to watch out for when using the fixed settings of the LAN medium for Industrial Ethernet CP?
The information given below applies for the following Industrial Ethernet communications processors:
- CP 343-1 A2 (6GK7 343-1EX10-0XE0)
- CP 343-1 IT (6GK7 343-1GX00-0XE0)
- CP 443-1 A2 (6GK7 443-1EX10-0XE0)
- CP 443-1 IT (6GK7 443-1GX10-0XE0)
These multiprotocol CPs support a data transfer rate both of 10 Mbps (Ethernet) and of 100 Mbps (Fast Ethernet).
If you connect such a CP to a network component, then normally the transfer rate is "negotiated" automatically by the users concerned via the autonegotiation mechanism. Here both the data transfer rate (10 Mbps or 100 Mbps) and the operating mode (half duplex or full duplex).
- Half duplex: the data exchange can be bidirectional. On one transfer route between two components there is always only one data transport possible in one direction at one time.
Example: if a message is sent from A to B, then user B can only send a message to A when user A has terminated its send process.
- Full duplex: the data exchange can be bidirectional. On one transfer route between two components each of the two users can send independently from each other.
Messages can be sent simultaneously from A to B and vice versa. There is a separate line pair for each send direction.
If one of the users does not support the autonegotiation mechanism, the partner sets the default operating mode (10 Mbps and half duplex).
If you implement components which support the transmission type 10 Mbps, full duplex but not the autonegotiation function, then you can set the full duplex operating mode by hand.
Example: OSM_V1 (Optical Switch Module)
With this network component the operating mode can be permanently set. If you link an OSM_V1 with one of the above-mentioned CPs, the CP sets itself to the transmission mode 10 Mbps - half duplex, because the OSM_V1 doesn't support autonegotiation.
Fixed setting of the LAN medium:
The LAN medium can be set in the hardware configuration of the CP under "Properties" in the "Options". This is illustrated by the relevant configuration window below:
The default value after inserting the CP is "Automatic setting". As an option you can permanently set any operating mode. The CP adapts to the operating mode set and does not do an autonegotiation.
You must be careful with the permanent setting.
A direct link between two of the above-listed modules is used. Here you connect the two communications processors without network components using a "crossed" cable. You set the setting of the LAN medium on one of the two CPs to "Automatic setting" and to "10 Mbps - full duplex" as a fixed setting on the partner CP. Then both CPs are activated in the relevant S7 stations.
- The CP with the fixed setting will set itself as requested to "10 Mbps - full duplex".
However at startup it doesn't start the autonegotiation mechanism and does not react to an attempted autonegotiation from the partner CP.
- The CP with the automatic setting will attempt to negotiate the operating mode through autonegotiation. In any case it doesn't get any response from the partner (which is permanently set). As a consequence it sets its LAN operating mode to the default operating mode of "10 Mbps - half duplex".
The setting finally selected by the communications processor can be read in the NCM diagnostics in the diagnostics buffer of the CP. In addition the selected operating mode is indicated by the LEDs on the front of the CP.
Diagnostics buffer of the CP with the fixed setting:
The marked entry clearly shows the settings selected: "Fixed Setting - 10 Mbps - full duplex".
Diagnostics buffer of the CP with the automatic setting:
The same for the partner CP: "Automatic setting - 10 Mbps - half duplex".
The two linked terminal devices are now working with different LAN operating modes. This results in an increased number of collisions. These collisions cause a vast number of message repeats. Depending on the frequency these can lead to connection interruptions and thus to impaired performance.
Take great care when using the fixed setting. Normally the automatic setting is sufficient, because the components concerned then agree upon the maximum possible operating mode. If you still want to operate the modules listed above with a fixed setting, then always select the same fixed setting for each partner.