COP888EB datasheet(32/75 Pages) NSC

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Frame Formats (Continued)

ERROR FRAME

The Error Frame consists of two bit fields: the error flag and

the error delimiter. The error field is built up from the various

error flags of the different nodes. Therefore, its length may

vary from a minimum of six bits up to a maximum of twelve

bits depending on when a module detects the error. When-

ever a bit error, stuff error, form error, or acknowledgment er-

ror is detected by a node, this node starts transmission of the

error flag at the next bit. If a CRC error is detected, transmis-

sion of the error flag starts at the bit following the acknowl-

edge delimiter, unless an error flag for a previous error con-

dition has already been started.

Figure 24 shows how a local

fault at one module (module 2) leads to a 12-bit error frame

on the bus.

The bus level may either be “dominant” for an error-active

node or “recessive” for an error-passive node. An error ac-

tive node detecting an error, starts transmitting an active er-

ror flag consisting of six “dominant” bits. This causes the de-

struction of the actual frame on the bus. The other nodes

detect the error flag as either a violation of the rule of bit-

stuffing or the value of a fixed bit field is destroyed. As a con-

sequence all other nodes start transmission of their own er-

ror flag. This means, that the error sequence which can be

monitored on the bus as a maximum length of twelve bits. If

an error passive node detects an error it transmits six “reces-

sive” bits on the bus. This sequence does not destroy a mes-

sage sent by another node and is not detected by other

nodes. However, if the node detecting an error was the

transmitter of the frame the other modules will get an error

condition by a violation of the fixed bit or stuff rule.

Figure 25

shows how an error passive transmitter transmits a passive

error frame and when it is detected by the receivers.

After any module has transmitted its active or passive error

flag it waits for the error delimiter which consists of eight “re-

cessive” bits before continuing.

DS012837-27

FIGURE 22. Interframe Space for Nodes Which Are Not

Error Passive or Have Been Receiver for the Last Frame

DS012837-28

FIGURE 23. Interframe Space for Nodes Which Are Error Passive

and Have Been Transmitter for the Last Frame

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