Zilog Z16C35 - Manual (Page 130)

ISCC
User Manual
UM011002-0808
124
the next character is received. When used for CRC error status in Synchronous or SDLC
modes, this bit is usually set since most bit combinations, except for a correctly completed
message, result in a non-zero CRC.
Bit 5 is the Receiver Overrun Error status
This bit indicates that the receive FIFO has overflowed. Only the character that has been
written over is flagged with this error, and when the character is read, the Error condition
is latched until reset by the Error Reset command. The overrun character and all subse-
quent characters received until the Error Reset command is issued causes a Special
Receive Condition vector to be returned.
Bit 4 is the Parity Error status
When parity is enabled, this bit is set for the characters whose parity does not match the
programmed sense (even/odd). This bit is latched so that once an error occurs, it remains
set until the Error Reset command is issued. If the parity in Special Condition bit is set, a
parity error causes a Special Receive Condition vector to be returned on the character con-
taining the error and on all subsequent characters until the Error Reset command is issued.
Bits 3, 2, and 1 are the Residue Codes, bits 2, 1, and 0
In those cases in SDLC mode where the received I-Field is not an integral multiple of the
character length, these three bits indicate the length of the I-Field and are meaningful only
for the transfer in which the end of frame bit is set. This field is set to “011” by a channel
or hardware reset and is forced to this state in Asynchronous mode. These three bits can
leave this state only if SDLC is selected and a character is received. The codes signify the
following (Reference Table 5-12) when a receive character length is eight bits per charac-
ter.
I-Field bits are right-justified in all cases. If a receive character length other than eight bits
is used for the I-Field, a table similar to Table 5-12 can be constructed for each dif-ferent
character length. Table 5-13 shows the residue codes for no residue (The I-Field boundary
lies on a character boundary).
Table 5–34. I-Field Bit Selection (8 Bits Only)
Bit 3 Bit 2 Bit 1
I-Field Bits
in Last Byte I-Field Bits
in Previous Byte
1 0 0 0 3
0 1 0 0 4
1 1 0 0 5
0 0 1 0 6
1 0 1 0 7
0 1 1 0 8
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