Record Control Byte (RCB)

Each logical record begins with a record control byte (RCB), which contains the stream identifier (type and number). In non-SNA transmissions, the end of the record is defined by a null string control byte (SCB) for compressed records, or by the data length byte for non-compressed records (signon, signoff, and path manager records).The initial signon record NCCIMRCB flag allows you to send mixed RCBs in a single transmission buffer. To use this flag, you must use the wait-a-bit only and ignore the FCS stream flags that indicate there is more tham one stream in a buffer. These messages cannot be avoided. See 2-93 for a description of this field.

If you do not use the initial signon record NCCIMRCB flag, your transmission buffer cannot contain records with different RCBs. The connection must be terminated (all streams) if a transmission buffer is received that contains an unexpected, unrecognized, or incorrect RCB. This includes:

RCBs for streams that have not been started.
RCBs for different streams in the same buffer.
Undefined RCB values.

When the next compressed transmission record will not fit in the buffer, a special “stand-alone” RCB of X'00' (EOB) is placed after the last record and the buffer is truncated at this point for transmission.

Segments are never spanned across transmission buffers, however, uncompressed records may be.

Valid RCBs received out of sequence will also cause a connection to terminate. For example, a X'B0' receiver cancel with an unstarted stream referenced in the SRCB constitutes an error situation.

A request to start a stream that has already been started is handled differently. In this case, the request is rejected with the X'B0' (permission denied) RCB. The connection is not terminated; instead, the transmitting system can terminate the stream upon receipt of the X'B0' RCB. Table 1 depicts possible binary and hexadecimal values and their meanings in a RCB.

Table 1. RCB Definition
Binary	Hex	Meaning
0000 0000	00	End-of-block (BSC)
rrrr rrrr	01-8F	Reserved
1001 0000	90	Request to initiate stream (SRCB=RCB of stream to be initiated)
1010 0000	A0	Permission to initiate stream (SRCB=RCB of stream to be initiated)
1011 0000	B0	Negative permission or receiver cancel (SRCB=RCB of stream to be denied)
1100 0000	C0	Acknowledge transmission complete (SRCB=RCB of stream received)
1101 0000	D0	Ready to receive stream (SRCB=RCB of stream to be received)
1110 0000	E0	BCB sequence error
1111 0000	F0	General control record
1001 0001	91	Reserved
1rrr 0001	A1-F1	Reserved
1001 0010	92	Reserved
1rrr 0010	A2-F2	Reserved
1iii 0011	93-F3	Reserved
1iii 0100	94-F4	Reserved
1iii 0101	95-F5	Reserved
1iii 0110	96-F6	Reserved
1rrr 0111	97-F7	Reserved
1iii 1000	98-F8	SYSIN record
1iii 1001	99-F9	SYSOUT record
1001 1010	9A	Operator command/console message
1rrr 1010	AA-FA	Reserved
1rrr 101	9B-FB	Reserved
1rrr 1100	9C-FC	Reserved
1rrr 1101	9D-FD	Reserved
1rrr 1110	9E-FE	Reserved
1rrr 1111	9F-FF	Reserved

i: represents 1 or 0 provided this produces a value within the range shown in the hex column.
iiii: may be from 1 to 7 and corresponds to the stream number.
r: represents 1 or 0 provided this produces a value within the range shown in the hex column. All r values are reserved.

For complete system-dependent and product-specific information, see System-Dependent Considerations.