Envoy ACP/XF Application Programming Manual

Features Common to Bit-Synchronous Protocols

EnvoyACP/XF Application Programming Manual–132179

B-9

Information Field

The information field, if present, follows the control field. The most common function

of the information frame is to hold the data being transmitted. In general, the use of the

I-frame is:

The information field is transmitted in the application buffer, following the first two

bytes, which are reserved for the message control word (MCW). Additional information

on the MCW is provided with the READ, WRITE, and WRITEREAD procedure call

descriptions in Section 4, System Procedure Call Specifics for EnvoyACP/XF

Frame-Check Sequence Field

The frame-check sequence is a 16-bit field containing the cyclic redundancy check

(CRC) value for the frame, which is used for error-checking. The sending hardware

computes the CRC value using the address field, control field, and information field (if

present) as a continuous bit stream. The ending flag follows the frame-check sequence

field.

U-Frame Commands

U-frame commands are categorized by function:

Recovery responses Frame reject (FRMR)

Other commands Exchange identification (XID)

Nonreserved 0-3

Other responses Exchange identification

Request disconnect (RD)

Nonreserved 0-3

Frame Format Type Information Field

I-frame Contains the data being transmitted

U-frame May be used for additional control information (optional)

S-frame Never appears

U-Frame Command Category Commands

Mode-setting SNRM, SABM, SNRME, SABME, DISC, SIM

Information-transfer UI, UP

Recovery RSET

Other XID

Nonreserved Nonreserved 0-3

Table B-2. U-Frame Command/Response Codes

U-Frame Category Command