Network Card User Manual

Adapter Internal Registers

A-37

A.3.14 Maximum Rx Frame Size Register–MaxRx @ 0x44 (DIO) (Bytes 2+3)

Byte 3 Byte 2

31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16

Maximum Rx frame size (in units of 8 bits)

This register is used to set the maximum size of received network frames.

Frames larger than this size are not copied and are counted as Rx-overrun er-

ror frames. Setting this parameter prevents an oversize frame from overrun-

ning the buffer space allocated in its receive list and, thereby, causing an

adapter check.

If you chose to pass the four-byte CRC field along with your data by asserting

the RxCRC in the NetConfig register, MaxRx should be large enough to ac-

commodate this additional field. In this case, the CRC is treated exactly as the

data, and failure to accommodate this field could result in frames not being co-

pied and an Rx overrun.

The register holds the maximum frame size in bytes. The value in the register

indicates the maximum frame size that can be accommodated in the host buff-

ers. Frames larger than this value are discarded, except in PEF mode. A value

of 0 in this register (default) is equivalent to a maximum frame size of 64K by-

tes; the adapter attempts to receive all frames.

In pass-errored-frame mode, oversize frames are written to the host. The

frame size reported in the list CSTAT field is the size of the buffer, not the size

of the frame received. The last eight bytes written are corrupt and should be

ignored.

For example:

If a 133-byte frame is received in PEF mode with MaxRx set to 128, then:

The CSTAT frame length indicates 128 bytes

The first 120 bytes in the buffer contain the first 120 bytes of the frame

The contents of the last eight bytes is not important

It should be noted that frames that exceed MaxRx are always counted as Rx

overrun errors, regardless of PEF mode.