Ff Mode; Send And Receive Buffer Structure; Sending; Receiving - Siemens SIMATIC NET SPC 4-2 LF Manual

7.7

FF Mode

After activating the FF mode with the FF-Mode bit of mode register 3 (the FF mode
is a variant of the PA mode but does not work in the PA mode !), the user must
activate the Start-SPC4 bit in mode register 1 as was the case on the SPC4/SPC4-
1. The microprogram of the SPC 4-2 then changes to a state in which it waits for an
FF send job for a received FF. The only way out of this state is to reset the SPC 4-
2. The SPC 4-2 has one buffer for sending and one for receiving in the internal
RAM.
7.7.1

Send and Receive Buffer Structure

Each buffer begins with 2 bytes of length information. The lower-order byte is
located in the first byte of the buffer and the higher-order byte with the frame length
is located in the second byte. These are followed by the data (caution: if a frame
has 10 bytes of data, the length word in the send buffer also has the value 10, in
the receive buffer, however, the value 12 is entered since this includes the two
received CRC bytes). The buffers must begin at a segment boundary, although
they themselves are byte-oriented; in other words, they never have gaps
regardless of the segment size.
7.7.2

Sending

The user can trigger sending by writing a 1 to the Start-FF-Send bit in mode
register 4. The SPC 4-2 6 then requires 6 bit times before the frame starts on the
bus. If the Start-FF-Send bit is activated while reception is still active, there is a
frame gap of 23 bit times on the bus. Half bit time after the end of the transmitted
frame the SPC 4-2 activates the MAC_Reset interrupt, after a further bit time, it
opens its receiver. The user must store the start address of the send buffer as a
segment number in the IND_RD pointer.
7.7.3

Receiving

The user stores the start address of the receive buffer once again as a segment
number in the IND_WP_PRE RAM memory location. The user defines the end of
the receive buffer using the highest permitted segment number in the IND_WP
RAM memory location.
If the microprogram attempts to write to the memory area that does not belong to
the receive buffer, writing is suppressed although reception continues.
When a valid start delimiter is received, the and CORRECT_SD interrupt becomes
active. This interrupt is therefore used for carrier detection.
PROFIBUS Controller SPC 4®-2 LF
C79000-G8976-C157-3
ASIC Interface
89