BECKHOFF EtherCAT Technology Section I - Manual (Page 59)

FMMU
Slave Controller – Technology I-39
7 FMMU
Fieldbus Memory Management Units (FMMU) convert logical addresses into physical addresses by
the means of internal address mapping. Thus, FMMUs allow to use logical addressing for data
segments that span several slave devices: one datagram addresses data within several arbitrarily
distributed ESCs. Each FMMU channel maps one continuous logical address space to one continuous
physical address space of the slave. The FMMUs of Beckhoff ESCs support bit wise mapping, the
number of supported FMMUs depends on the ESC. The access type supported by an FMMU is
configurable to be either read, write, or read/write.
Figure 20: FMMU Mapping Principle
The following example illustrates the functions of an FMMU configured to map 14 bits from logical
address 0x00010011.3 to 0x00010013[0] to the physical register bits 0x0F01[1] to 0x0F02[6]. The
FMMU length is 3 Byte, since the mapped bits span 3 Bytes of the logical address space. Length
calculation begins with the first logical byte which contains mapped bits, and ends with the last logical
byte which contains mapped bits.
Table 22: Example FMMU Configuration
FMMU configuration register FMMU reg. offset Value
Logical Start Address 0x0:0x3 0x00010011
Length (Bytes) 0x4:0x5 3
Logical Start bit 0x6 3
Logical Stop bit 0x7 0
Physical Start Address 0x8:0x9 0x0F01
Physical Start bit 0xA 1
Type 0xB read and/or write
Activate 0xC 1 (enabled)
NOTE: FMMU configuration registers start at address 0x0600.
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Telegram structure
Ethernet HDR HDR 1 PLC Data HDR 2 NC Data HDR n Data n FCS
PLC Data
Data n
NC Data
DVI
IPC
.. ..
Sub-
Telegram 1 Sub-
Telegram 2 Sub-
Telegram n