The FEE Server Control Engine of the ALICE-TRD - Westfälische ...

4 The ALICE Transition Radiation Detector
Startbit (1 Bit) + Frame (85 Bit)
Frame
(69 Bit + 16 Bit Checksum)
Header (16 Bit)
Slave address
src/dest Bit
Hop Counter
Data (53 Bit)
(7 Bit)
(1 Bit)
(8 Bit)
Command (5 Bit)
Memory addr. (16 Bit)
Data Word (32 Bit)
CRC Checksum (16 Bit)
Figure 4.13: Structure of a SCSN Bus frame. The two outermost parts are added by the SCSN
driver on the DCS board before sending and are removed after receiving a frame.
Therefore the data structure representing a frame in the control software running on
the DCS board only contains the header and data fields (drawing based on [Gar02]).
typedef s t r u c t {
unsigned i n t mcmaddress ; / / s l a v e a d d r e s s o f t h e t a r g e t MCM
unsigned i n t s o u r c e _ b i t ; / / s o u r c e b i t , must be s e t t o 1
unsigned i n t hopcounter ; / / hopcounter , must be s e t t o 0
unsigned i n t command ; / / Command t o be e x e c u t e d w i t h i n t h e
MCM
unsigned i n t address ; / / Memory a d d r e s s in t h e t a r g e t MCM
i n t data ; / / Payload ( d a t a f o r t h e MCM)
} FrameSend ;
typedef s t r u c t {
unsigned i n t e r r o r _ f l a g ; / / e r r o r f l a g
FrameSend the_frame ; / / t h e r e c e i v e d frame
} FrameReceived ;
Listing 4.1: C/C++ structure representing a frame. Before sending the frame the SCSN driver
calculates and adds the CRC sum and the start bit (compare fig. 4.13). When the
frame has returned the driver checks the CRC sum and sets the error flag if the CRC
sum does not match.
The addressing scheme is quite simple. Each frame contains an address field and a
hop counter (see figure 4.13 and listing 4.1). A slave receives a frame either from the
bus master or from another slave. It compares the address field with the hopcounter
field. If both values are not equal or the src/dest bit is set to zero it just increases the
hop counter by one and forwards the frame to the next slave. If the values are equal and
the src/dest bit is set the slave processes the information in the data section of the frame.
After processing, the slave sets the hop counter and the src/dest bit to zero and forwards
the frame to the next slave.
In case of TRD quite often all MCMs in the bus should process the same data frame.
Therefore the SCSN bus supports broadcasts. If a slave receives a frame with an address
field set to 127 and a src/dest bit of one, it processes the information in the data section.
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