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For the purpose of the software, and more specifically, of the Windows Application
Programming Interface (API) routines, the data are packed in byte-wide buffers, a process
that may remain transparent to the user when proper set of routines (DLLs) is used. Also,
much of the technical information on writing and reading back data from the internal devices
of the VM-USB may be considered redundant, when a set of routines is available to perform
the task. This information is, however, necessary for writing such routines.
First (16-bit) word in an Out Packet identifies the internal device/address for which the
packet is intended and whether the packet represents a request for data or it contains the data
to be stored/interpreted to/by the target device. The latter information is coded in bit 2
(value=4) of the header word, with bit 2 set for write operations and reset for read operations
(request for data). The meaning of the second word in the Out Packet depends on the address
and represents the sub-address in the case of the Register Block and the number of words to
follow, in the case of VME Stacks (VDS and VSS) and the VME Generator (VCMD). The
subsequent words in the buffer, if any, represent the data to be stored in the target device or
the data to be interpreted and acted upon by the target device (in the case of the VCMD). A
detailed description of Out Packets for the four target devices is given below.
4.2 Writing Data to the Register Block
The Out Packet for writing data to the Action Register (the single register) of the Register
block is composed of the following words:
1. Target Address = 5 (1 + 4) the target address identifying the register block + the "write"
bit.
2. Register Sub-Address = 10 secondary address of the Action Register
3. Data To be Written
4.3 Reading Back Data from the Register Block
To read back data from the Register block, one must first send a request Out Packet to the
Register Block consisting of two words:
1. Target Address = 1
2. Register secondary address =10 of the Action Register.
4.4 Writing Data to the VME Command Stacks / VME Command Generator
Given the width of the VME address and data buses, the Stacks and the VME commands are
organized as a sequence of 32-bit words but coded in lines of 16-bit words.
The Out Packets targeting the 8 VME Command Stacks (VCS) for the DAQ mode and the
VME Command Generator (VCG) have identical structure, differing only in the Target
Address and in the allowed length. Writing a stack to the VM-USB the first line of the USB
out packet contains the Target Address (see table below). The following stack data define
first the length (number of following lines) as well as the starting address (0 – 1023, with
wrap-around) of the particular VCS stack within the 1kWord stack memory space. For the
starting address only the 9 least significant bits are relevant. The starting address is
WIENER, Plein & Baus GmbH
an 8-bit Action Register data word.
the target address of the Register Block
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