National Instruments 320030-01

Chapter 6 Theory of Operation

Start of array

# of entries = 3

BAR

BTC

DMAC

Memory Address A

Transfer Count A

Memory Address B

Transfer Count B

Memory Address C

Transfer Count C

Data Block

Address and

Transfer Count Array Data Blocks

Figure 6-2. Array Format for Array Chaining Modes

Linked Chaining Operations. This type of chaining is very similar to array chaining. The

difference is that it uses a list in memory consisting of memory address, transfer counts, and

linked addresses. While array chaining requires a continuous address & transfer count array, the

linked address in each entry allows a non-continuous address & transfer count array. Each entry

in the chain list is 10 bytes long (four bytes to hold the address of the data block, two bytes to

hold the transfer count, and the last four bytes to hold the linked address of the next array entry.

The link address of the last array entry must be set to zero. Similar to array chaining, the BAR

points to the first array entry, but the BTCR is unused. Before starting any block transfers, the

DMAC fetches the entry currently pointed to by the BAR. The address information is placed in

the MAR, the count information is placed in the MTCR, and the link address replaces the current

contents of the BAR. The channel then begins a new block transfer. As each chaining entry is

fetched, the updated BAR is examined for the terminal link, which has all 32 bits equal to zero.

When the new base address is the terminal address, the chain is exhausted and the entry just

fetched determines the last block of the channel operation.

Linked chaining allows entries to be easily removed or inserted without having to reorganize data

within the chain. Since the end of the chain is indicated by a terminal link, the number of entries

in the chain need not be specified. The last four bytes in each entry (the link address) must be

even or the entry fetch results in an address error. The middle two bytes (the transfer count of

the data block) must be non-zero. The first four bytes in each entry (the starting address of the

data block) can be even or odd. Altogether, the address of first array entry must be even. If a

terminal count is loaded into the memory transfer counter, the count error is signaled.

While it is not currently implemented in the GPIB-1014 software, the linked chaining method

could be used on Channel 0 to transfer large data blocks arbitrarily just like array chaining.

Similarly, you can use linked chaining on Channel 1 to implement the carry cycle feature, but

array chaining is much simpler. An example of the array format for linked chaining is shown in

Figure 6-3.