Galil DMC-1700 Interrupt Communications Method, Data Record Cache Depth, Data Record Refresh Rate

Interrupt Communications Method

The interrupt method overall is the most efficient of the three methods. The interrupt communications method uses a hardware interrupt to notify the driver that a response or unsolicited data is available. This allows for greater efficiency and response time, since the drivers do not have to “poll” the buffers for the data. Additionally, the interrupt method allows for data record caching.

The interrupt method uses bus level interrupts (IRQ) from the controller to notify the PC that data is available. This requires that the Controller be configured with a valid interrupt line. For DMC-1800 controllers the interrupt is configured automatically. For DMC-1700 controllers, the interrupt is manually set with a jumper specified during the installation procedure (see Ch.2). Firmware version 2.0m (and greater) is required for the “communications interrupt” method to be available. For complete information on the different communications methods, select the More Info button on the Communications parameters dialog box.

Data Record Cache Depth

With the secondary FIFO of the DMC1700 or DMC1800 selected and “interrupt communications” method enabled, the driver will cache data records for retrieval via API function calls. This makes it possible to not 'miss' any data records, even if the DR command has been configured to refresh the data record every two milliseconds. For example, a program could poll at a relatively long frequency (say every 50 milliseconds), and not miss any data. The cache depth can be set when the interrupt communication method is selected. The data record cache functions like a FIFO. Reading the data records removes them from the cache. If the cache is full and a new data record arrives from the controller, the new data record is placed in the cache and the oldest data record in the cache is discarded. If multiple handles to a controller are open, the first handle to retrieve the data record(s) will possess the only copy available. When an application needs only the most recent data record available, the cache depth should be set to 1.

Stall Thread and Delay Thread Methods

Users can also choose between "Delay" and "Stall" methods. These two methods are available for both the DMC-1700 and DMC-1800 controllers and affect how the software "waits" for a response from the controller when a command is sent. If a controller is configured with the "Delay" method, the thread waiting for a command response gives up its time slice, allowing other processes running on the operating system to proceed. This method can slow communication, but results in negligible CPU utilization. The second method, the "Stall" method, uses the opposite strategy. The thread that performs I/O with the controller maintains ownership of the CPU and polls the controller until a response is received. This approach is essentially the same method employed in previous versions (< V7) of the

Galil communication DLLs and drivers. While the "Stall" method does not have to wait for its thread to become eligible for execution, it does result in 100% CPU utilization while communicating with the controller.

Data Record Refresh Rate

Under the PCI or ISA Bus Parameters tab, the rate at which the data record is sent to the software drivers can be configured. The period between refreshes can be set from 2 - 256 ms (assuming the standard TM setting of 1000 is set). The Galil communications .DLL will use this value to send the appropriate DR command to the controller when a communications session is opened. Additionally, for DMC-1700 users, the dialog box (shown in Fig 4.7) allows the user to select between two Data Record Access methods: DMA or Secondary FIFO.

DMC-1700/1800 Chapter 4 - Software Tools and Communications • 59