How Does the VXI Resource Manager Use the MODID Lines to Perform Slot Detection of Statically Configured VXI Devices?
Hardware: VXI/VME/MXI
Problem: I built my own VXI card. When I put it in my VXI chassis, Resource Manager (Resman) returns errors about conflicting devices in conflicting slots. How does Resman detect which cards are in which slots?
Solution: According to the VXI specification, the slot 0 controller can detect the presence of devices in a slot by using the MODID lines on the VXI backplane.
For statically configured VXI devices, the MODID lines serve two purposes. First, they identify whether a card is in the slot, even if it is nonoperational. Second, the MODID lines uniquely associate a VXI card's statically configured logical address with the slot where it is found.
- By default, the state of the MODID line for a given slot is high (+5 V) when no card is found in the slot, because the slot 0 controller is required to provide a 16.9 kOhm pull-up resistor for each MODID line. In compliance with the VXI specification, a card must provide a 825 kOhm pull-down resistor to pull its MODID line back to ground. In this situation, even if the card is nonoperational or damaged, the slot 0 controller can detect the presence of a device by monitoring the state of the MODID line for a given slot because if a MODID line is at ground, there is a VXI card present in that slot.
- During initialization, Resman pulls up in succession the MODID line of each slot that was previously pulled down by the presence of a VXI card. After pulling down a MODID line to ground, Resman queries each logical address and checks bit 14 at offset 0x4 (the MODID bit) of the device's A16 configuration registers. This bit reflects the state of that corresponding slot and device pair's MODID line (1=LOW, 0=HIGH). If Resman reads 1 at a given logical address, it then can associate that logical address with the slot whose MODID line Resman is currently driving low. The process is repeated for each slot with a device present until all devices with statically configured logical addresses are detected.
Related Links: KnowledgeBase 0OM7OT52: The Resource Manager (Resman) Explained and Useful Tips
KnowledgeBase 13KBG6SR: How Does the VXI Resource Manager (Resman) Work?
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Report Date: 03/27/2002
Last Updated: 04/03/2002
Document ID: 2JQCE7ON
