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Revision B User Manual
for the VME-SIO4: Board Revision: A
General Standards Corporation
8302A Whitesburg Drive Huntsville, AL 3580, Phone: (256) 880-8787
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run at full speed without interfering with each other. The VME can read and write the Zilog during DMA cycles.
However, it must wait until the end of the current DMA access when the DMA finishes its current access, then the
VME will be allowed onto the Zilog bus.
When the Zilog bus is free from the VME, the DMA will restart. The same holds true for interrupts. If a VME or
interrupt access to or from the Zilog is currently taking place, and a DMA request is made, the DMA will wait until
the current access cycle has completed and the bus for the Zilog is free before it starts the DMA transfers.
2.3
CABLE
The Cable is configured as upper and lower so that one cable can be used for both transmit and receive, therefore
allowing full duplex capabilities for each channel. Each channel has a control register that can be set to transmit
upper or lower and to receive upper or lower. It is not possible to receive both but it is possible to transmit both. If
the channel control register is not told to transmit or receive upper and not told to transmit or receive lower, then this
board will not drive the cable, nor will it load the cable, i.e., this channel will be tri-stated.
If an external loopback test is desired to be performed without a cable, the software can set it up to do transmit,
upper or lower, and to receive the same. The effect given will be an external loopback without a cable.
2.4
TRANSMIT RECEIVE CLOCK
The transmit receive clock is controlled via a 3 x 5 jumper. If the jumper is removed, for a particular clock, it is
expected that the Zilog will produce the transmit receive clock. It cannot output a clock to the half of the cable that
it is receiving from. It can output a clock to the cable it is transmitting to. If the jumper is selected for “receive
clock”, the Zilog will get its’ clock from the cable, given that the software has chosen the cable as receive. If the
Zilog clock is chosen for the onboard transmit receive clock, then the Zilog will get its’ clock from the local
oscillator, factory installed at 20.0 megahertz.
NOTE:
Care must be taken when setting these jumpers. If the jumper is installed and the Zilog drives a clock out,
then a conflict between clocks will exist.