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Source: unknown
Note: The VMC interface is available on the Lacuna MediaBurst Module.
VMC, like VAFC, offers a 32-bit data path. But VMC supports up to 15 video
streams simultaneously and offers a more long-term solution for video computing
than VAFC. One developer described VMC as "a video superhighway that bypasses
the already-crowded system bus." Since VMC is a dedicated channel for real-time
video, peripherals can communicate independently and without slowing the system
CPU. VMC decouples the memory subsystem from the video transfer specification,
allowing graphics board manufacturers to offer a variety of boards with
differing types of graphics memory--DRAM, VRAM (video RAM), synchronous DRAM,
RAMBUS, and other future memory standards.
The VESA Media Channel (VMC) is a dedicated 132 Mbytes-per-second multimedia
bus that provides an independent path for the simultaneous processing of
several high bandwidth video streams. The VMC directly addresses the current
limitations of running video across a computer's system bus. This design solves
the universal bandwidth bottleneck and latency issues that exist in all system
or processor bus architectures including ISA, EISA, Micro Channel, VL-Bus, and
PCI.
To correct these problems, the VESA Media Channel is designed to allow the
transparent integration of video and graphics without the interference of
processor interrupts or bus contention. The VESA Media Channel provides the
option for a 68-pin multi-drop cable, allowing multiple devices to be combined
in a modular fashion. For example, a graphics system supporting the VESA Media
Channel can easily and cost-effectively be configured as a capture, ecode-only,
encode-only, or a full encode/decode video system. This is important in
applications such as video teleconferencing, and provides flexible cost
effective engineering of a particular system.
TIP: For any high performance video adapter, make sure that it supports at
least the 80-pin VAFC connector or the 68-pin VMC connector. If you see only a
26-pin connector on the card, then the card would not be recommended as that is
the standard VFC. Most of the higher-quality multimedia adapters will require a
VAFC connection for high-performance video signal transfer.
VMC Bus pinout HERE
To move uncompressed video through computer systems and multimedia equipment
derived from PC technology in real time, VESA developed a new, high speed bus
interface. Called the VESA Media Channel to represent its wide range of
applications well beyond traditional PC video systems, the interface was
designed with two conflicting goals in mind - high bandwidth and low cost. The
result is elegant in its simplicity, carrying few signals beyond video
data.
VM Channel is designed as a link between multimedia devices within a PC or
other equipment. Although it is aimed primarily at expansion boards built under
the PCI standard, its transfers are independent of the PCI bus and are not wed
to the PCI design at all. Two devices that use the VM Channel can transfer
commands and data between themselves without affecting PCI at all.
Because it is designed as a supplement to normal expansion buses, the VM
Channel depends on a traditional expansion bus to service the basic needs of
expansion boards. The VM Channel has no provision for supplying power to the
devices linked through it. Nor does it have any provision for addressing memory
or input/output ports.
VM Channel is a 32-bit bus. The signals it uses are listed in Table 16.11.
It is designed to move video data only in 32-bit double-words. The VM Channel
also uses the same 32 data lines to send commands throughout the system.
Because the VM channel is designed to accommodate devices of limited data
handling abilities, such as those with only 8- or 16-bit bus width, all
commands are limited to an 8-bit width. The channel thus requires one I/O cycle
to transfer each command, despite its 8-bit width. When the bus carries these
commands, only the lowest bits (on data lines 0 through 7) are significant; the
channel devices ignore the upper bits.
Table 16.11. VESA Media Channel Connector Signal Assignments:
Card pin Cable Pin Function Card pin Cable Pin Function
1 1 -SA 35 2 +EVST(0)
2 3 +EVST 36 4 Ground
3 5 -BS(0) 37 6 -BS(1)
4 7 Ground 38 8 -SNRDY
5 9 +Control 39 10 Ground
6 11 -Reset 40 12 Ground
7 13 Clock 41 14 Ground
8 15 Unused 42 16 Ground
9 17 +MASK0 43 18 +Mask1
10 19 Ground 44 20 Data 0
11 21 Data 1 45 22 Ground
12 23 Data 2 46 24 Data 3
13 25 Ground 47 26 Data 4
14 27 Data 5 48 28 Ground
15 29 Data 6 49 30 Data 7
16 31 Ground 50 32 Data 8
17 33 Data 9 51 34 Ground
18 35 Data 10 52 36 Data 11
19 37 Ground 53 38 Data 12
20 39 Data 13 54 40 Ground
21 41 Data 14 55 42 Data 15
22 43 Ground 56 44 Data 16
23 45 Data 17 57 46 Ground
24 47 Data 18 58 48 Data 19
25 49 Ground 59 50 Data 20
26 51 Data 21 60 52 Ground
27 53 Data 22 61 54 Data 23
28 55 Ground 62 56 Data 24
29 57 Data 25 63 58 Ground
30 59 Data 26 64 60 Data 27
31 61 Ground 65 62 Data 28
32 63 Data 29 66 64 Ground
33 65 Data 30 67 66 Data 31
34 67 Ground 68 68 -SB
The clock of the VMChannel operates as high as 33 MHz, matching the PCI
standard. This speed allows the peak throughput of the channel to reach
132MB/sec.
Data moves through the VM Channel in packets of double-words. Each transfer
of one double-word is termed a cycle. The VM Channel design defines two types
of cycles, control and data. These are distinguished by the dedicated +Control
signal on the bus. When this signal is active during a transfer, it indicates
that the next cycle after the transfer is a control cycle.
Exchanges across the VM Channel are broadcast rather than exchanged. That
is, when a source sends out video data, the bus carries the data to all devices
connected to the bus. Each device determines whether it should accept and use
the data.
To achieve its high data rate, the VM Channel does not use handshaking
between devices or any acknowledgment that the video data was in fact received
by its target. The nature of video data underlies this design decision: if real
time video doesn't arrive at its destination at the proper time, it is
worthless. Its moment on the screen is lost forever. Re-transmitting old video
data would merely be a waste of time and the bus.
On the other hand, VM Channel uses a not ready signal to indicate that the
target to which a device wants to send data is incapable of receiving or
processing that data. The not ready signal inhibits the transmission of data
from the source device so that unused or superfluous information does not waste
bus bandwidth. When a device takes control of the VM Channel then receives a
not ready indication, control passes from that device to the next one that
needs to transfer data across the channel.
Despite its broadcast design, the VM Channel operates as an arbitrated bus
with multiple bus masters. The VMChannel specification allows for up to 15
devices to share data. Several devices may be integrated into a single
assembly. When a device takes control of the VM Channel, the first data it
sends out must be a command called the Token Direction, which indicates which
device will take command of the bus following the transfer.
The standard VMChannel connector is an edge connector with the contact
fingers spaced on centers measuring 0.05 inches. You'll find the connector on
the top of both ISA and PCI video boards. Figure 16.3 illustrates this
connector.
Figure 16.3 VESA Media Channel card edge connector. (missing)
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