VESA Media Channel
Following text is extracted from "The Micro Channel Architecture
Handbook" Chet Heath and Winn L. Rosch, pages 125-127 Simon & Schuster
NY, NY. ISBN 0-13-583493-7 1990.
The video extension gives Micro Channel expansion boards
direct access to the various inputs of the digital-to-analog convertor
or DAC used by VGA. As it's name implies, the DAC makes analog signals
compatible with VGA monitors from the digital signals generated by the
computer's circuitry. By allowing access to the inputs of the DAC, the
Micro Channel video extension allows changes and improvements to be made
in the computer's video system without sacrificing monitor compatibility...
The video extension (Ed. AVE)
uses several important signals. Present here are horizontal and vertical
synchronizing signals plus a special control line called ESYNC or enable
sync line. This line determines whether the synchronizing signals used
in the video system original on the planar board or from an adapter plugged
into the Micro Channel. ESYNC is normally held to logical high. Bringing
it low enables the system to use the synchronizing signals from the Micro
Video data are transferred across the Micro Channel video
extension in digital form using eight video data lines. The data here are
used to drive the VGA Digital-to-Analog convertor on the system board.
Video Extension Signal Descriptions
The following are signal descriptions for the auxiliary and base video
extensions of the channel connector.
VSYNC: Vertical Synchronization:
This signal is the vertical synchronization signal to the display. Also
see the ESYNC description.
HSYNC: Horizontal Synchronization:
This signal is the horizontal synchronization signal to the display. Also
see the ESYNC description.
BLANK: Blanking Signal: This signal
is connected to the BLANK input of the video DAC. When active (0 V dc),
this signal tells the DAC to drive its analog color outputs to 0 V dc.
Also see the
P7- P0: Palette Bits: These eight
signals contain video information and comprise the PEL address inputs to
DAC. See also the EVIDEO description.
DCLK: Dot Clock: This signal is
the PEL clock used by the DAC to latch the digital video signals, P7 through
P0. The signals are latched into the DAC on the rising edge of DCLK. This
signal is driven through the EXTCLK input to the VGA when DCLK is driven
by the adapter. If an adapter is providing the clock, it must also provide
the video data to the DAC. Also see the EDCLK description.
ESYNC: External Synchronization:
This signal is the output-enable signal for the buffer that drives BLANK,
VSYNC, and HSYNC. ESYNC is tied to + 5 V dc through a pull-up resistor.
ESYNC is high, the VGA drives BLANK, VSYNC, and HSYNC. When ESYNC is
pulled low, the adapter drives BLANK, VSYNC, and HSYNC.
EVIDEO: External Video: This signal
is the output-enable signal for the buffer that drives P7 through P0. EVIDEO
is tied to + 5 V dc through a pull-up resistor. When EVIDEO is high, the
VGA drives P7 through P0. When it is pulled low, the adapter drives P7
EDCLK: External Dot Clock: This
signal is the output-enable signal for the buffer that drives DCLK. EDCLK
is tied to + 5 V dc through a pull-up resistor.
When EDCLK is high, the VGA is the source of DCLK to the
DAC and the adapter. The Miscellaneous Output register should not select
clock source 2 (010 binary) when EDCLK is high.
When EDCLK is pulled low, the adapter drives DCLK. If
the adapter is driving the clock, it must also provide the video data to
the DAC, and the Miscellaneous Output register must select clock source
2 (010 binary).
Vesa Media Channel
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, MicroChannel,
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
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
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
Figure 16.3 VESA Media Channel card edge connector. (missing)