With the introduction and widespread use of 32 bit microprocessors such as the Intel 80386 and 80486, the 16 bit ISA bus faced a serious data throughput bottleneck. Passing a 32 bit word across the expansion bus in two 16 bit halves presented a serious waste of valuable processing time. Not only was data and CPU speed an issue, but video and audio systems in PCs had also been improving - and demanding an increasing share of bus bandwidth. By early 1987, IBM concluded that it was time to lay the ISA bus to rest, and unleash an entirely new bus structure which it dubbed the MicroChannel Architecture (MCA). IBM incorporated the MCA bus into their PS/2 series of personal computers, and also in their System/6000 workstations. This chapter shows you the layout and operations of the MCA bus.

All things considered, the MCA bus was a revolutionary - and superior - design. One of the most substantial advantages is a reduction in electrical noise due to a radical re-arrangement of bus signals. Unlike the ISA or EISA bus (which had only a few ground lines), the MCA bus provides an electrical ground every fourth pin. Superior grounding and the corresponding reduction in electrical interference also meant that the MCA bus can operate at higher frequencies than XT or AT busses (10MHz as opposed to the ISA/EISA 8.33MHz). The MCA bus also offers extended performance in data and addressing. You already know that an MCA bus can work with up to 32 bits of data. However, the bus also has an increased number of address lines (32 instead of 24). This increases the amount of directly addressable memory from 16MB to 4GB.

The MCA also brings sound and video to the bus. A single analog audio channel is added to the 8-bit bus segment. The audio channel can handle voice and music, and is intended to be almost as good as FM radio (roughly about 50Hz to 10kHz). Since the audio channel is available to all expansion devices, the signal can be exchanged and processed among each device independently. A VGA video extension is also provided with the MCA bus. This allows expansion video boards to be installed and work in concert with the VGA circuitry already existing on the MCA motherboard. An 8-bit video data bus and all necessary synchronization signals are available to an expansion board. Typically, only one video extension connector is included on an MCA motherboard.

Still more advances include such features as matched memory cycles, burst and streaming data modes, data multiplexing, and bus mastering. A matched memory cycle is supported with a small expansion connector. When a device is capable of sustaining matched memory transfers, the typical memory transfer cycle of 250nS is increased 25% to only 187nS. The burst data transfer mode allows data to be transferred in blocks without the intervention of a CPU (unlike ordinary data transfers which require multiple CPU cycles for each transfer). The streaming data transfer mode allows even faster transfers during bus-mastering operation. Using a data multiplexing technique, the MCA bus can accomplish 64-bit data transfers by multiplexing the upper 32 data bits on the 32 idle address lines. Finally, the MCA bus supports bus mastering - a technique which allows other devices besides the main CPU to take control of the system busses to accomplish their respective tasks.

Although MCA offers many tangible enhancements over the ISA bus, computer users refused to abandon their hardware and software investment in order to scramble for limited MCA-compatible peripherals to fill their needs. As a result, the MCA bus has never become the new standard that IBM hoped it would be. Though the number of PS/2 systems still in service are dwindling rapidly, you may still encounter PS/2s in many home and school environments.

The layout for an MCA bus slot is shown in Fig. 28-1. Note that there are up to three segments to the bus connector; an 8-bit portion, a 16-bit portion, and a 32-bit portion. There is also an auxiliary video extension connector which is usually available on only one slot. The first thing you should realize about the MCA bus is that it is physically much smaller than an ISA or EISA bus - as a result, it is totally incompatible with ISA or EISA expansion products.

The pinout for a 16-bit MCA slot is shown in Table 28-1. This is the primary type of MCA connector which combines video and audio signals in the expansion bus. The connection itself can be divided into three sections; the video section (pins xV00 to xV10), the 8 bit section (pins 1 through 45), and the 16 bit section (pins 48 to 58). Power, ground, and interrupt lines are easy to spot, but most other signals are new. The signal pinout for a 32-bit MCA slot is shown in Table 28-2. The 32-bit bus replaces the video section with a smaller matched memory control section (pins xM4 to xM1), but 8 bit and 16 bit sections remain the same. The 32 bit MCA slot also includes a 32 bit section (pins 59 to 89).

Enable Synchronization (ESYNC) controls VGA signals (VSYNC, HSYNC, and BLANK) on the motherboard. When ESYNC is true, the Vertical Synchronization (VSYNC) pulses, Horizontal Synchronization (HSYNC), and Blanking (BLANK) signals control the display. An independent 8 bit video data bus (P0 to P7) support 256 colors on the VGA display. VGA timing signals are controlled by the Enable Data Clock (EDCLK) and Data Clock (DCLK) signals. The Enable Video (EVIDEO) signal switches control of the palette bus allowing an external video adapter to provide signals on P0 to P7. Audio (AUDIO) and Audio Signal Ground (Audio GROUND) allow the expansion board to send tone signals to the motherboard speaker.

There are 32 address bits (Address Bit 0 to Address Bit 31), 11 interrupts, and 32 data bits (Data Bit 0 to Data Bit 31). The - Address Latch (- ADL) signal is true when a valid address exists on the address lines. A -Channel Check (-CHCK) signal flags the motherboard when an error is detected on the expansion board. When data on the data bus is valid, the -Command (-CMD) is true. The Channel Ready Return (CHRDYRTN) signal is sent to the motherboard when the addressed expansion board I/O channel is ready. A Channel Reset (CHRESET) signal can be used to reset all expansion boards. The -Card Setup (-CDSETUP) instructs an addressed board to perform a setup. The Memory Address Enable 24 (MADE24) line activates address line 24. The Channel Ready (CHRDY) line that the addressed board is idle after completing its access. When -Burst (-BURST) is true, the system bus will execute a burst cycle.

The -Data Size 16 Return (-DS16RTN) and -Data Size 32 Return (-DS32RTN) tell the motherboard whether the board is running at a 16 or 32-bit bus width. The System Byte High Enable (SBHE) signal is true when the upper 16 data bits are being used, but the Card Data Size 16 (CDDS16) signal is true when only 16 data bits are being used. If all 32 bits of data are being transferred, the -Card Data Size 32 (-CDDS32) signal is true. When the main memory is being refreshed, the -Refresh (-REF) line is true. This allows any dynamic memory on expansion boards to be refreshed as well. The Memory/ -I/O (M/ -I/O) signal defines whether the expansion board is accessing a memory or I/O location. Signals -S0 and -S1 carry the status of a MicroChannel bus.

The -Preempt (-PREEMPT) signal is true when a bus arbitration cycle begins. The Arbitration signals ARB00 to ARB03 indicate (in BCD) which of the 16 possible bus masters has won arbitration. The Arbitration/ -Grant (ARB/ -GNT) is high when the bus is in arbitration, and low when bus control has been granted. When a DMA transfer has finished, the Terminal Count (TC) signal is true. -Byte Enable signals 0 to 3 (-BE0 to -BE3) indicate which four bytes of a 32-bit data bus are transferring data. When an external bus master is a 32-bit device, the Translate 32 (TR32) line is true. The -MMCR, -MMCCMD, and -MMC lines are matched memory control signals.

MicroChannel (PS/2) computers require the use of a "reference" (or "startup") disk whenever configuring the system, or exchanging MCA cards in their bus slots. Each time you add, remove, or exchange an MCA card, you’ll need to run the reference disk. However, reference disks are often among the first items to be misplaced as a PC is sold or passed from owner to owner. When servicing a PS/2 system, you may easily find yourself without a reference disk. Fortunately, the reference disks for many PS/2 systems (along with a variety of diagnostic disks) can be downloaded from the IBM FTP site. Table 28-3 highlights the most popular FTP download addresses. Download the file to an empty directory on the hard drive, then decompress the file. This will usually result in several individual files, with specific instructions for creating a reference/startup disk.

NOTE: For files with a .TG0 extension, you will also need to download Teleget - a utility which will extract .TG0 files: ftp://ftp.pc.ibm.com/pub/pccbbs/os2_fixes/tgsfx.com

For PS/2 55sx systems, a password will prevent the reference disk from running. You’ll need to clear the CMOS RAM (including the password). Ordinarily, you can remove the CMOS backup battery and allow the memory to clear. But for 55sx systems, you can use the speaker cable to clear the CMOS RAM:

• Turn off your system.
• Unplug the speaker cable from the riser card and plug it in upside down.
• Turn it back on, wait for memory to count, and listen for the beep.
• Turn the system off, and plug the speaker cable right side up again.
• The password will be cleared when you power the system up again.

When running the PS/2 reference disk, you’ll need to have an .ADF file for each MCA board in the system. If you encounter a "165" error when running "autoconfigure" with the reference disk, chances are that the .ADF file for one or more MCA devices in the system is missing from the reference disk. Check with the driver software that came with the particular MCA card (its diskette may contain the needed .ADF file).

If you’ve got IBM-brand MCA cards in the system, you can obtain current .ADFs (Adapter Description Files) directly from IBM. Download the ALLFILES.TXT file from; ftp://ftp.pc.ibm.com/pub/pccbbs/allfiles.txt and locate the .ADF file for your particular device(s). You can then navigate the FTP site and obtain the .ADF file (usually somewhere under ftp://ftp.pc.ibm.com/pub/pccbbs/).

If you’re using third-party MicroChannel cards, you’ll need to contact each particular manufacturer and download the current .ADF file from their "tech support" areas or FTP sites. One very good source for .ADF files for older, non-IBM MicroChannel cards is at NCR’s web site (http://www.ncr.com/support/pc/pcdesc/library/adfs.shtml). You should also check Peter H. Wendt's Web site (http://members.aol.com/phwimage1/mcaindex.htm) for free software that will identify MicroChannel controllers, and give you the ADF file name. The site also has a large library of .ADF files for download.

In most cases, you will not be troubleshooting a bus - after all, the bus is little more than a passive connector. However, the major signals that exist on an MCA bus can provide you with important clues about the system's operation. The most effective bus troubleshooting tool available to you is a POST board (such as the ones discussed in Chapter 28). Many POST boards are equipped with a number of LEDs that display power status, along with important timing and control signals. If one or more of those LEDs is missing, a fault has likely occurred somewhere on the motherboard. Refer to Chapter 28 for detailed POST board instructions. While most POST boards are designed for ISA bus work, Micro2000 provides an MCA adapter for their POST-Probe.

Another point to consider is that bus connectors are mechanical devices - as a result, they do not last forever. If you or your customer are in the habit of removing and inserting boards frequently, it is likely that the metal "fingers" providing contact will wear and result in unreliable connections. Similarly, inserting a board improperly (or with excessive force) can break the connector. In extreme cases, even the motherboard can be damaged. The first rule of board replacement is; always try removing and re-inserting the suspect board. It is not uncommon for oxides to develop on board and slot contacts that may eventually degrade signal quality. By removing the board and re-inserting it, you can wipe off any oxides or dust and possible improve the connections.

The second rule of board replacement is; always try a board in another expansion slot before replacing it. This way, a faulty bus slot can be ruled out before suffering the expense of a new board. If a bus slot proves defective, there is little that a technician can do except:

1. block the slot and inform the customer that it is damaged and should not be used.
1. replace the damaged bus slot connector (a tedious and time-consuming task) and pass the labor expense on to the customer.
1. replace the motherboard outright (also a rather expensive option).

That’s it for Chapter 28. Be sure to review the glossary and chapter questions on the accompanying CD. If you have access to the Internet, take a look at some of the MCA (or PS/2) resources listed below (in addition to the FTP references in Table 28-3):

General Technics: http://gtweb.net/mi151.html (MicroChannel Sound Card)

IBM PS/2 Reference Disks: ftp://ftp.pc.ibm.com/pub/pccbbs/refdisks/

Indelible Blue: http://www.indelible-blue.com/ (MicroChannel add-on boards)

PS/2 Parts: http://www.can.ibm.com/parts/catalogue/indexes/ps2indx.htm

Vintage PCs: http://www.can.ibm.com/helpware/vintage.html

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