Power Mac 6400-series

Motherboard Diagram by MacGurus
We appreciate your technical corrections and comments !

The Performa 6400, like the related Performa 6360 and Power Mac 5400, boasts a 32-bit PCI bus architecture and a 64-bit memory bus. With two (2) DRAM DIMM slots, some variants prefer JEDEC-standard 5-volt, 64-bit, 168-pin, fast-paged mode (FPM) 70ns DIMMs (or faster), which must refresh at a rate of 2k or better.

As is true with other Alchemy-based Power Macintosh like the 5400 & 6360-series, however, EDO memory is often preferred, even though no performance benefit accrues in the case of those 6400's designed to use FPM DRAM.

It should be emphasized that the 6400-series can be distressingly finicky about DRAM, and in some cases, standardizing on EDO yields superior results in terms of compatibility and system stability.

Those variants of the Performa 6400 which Apple shipped with internal Zip drives, for example, reap a modest performance bonus by utilizing 5v, 60ns, JEDEC-standard, buffered EDO DIMMs. Apple TIL articles Macintosh Performa 6400/200 Zip: Supported Memory Types, and Power Macintosh Computers: Using FPM, EDO, SDRAM & SGRAM confirm that FPM DIMMs are also acceptable, but any performance benefit derived from the use of EDO DIMMs will be sacrificed.

MacGurus has noted that optimum results are achieved by utilizing high-quality EDO DRAM in these machines, although it is also clear that EDO and FPM memory should not, as a rule, be mixed if optimum performance is desired. The effect of mixing DRAM types on system stability is less clear, and still under scrutiny.

As mentioned, and with Apple technical documents notwithstanding, it should be stressed that the 6400-series can be exasperatingly picky, and the cautious owner will strive to standardize on one type of memory--ideally that which is most appropriate for the particular logic board. Those 6400's which prefer EDO DRAM can be identified thusly:

  1. The logic board utilizes soldered EDO DRAM;

  2. A resistor is present at MLB (Main Logic Board) reference U34;

  3. The MLB lacks a resistor at board reference R77;

  4. The 6400 shipped with an internal Iomega Zip drive.

    Those 6400's which utilize soldered EDO DRAM typically have two chips situated between the processor and ROM slot, as shown in the drawing above (labelled "soldered-on DRAM"). These chips are typically marked as follows:

  1. Flipping the logic board upside down;
  2. Locating MLB reference U34. With the rear of the logic board oriented towards you, U34 is located approximately half-way up on the right-hand side.
  3. If a resistor (SIP) is present (typically five-pinned ceramic), the MLB supports EDO DRAM.1

    Finally, those logic boards which lack resistors at board reference R77 also prefer EDO DRAM, and are readily identifiable by inspecting the three small groupings of microresistors just south of the processor and heat sink, as identified in the drawing above (labelled "Resistor R77"). If there is no resistor at board reference R77, the logic board supports EDO.

    In the event FPM memory is preferred, it may be helpful to know that the more common 4k refresh DIMMs acceptable in other PCI Power Macs are not compatible with the 6400 motherboard, insofar as they will register only half of their engineered memory density. It should be noted, however, that DIMMs supporting 1k or 256k refresh are compatible with the 6400's PSX memory controller, as are 4k devices based on a 2M x 8 architecture with 12 x 9 addressing. Additional information is available in Power Macintosh 5400 & Performa 6400: Adding Memory (TECHINFO-0019598).

    With 8MBs of DRAM soldered to the logic board, the 6400-series typically ships with an 8MB DIMM installed in one of its slots, for a default configuration of 16MB. Additional memory may be installed in any slot, in any order, as the PSX memory controller supports linear memory organization only, and cannot interleave. No performance benefit whatsoever derives from installing DIMMs in pairs in this machine.

    The maximum amount of usable memory in the 6400-series is 136MB, in the form of two (2) 64MB DIMMs, including the 8MBs of DRAM soldered on the logic board. Please see Power Macintosh 6400/180 & 6400/200: Specifications (11/96) in Apple's Technical Information Library (TIL) for more information.

    At the present time, the only approved L2 cache available for the 6400-series is Apple's 160-pin L2 DIMM, part number M4505LL/A, High-Performance Module for Power Macintosh 5400 and Performa 6400, available from most Apple Authorized Service Providers. It should be noted however that other memory vendors also offer a 512k L2 module which is fully compatible with the Alchemy-based 6400, 5400, and Power Computing PowerBase-series. Performance gains between 256k L2 and 512k L2 are similar to those observed in other models of Power Macintosh: on the order of 8-15% system-wide.

Logic Board RAM 8MB.
Supported DIMMs 32MB, 64MB
Number DIMM Slots Two (2).
Supported VRAM 1MB non-upgradeable DRAM.
Number VRAM Slots None.
L2 Cache Supported High-Performance Module for Power Macintosh 5400 and Performa 6400, part number M4505LL/A.
DIMM Type 168-pin, 5v, FPM, 2k refresh, 70ns or faster; or buffered 168-pin 5v EDO at 60ns or faster.

As noted, the 6400-series have no VRAM slots on the logic board, and minimal on-board video circuitry, although this Mac's multimedia pedigree is intriguing. Fortunately, this architecture supports 7-inch PCI video cards, including accelerated third-party video cards. VRAM configurations and machine capabilities are hence only limited by third-party offerings.

1The Apple Authorized Technician's Handbook Macintosh Computers, Volume V, as well as other Apple technical sources, were consulted in writing this page. It should be stressed that significant portions of the information on this page are also derived from the observations of MacGurus systems engineers. Drawing a hard, clear distinction between which data is derived from which source in this case is probably impossible. We have attempted to cite sources where feasible, as an aid to other service providers.

Back to Motherboards Directory