For the Commodore 64 & 128 Computers

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Multiplexer Unit


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Multiplexer Unit

Xetec's MUXer:

In 1987, Xetec released a supplemental device, a Multiplexer (MUX) unit. The MUX unit provides the means of connecting multiple C64 or C128 Commodore computers (each with Host Adapters) to a single hard drive. This capability was useful in a business environment or school computer lab. The MUX unit was also used by BBS SysOps to connect multiple computers (each having their own modem) to one drive, which provided a multi-phone line BBS system with one central storage device.

You can connect up to four Commodores to a single MUX unit and gang a total of four MUX units together, which provides the capability of connecting sixteen (16) Commodores to one hard drive. No other Commodore hard drive system manufacturer ever offered 'computer multiplexing' capability for their products.

As mentioned in the Host Adapter section, the host adapter is fitted with a 4-position DIP switch used to set the Binary 'number' of the Host Adapter - 0 through 15. This 'address' is identified as the Port Number (seen in the F3 page in "config"). When connecting multiple Commodores (with different Port numbers) to a Mux unit, one of these computers must have its Host Adapter set to Port address Zero. This computer then becomes a kind of Administrator's station because Port Zero has more control in hard drive Configuration.

For example, only Host Adapter Port Zero can change LU sizes and Activate an LU while all Port numbers can change their own screen colors.  However, one of LK DOS routines (SWPOUT) uses the Port Number to define a different 'Port buffer' on the disk for data transfers.  Therefore, if two or more Host Adapter's Port Number are set the same, this would cause SWPOUT to write data to the same buffer resulting in one of the identically IDed stations to crash.

In the same way Host Adapters need to be set to different Port Numbers, The Mux unit also has an internal Address switch. The Mux address switch is actually a set of four different jumper pins, labeled M1, M2, M3 and M4.  Therefore, if you were using four Mux units, each would be set to one of four different Mux unit numbers.  If you are using only one Mux unit, there is no performance difference if you set the Mux unit to M1, M2, M3 or M4 address.

How does it works:

As shown in the photo below, the MUX design is fairly simple. All 8-Bit Data and SCSI Control lines are all connected via Mux circuitry to each of the DB-25 connectors. There are four Host Adapter inputs (referred to as Slave 1 - 4; note that Slave 1 is the Master Host Adapter), one Input from another MUXer unit (referred to as Multiplexer Output) and one input from the hard drive referred to as Drive Input (Multiplexer Input). Computer configurations where more than one MUXer is used, the 'Multiplexer Input' would connected to the 'Output' of the other MUXer (instead of the hard drive). Regarding Mux power, each unit is powered by its own power supply (9vdc @ 500ma - center positive).

Lt. Kernal Multiplexer Unit

Simply stated, The MUXer uses a round-robin access to the hard drive to prevent simultaneous drive access. It does this by using an internal free-running Counter that decodes control to one Host Adapter at a time.  If this hard drive accessible time-slot is grabbed by a Host Adapter, the Counter stops and exclusive hard drive access is given to that Host Adapter.  When SCSI communication is finished, the Counter continues and other Host Adapters are offered drive access.

The MUXer itself knows nothing about Host Adapter Port numbers, just that there are different Host Adapters connected. The Mux unit manipulates the SCSI control lines in a way that gives each computer a period of exclusive access to the drive's SCSI bus that extends from the MUXer to the hard drive. This action is totally transparent to the computers. Electronically, MUXing would work even if all Host Adapters were set to Port 00.  However, LK DOS would overwrite data in All like-numbered Port Number Buffers - causing data corruption or a system crash!

Where the Port number assignments become important is in preventing simultaneous writes to common data areas on the drive. The LTK DOS, in an incredibly glaring oversight, did not have any provisions for preventing a simultaneous write situation. It is the lack of write conflict arbitration that can result in LU and/or file corruption on a multiplexed system. Amazingly enough, none of the BBS developers ever addressed this aspect of the LTK DOS, which meant they couldn't get their programs to work right on MUXed systems.

However, if you use multiple computers and assign each to a different LU section of hard drive storage, the system works perfectly! The only problem that could occur would be if Any computer operator decided to run 'config' and change storage to someone else's in-use LU#!

For detailed Mux unit troubleshooting instructions, schematic and timing diagram, click HERE.

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