MicroVAX 3300/3400/3500/3600/3800/3900

Introduction
The MicroVAX Mayfair series of computers were intended to compete with the IBM AS/400. They all featured the DIGITAL Small Storage Interconnect (DSSI) interconnect that connected to RF series disks and another computer using the DSSI bus. The difference between the complementary models, for example a MicroVAX 3500 versus a 3600, was the cabinet. The MicroVAX 3500 came in a compact 27 inch high cabinet (BA213 enclosure), perfect for an office environment, whereas the MicroVAX 3600 was housed in a 40 inch high cabinet designed for a computer room environment. Both enclosures supported twelve slots for Q-bus devices. The taller enclosure could accommodate two additional disk such as 1.2 GByte RA90s for additional storage capacity. The CPU processor was common to both MicroVAX 3500 and MicroVAX 3600.

Processor Performance

Processor	CPU	Clock Rate	VUP Rating	Year
MicroVAX 3500/3600	KA650	11.11 MHz	2.8	1987
MicroVAX 3300/3400	KA640	20 MHz	2.4	1988
MicroVAX 3800/3900	KA655	16.67 MHz	3.8	1989

MicroVAX 3500/3600
MicroVAX 3500 and MicroVAX 3600 The MicroVAX 3500 and MicroVAX 3600, code named Mayfair, were introduced in September 1987 and were meant to be the higher end complement of the MicroVAX family. These new machines featured more than three times the performance of the MicroVAX II and supported 32 MB of ECC main memory. The performance improvements over the MicroVAX II resulted from the increased clock rate of the CVAX chip set, which operated at 11.11 MHz (90ns cycle time) along with a two-level, write-through caching architecture. It used the KA650 CPU module.

MicroVAX 3300/3400
The MicroVAX 3300 and MicroVAX 3400, code named Mayfair II, were entry-level to mid-range server computers introduced on 19 October 1988 intended to compete with the IBM AS/400. They used the KA640 CPU module. MicroVAX 3400 and VAXserver 3400 base system components resided in a BA213 enclosure, with a 12-slot backplane. The base system included a central processor unit (CPU) module, and one to three memory modules. The CPU module resided in slot 1 of the backplane. Memory modules were in slots 2 through 4.

The VAX3300 was housed in a smaller BA215 enclosure which accommodated only 6 Q-bus devices and was targeted for small offices. It could accommodate two RF30 ISE (150 MBytes) disks and a TK70 tape drive. It had limited expansion capability and had a base memory of 4 MBytes (ECC). The VAX3400 provided more device and memory expansion capability as it was housed in the wider BA213 enclosure.

MicroVAX 3300/3400 SPECIFICATIONS

CPU CPU Module: KA640 with 1Kbyte on-chip cache, 64KByte on-board cache and a Floating Point Accelerator (FPA) chip Number of Processors: 1 CPU Cycle Time: 20MHz MEMORY Maximum Memory: VAX3300 was limited to 4MBytes due to the smaller cabinet and limited expansion, the VAX3400 had 4MByte, 400-nanosecond, 39-bit wide memory array expandable to 28MBytes (3 memory card slots available in chassis) I/O INTERFACES QBUS: Q22 LAN Support: 10Mbit Ethernet, standard AUI and thinwire Storage Bus: DSSI bus adapter PERFORMANCE VUPS: 2.4

MicroVAX 3800/3900
The MicroVAX 3800 and MicroVAX 3900, code-named Mayfair III, were introduced in April 1989. They were high-end models in the MicroVAX family, replacing the MicroVAX 3500 and MicroVAX 3600. At introduction, the starting price of the MicroVAX 3800 was US$81,000 and that of the MicroVAX 3900 was US$120,200. A variant of the MicroVAX 3800, the rtVAX 3800, was intended for real-time computing (RTC) applications such as computer-aided manufacturing (CAM). These systems used the KA655 CPU module, which contained a 16.67 MHz (60ns cycle time) CVAX chip set. They supported up to 64 MB of memory.

DSSI
The DIGITAL Small Storage Interconnect (DSSI) bus had the following characteristics:

• 4 Mbytes-per-second bandwidth
• Single-ended bus transfers
• Up to eight nodes (up to two host adapters and six mass storage devices)
• Eight data lines
• One parity line
• Eight control lines

DSSI architecture featured a DSSI host adapter and DSSI mass storage devices. The DSSI host adapter, located on the KA640 CPU module, was the interface between the DSSI mass storage devices and the CPU. The DSSI mass storage devices, such as the RF30, contained a built-in controller. DSSI architecture improves system performance through the following:

• DSSI bus handled all mass storage transactions (Q-bus is free for other activity).
• DSSI bus was faster than the Q-bus.
• Mass storage devices could act independently (since each device had its own controller, several devices could work simultaneously).

The DSSI supported up to eight daisy-chained nodes. These eight nodes might be one DSSI host adapter and seven DSSI mass storage devices, such as seven RF-series disk drives. Atypically, a DSSI mass storage device could maintain connections to more than one DSSI adapter. Since the DSSI adapter was located on the KA640 CPU module, a DSSI mass storage device could connect to more than one KA640 CPU module.

For example, an external DSSI cable could connect two MicroVAX 3400 systems. The DSSI bus, in this case, would consist of two DSSI host adapters and up to six RF30 disk drives. The two MicroVAX 3400 systems would have access to each drive on the DSSI bus.

Sources:

• wikipedia.org