005-038601 – SCSI II Controller (6786 / 6787)

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This is SCSI II Controller P/N 005-038601 which I am told had the name Inferno. The Data General Model number for the PCB was 6786 and 6787. The two model numbers denote a disk/tape controller and tape only controller, the only difference being a DIP switch setting on the PCB. 

The PCB presents the SCSI bus on a DB50 connection on the chassis and connects to the backplane via DG cable assembly P/N 005-00000. However it is worth noting that the pinout on the DB50 is not SCSI standard!

The PCB makes use of a number of interesting devices:

Motorola 68EC030 32Bit CPU
NCR 53C710 SCSI Controller 
On-Board Flash (microcode and menu driven console configuration)

The controller also has a set of switches that set the SCSI bus to Single Ended or Differential.

Technical Specification

Models 6786 and 6787 SCSI-2 Host Bus Adaptors (HBA’s) are high speed bus adaptors for ECLIPSE MV family that have Burst Multiplexer Channel (BMC) interfaces. The HBA’s occupy one slot in an ECLIPSE MV family computer chassis and connects the BMC to SCSI-2 making both SCSI-1 and SCSI-2 devices available to ECLIPSE MV family computers.

The HBA is a SCSI initiator, sending commands from the host computer over SCSI bus to initiate activity in a peripheral device. Disk and Tape protocol requests are issued from the host computer via the PIO interface. The requests are interpreted by an embedded controller (the IOP) in the HBA. Depending on the request the IOP can respond via the PIO registers or it can retrieve additional command block information via the BMC DMA channel. The Data path for access to host memory by the HBA is the BMC.

IOP Clock Speed: 32Mhz
Bus Capacity: Seven SCSI Targets
Controller Latency: <1ms
Power Requirements: +5v DC 12.25A and 12v DC 0.5A

Field Replacable Units

005-038601: SCSI-2 Host Bus Adaptor PCB
005-038905: SCSI-2 Host Bus Adaptor Internal Cable

Configuring the Controller

To configure the HBA you have to assign:

  • Hard and Soft Device Codes
  • Default Device Type
  • SCSI Bus ID
  • BMC Bus ID
  • SCSI Mode of Operation

In addition you may have to run the Peripheral Microcode Installer (PMI) program to ensure that the HBA firmware level is up to date and that the correct OS has been set on the controller.

There are 15 switches in the HBA PCB that are used to configure the PCB operating characteristics (except for soft device codes which are configured using the build in configuration program). Three of the switches set the basic configuration of the board and the other 12 switches select either Single Ended or Differential mode of SCSI operation. 

SW9, SW14 and SW15 select the basic board configuration.

Models 6786 and 6787 HBA’s support up to eight different device codes which are designated as ‘Device ID 0’ thru ‘Device ID 7’. Device ID 0 has the highest priority and is defined as a ‘hard’ device code. This hard device code is assigned to the HBA, is set by a six position switch on the HBA PCB, and cannot be changed or disabled by firmware on the HNA PCB. The other seven device ID’s supported by the HBA are defined as ‘soft’ device codes and are programmed by the HBA firmware. If there are two model 6786 or 6787 host adaptors on the same I/O channel, each must have a different hard and soft device codes.

Switch / Jumper Settings

SW9 is a six position DIP switch that selects the HBA hard device code referred to as Device ID 0. The switches are read by the HBA IOP through the default register (Switch ON is logic 0, Switch OFF is logic 1):


The recommended HBA hard device code is 13 (octal) or 001011 (binary) and the recommended secondary HBA hard device code is 53 (octal) or 101011 (binary).

SW9-1 thru SW9-6 = 110100 for hard device 13 (octal)
SW9-1 thru SW9-6 = 010100 for hard device 53 (octal)  

SW14 is a four position DIP switch that selects the HBA default device type and the SCSI bus device identification (DID). This default device type is specified by a single bit that indicates if the device is masked out as a tape or disk device. The default device type can be read along with the hard device code through the default device register. The SCSI bus DID is specified by three bits that select a SCSI DID number from 0 thru 7; the DID can be read through the SCSI status register. (Switch ON is logic 0, Switch OFF is logic 1):

SW14-1 ON = Disk, OFF = TAPE (configurations of both Disk and Tape should select DISK)
SW14-2 SCSI DID bit position 2 (MSB)
SW14-3 SCSI DID bit position 1
SW14-4 SCSI DID bit position 0 (LSB)

In most configurations the HBA should have a SCSI DID of 7 or 111 which is the highest priority device on the SCSI bus. To achieve this SCSI DID set SW14-2,SW14-3 and SW14-4 to 000

SW15 is a four position DIP switch that selects the HBA ID on the BMC bus. Most EXLIPSE MV family BMC computers permit as many as 8 BMC controllers to be configures on the host BMC bus. Models 6786 and 6787 HBA’s can be assigned any priority on the BMC bus from 0 thru 7. Three positions of SW15 are used to assign the BMC ID number to the HBA. (Switch ON is logic 0, Switch OFF is logic 1):

SW15-1 Not Used

SW1, SW2, SW3, SW4, SW5, SW6, SW7, SW8, SW10, SW11, SW12 and SW13 select either single-ended mode or differential mode of SCSI operation. The switches are arranged in two banks, one bank SW10 thru SW13 routes the SCSI bus signals from the SCSI controller either to differential transceivers or to terminators. The other bank of switches, SW1 thru SW8 selects either outputs of the differential transceivers or the single ended bus signals and routes the signals to the back-plane. 

Each of the switches should be set one way of the other as marked on the HBA PCB SE or DIFF. 

One bit in each switch is used to allow the hardware to determine the switch setting. The switch settings can be read by the IOP through the SCSI status register to check that all the switches are set the same way. 

Device Addressing

Soft device codes (those assigned to Disk, Tape and Arrays) are set using the HBA’s inbuilt configuration program. You access this program from the SCP CLI by booting to the HBA’s hard device code, for example:

SCP-CLI> boot 13

The following screen is displayed:

Data General SCSI-II HBA Main Menu

To set a soft device code select option 2 ‘Modify Current Device Code Assignments’. When the modify screen appears follow the directions to set the soft device codes for each SCSI device as desired.

Firmware and Fault Diagnosis

You must access the PMI program to update the HBA firmware revision level. You access the PMI program from the MV ADEX prompt by typing PMI. Proceed through the various screens until you get to the INSTALL SCREEN. Enter selection 0 (for unit 0) to install the firmware. The installation takes less than a minute – DO NOT INTERUPT THE PROCESS. When the INSTALL screen returns the entry under MICROCODE INSTALLED should read ‘Included’ rather than ‘No Activity’

There are two diagnostic tools to help diagnosing faults in models 6786 and 6787 Host Bus Adaptors (HBA) a diagnostic LED on the PCB and the Eclipse MV family diagnostic programs.

The diagnostic LED (CR3) indicates the general status of the HBA, when this green LED blinks steadily (approx. once per second) the HBA is operating without error. If the LED stops blinking i.e. shows a steady light there is an error condition on the HBA. 

If an error occurs during operation of the HBA the HBA reports a panic code to the Operating System.

MV ADEX is the diagnostic operating system used to diagnose faults in the host bus adaptor. Specific  programs under MV ADEX for this HBA are:


Backplane Connectivity
Signal SESignal DIFFBackplaneD50 SCSID50 DG
High Resolution Image