U28 may be an Emulex 2400150 or a  QLogic FAS236

BT-646S has RP2 and RP3 only. single-ended termination 
RP2 and 3 are 10-pin 100 ohm SIPs containing 9 bused 100 ohm resistors (CTS 4610X-1-101) 

BT-646D has RP6-RP13 differential termination 
RP6,8,10,12 are 10-pin SIPs with 5 isolated 150 ohm resistors (Bourns 4610X-2-151) 
RP7,9,11,13 are 10-pin SIPs with 9 bused 330 ohm esistors.(Bourns 4610X-1-331) 

DS36954AV The DS36954 is a low power, quad EIA-485 differential bus transceiver especially suited for high speed, parallel, multipoint, I/O bus applications. Five devices can implement a complete SCSI initiator or target interface. Three transceivers in a package are pinned out for data bus connections. The fourth transceiver, with the flexibility provided by its individual enables, can serve as a control bus transceiver.Datasheet

BT-646S/D SCSI Adapter for MicroChannel Systems
  32-bit bus master DMA transfers of up to 40MB/second across the MC bus, up to 10 MB/sec synchronous and up to 7 MB/set asynchronous SCSI data transfers. Support for up to seven SCSI devices, hard drives with up to 8GB capacity each. Enhanced SCSI-2 features: scatter-gather, disconnect-reconnect, and command-tag queuing 

BT-646x Host Interface Microprocessor
   A 20MHz Intel 80186 16-bit MPU is used to supply the speed for low command overhead. This MPU coordinates all activity on the BT-646S/D under the direction of the board’s firmware, including initialization, command decoding, interrupt generation, and control of the data flow among the board’s components. 

*This error is when single-ended SCSI drives are connected to the differential SCSI bus. 

I/O Port Address
  I/O Port of the host adapter 
   <"330h" (io 0330h-0333h)>, 230h (0230-0233), 130h (0130-0133), 334h (0334-0337), 234h (0234-0237), 134h (0134-0137) 

Interrupt Request
   Interrupt channel used to report status 
    <"IRQ-11">, 10, 15, 14, 9, 12 

Arbitration Level
   DMA channel used to transfer data 
    <"Level 5">, 6, 7, 4, 3, 1, 0 

Adapter SCSI Bus ID
   SCSI ID of the host adapter 
    <"Device ID 7">, 6, 5, 4, 3, 2, 1, 0 

BIOS Address
   Memory location of the host adapter's BIOS PROM. Note, if two SDC3211B are in the system one must have its BIOS PROM disabled. 
   <"DC000h"  (dc00-dfff)>, Disable, D8000h (d800-dbff), D4000h (d400-d7ff), D0000h (d000-d3ff), CC000h (cc00-cfff), C8000h (c800-cbff), C4000h (c400-c7ff) 

SCSI Parity Checking
   The host adapter will check SCSI Parity if 'On' 
     <"On">, Off 

Synchronous Negotiation
   Synchronous transfer on the SCSI bus will be initiated by the  host adapter if 'Enabled' 
   <"Enabled">, Disabled 

; The function of this bit was changed. The data width function is detected ;by the host adapter and this switch was ignored. 
; Slot Data Width
;    The adapter can be inserted into a 16 or 32 bit slot on the motherboard.  ;This option controls the data transfer size of 16 or 32 bits 
;    <"16">, "32" 

Data Streaming
   Streaming data procedure transfers data blocks by using a single address followed by multiple 16 or 32-bit data transfers within a single streaming data cycle. If data streaming is enabled for a Micro Channel bus that supports streaming, the host adapter can achieve a maximum data transfer rate of 40 MBytes/sec on the Micro Channel bus.  If data streaming is disabled or the Micro Channel bus does not support streaming, the maximum data transfer rate is 20 MBytes/sec. 
    <"Disable">, Enable 

Arbitration Fairness
   Bus Arbitration Fairness controls whether the host adapter  implements the fairness alorigthm after it is preempted from  the Micro Channel 
   <"Off">, On 

1Gbyte Translation See the following setion for more details
   Change CHS per track  to support drive capacity > 1 GB 
    <"On">, Off 

Disk > 1 GB and not SC0 UNIX.
    In the DOS environment, INT 13 calls are routed through the BT-64xA/S/D’s ROM BIOS. This on-board BIOS intercepts host interrupt 13H calls and dispatches a command to the BT-64x for all host to SCSI disk accesses. When the >lGB option is turned on, the BT-64x BIOS can access up to 8 GBytes per disk. Otherwise, it can only access the first 1 GBytes even if the formatted disk capacity is greater than 1GByte. This 1GByte restriction does not apply to other operating systems, such as NetWare, UNIX, SC0 UNIX 3.2.4, or OS/2 if the operating system can boot without accessing > 1 GBytes. If the operating system’s bootable image resides below 1 GBytes then it can boot via Interrupt 13H. Once any of these operating systems are booted, the disk accesses are not routed through Interrupt 13H and the operating system can access the entire disk space even if the > 1 GB option is not turned on.  

   Under SC0 UNIX 3.2.2, the >lGB option must be turned off because the operating system itself has a 1 GByte limitation. Otherwise, disk images may be corrupted when the 1 GByte boundary is reached. For SC0 UNIX 3.2.4, the operating system does not impose the 1 GByte limit, and this
option can be turned on or off accordingly. Consequently, the >lGByte support must be turned on under the following two conditions: (1) the combined space of all the DOS partitions exceeds 
lGByte, or (2) >lGByte disk accesses are required to boot the operating system. To enable the > 1 GB support, turn on this option. Note: If this option is changed, you must reformat the disk to avoid corrupting the existing file system.  

   {Ed. The >1GB translation scheme is incompatible with IBM SCSI controllers. After formatting a DPES-13080 1.08GB drive on a BT-646S (>1GB translation enabled), I was unable to access it after then attaching the drive to a Corvette. I had to reformat the drive with the Corvette.]