53C710/C720 AT Development Board Upgrade to Differential SCSI

This document will describe how to "upgrade" a 53C710/C720 Development 
board to allow for Differential SCSI operation.  This procedure is 
only valid if the development board is NOT a Rev A board.  
Additionally, when the modifications have been made, the board can 
only operate in differential mode.  In order to go back to 
single-ended mode, the differential-pair transceivers must NOT be 
populated on the board.  The board can run both modes, but only one 
mode at any given time.  This is very important!

In order to upgrade a single-ended AT development board, the following 
steps should be taken. 

1)  Populate U20-U24 only for 8-bit Differential SCSI bus operation.  
Populate U20-U26 for 16-bit Differential SCSI bus operation ONLY if 
using a 53C720.  The board has been laid out for using the 
National DS36954 differential-pair transceivers.  These should be the 
20-pin PLCC package.

2)  Remove the Single-Ended TERMPWR termination resistors - RP2-RP3 
(8-bit SCSI) or RP2-RP4 (16-bit SCSI & C720).  These should be socketed 
and should not be difficult to remove.  Note their orientation before 
removing.

3)  Populate RP14-RP15 for 8-bit differential SCSI, and RP14-RP15, and 
RP6 for 16-bit differential SCSI.  These 680W pullup resistors 
(use 1.5k pullup resistors if active negation is enabled) are needed 
to supply inactive high levels for the inputs to the differential pair 
transceivers.

4)  Populate RP7-RP9 and RP11-RP13  for 8-bit differential SCSI, and 
RP7-RP13 for 16-bit differential SCSI.  These are the differential 
termination resistors required for the differential SCSI bus.  Their 
values should be 330W/150W/330W.  NCR uses a Sprague terminator resistor 
pack part number 435CK506. These should be socketed to allow for removal.

5)  Populate the 50-pin header marked J3 for 8-bit differential SCSI.  
Populate the 68-pin high-density connector marked J4 for 16-bit 
differential SCSI.  If you are upgrading a 53C720 board, you can 
populate both J3 and J4 and use both on the same system.  This provides 
physical connector access for both 8-bit devices and 16-bit devices.

6)  Remove U18 (the voltage regulator).  Attach a wire between pins 
2 and 3 of where U18 used to reside.  

7)  The last thing to do BEFORE operating in differential mode is to 
enable the 53C710/C720 chip itself for differential mode.  To do this 
for the 53C710 simply write bit 0 of the CTEST7 register to a 1.  
To do this for the 53C720 simply write bit 5 of the STEST2 register 
to a 1.  This action forces you the user to operate in differential mode.