Re: Auction: SCSI and Startech adapter

...In this case the BOCA has a 16C452. Two 16C450-compatible
UARTs, one non-bi-di parallel port. The Model 25 286 & Model
30 286 planars have a 16C451 (one 16C450 UART, one non-bi-di
parallel). For the StarTech versions I have seen alot of 16C552,
which is two 16C550 UARTs with FIFO, one bi-di parallel. Less
common is the 16C554 (four 16C550 UARTs & one bi-di parallel).
This needs to be measured (or metered) out further on the topic of
the 16C451/16C452/16C552 uni- or bi-directionality of the parallel
port. I had forgotten a pinout change that can be done to the chips.
Further tests bear out the conclusion I have for now.
We know the Model 25 286 / Model 30 286 planar to be
bi-directional (considered to be the "PS/2" innovation on the parallel
port that all models had) but it has the 16C451 part. When I had the
16C552 data*** it marked pin 1 (in the center of the beveled edge) to
select whether the part would be set to a uni or bi-directional
parallel port mode.
Sure enough, I have a BOCA ISA adapter (with 16C552) that has that
jumper selection. The 16C451 & 16C452 are supposed to be pin-compatible
lower level versions (non-FIFO UARTs in the case of the 16C452,
non-FIFO UARTs & one less serial port for the 16C451). If pin 1 is tied
to +5VDC then the parallel port is set to bi-directional mode. Pin 1
being grounded means uni-directional (classic) mode.
On the Model 25 286 / Model 30 286 planar the 16C451 pin 1 is
connected to +5VDC. In the case of the BOCA MCA adapter the 16C452 pin
1 is grounded, to be set to uni-directional mode (why cripple your
adapter slightly). By a wiring change you can set the port to
bi-directional mode.
Years ago I tested out a BOCA ISA adapter with a 16C452 I made the
change to & think it tested successfully. This weekend I will repeat
the test to be sure. For those interested in the BOCA cards, here is a
table:

PN4196: ISA w/ 16C452 pin 1 GND (uni-directional)
PN4197: ISA w/ 16C552 pin 1 switchable (uni or bi-directional)
PN4234: MCA w/ 16C452 pin 1 GND (uni-directional)
PN4250: MCA with single parallel (most likely uni-directional)
PN4821: ISA w/ 16C452 pin 1 GND, 2nd LPT using 8211 chip.

Note that PN4197 & PN4821 lose any "Made in USA" labels.

The concept of pins tied high or low to change chip functionality
exists at least in one other place in PS/2s. On the Model 50 (021 & Z)
pin 39 of the 287 socket is tied high (+5VDC) to run the NPU at the
same frequency as the 286 CPU (10MHz). For the Model 25 286 / Model 30
286 the same pin 39 is tied low (GND) to have the 287 NPU run at 2/3rds
CPU clock (the 286 is also at 10MHz here, so the NPU uses 6.66MHz for a
much less expensive, but lower performing 287).
David
David@xxxxxxxxxxxxx

.