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Philips P2000C SASI interface, what are the options?

JonB

JonB

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Hi

I'm looking at a CP/M machine with a SCSI HDD interface. I know this is the forerunner to SCSI, but not how compatible it is.

The machine is a Philips P2000C. What's the easiest way to connect a hard drive to it? Is there a CF or SD card solution that is bootable? Or can I connect a SCSI drive? And what is the maximum size drive that is likely to be supported?

Thanks
JonB
 
You cannot connect a SCSI drive directly. I tried once, and some component (a diode?) promptly desoldered itself. SCSI has one +5V line between the GND side of the connector, SASI as in the P2000C apparantly doesn't. It could be the SASI bus is properly documented, but I haven't checked. When I finally could scan the Service Manual document, the P2000c itself was long gone.

See http://electrickery.xs4all.nl/comp/p2000c/doc/index.html

Fred Jan
 
I believe this can happen on "normal" SCSI if you get the TermPwr wrong. I am pretty sure it should be possible to connect old single ended SCSI drives to SASI. The reverse is certainly possible. I have used the XEBEC S1410 controller which is basically a SASI controller on my Atari ST SCSI bus.

http://www.ebay.com/itm/200910319796

and Wikipedia says that SASI and SCSI are the same electrically

http://en.wikipedia.org/wiki/Hard_disk_drive_interface

and the SCSI history says it should be possible to connect SCSI I drives to SASI

http://en.wikipedia.org/wiki/SCSI#History

BUT you will need an old drive <1Gb...
 
Sorry to resurrect this post but.. you may know by now that I actually acquired a P2000C in non working state - cue the various discussions in different places round the 'net about how to fix it.

Well, it boots into CP/M now and I have all the downloadable disk images on floppy, plus the original disks that all seem to work fine (albeit noisily).

With it being a working CP/M machine (linked to a PC or terminal via the internal serial port), my interest in using its SASI interface has returned (in this thread I was asking prior to bidding on, and losing, another P2000C that worked - oh well). So, here is a 50 pin edge connector looking for a drive to connect to. What suggestions have you? Are there any modern day emulators that use, say, CF cards? I do have ULTRASATAN drives for my ST, which are essentially SD card adapters that emulate the ST's ACSI interface (plug right in). Maybe I could adapt one? The pinout is attached.

Cheers
JonB
 

Attachments

  • P2KC SASI Pinout.JPG
    P2KC SASI Pinout.JPG
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I seem to remember that the ACSI interface is very similar to SCSI, but the polarity of a few signals is inverted. I recall designing a board to convert a SCSI drive (actually an OMTI MFM-to-SCSI bridge) to interface to ACSI and that's pretty much all it was.
 
I can see why there might have been some problem with attaching SCSI directly. According to http://pinouts.ru/HD/ScsiInternal_pinout.shtml , the termination power line (TMPWR) is connected to pin 25, which appears to be grounded... boom!

So, it looks like this:

Code: 
P2K SASI              SCSI
--------              --------------------------
 2 D0 ---------------  2 D0
 4 D1 ---------------  4 D1			
 6 D2 ---------------  6 D2
 8 D3 ---------------  8 D3
10 D4 --------------- 10 D4
12 D5 --------------- 12 D5
14 D6 --------------- 14 D6
16 D7 --------------- 16 D7
18 GND       X        18 PARITY
20 GND -------------- 20 GND
22 GND -------------- 22 GND
24 GND -------------- 24 GND
26 TPWR      X        26 GND
28 GND--------------- 28 GND
30 GND--------------- 30 GND
32 GND       X        32 ATN
34 GND -------------- 34 GND
36 /BSY ------------- 36 /BSY
38 /ACK ------------- 38 /ACK
40 /RES ------------- 40 /RST (reset)
42 /MSG ------------- 42 /MSG
44 /SEL ------------- 44 /SEL
46 /C/D ------------- 46 /C/D (control / data)
48 /REQ ------------- 48 /REQ
50 /I/O ------------- 50 /I/O (input / output)

1 - 49 (Odd) GND ---- 1-49 (Odd) GND

Obviously TPWR is not connected, but what about Parity and ATN?
 
Not sure if Atari ACSI devices really can be used, though:

Code: 
Pin	Name	Description
1	D0	Data 0
2	D1	Data 1
3	D2	Data 2
4	D3	Data 3
5	D4	Data 4
6	D5	Data 5
7	D6	Data 6
8	D7	Data 7
9	/CS	Chip Select
10	IRQ	Interrupt Request
11	GND	Ground
12	/RST	Reset
13	GND	Ground
14	ACK	Acknowledge
15	GND	Ground
16	A1	Address bit
17	GND	Ground
18	R/W	Read/Write
19	REQ	Data Request

Would have to invert some signals and work out what to do with A1.
 
If I made a SCSI adapter what would I need? Some sort of drive controller as well as a drive, or does this provide all the signals to connect to a SASI drive on its own? I'm guessing I need a controller.
 
I'm not sure about that--perhaps if the adapter were early SCSI; but later controller firmware depends somewhat on commands not supported by ACSI devices. Really, the closest thing to ACSI is SASI.
 
OK, I have scored a 160MB 50 pin SCSI drive from "a well known auction site" for the princely sum of £22.

So, referring back to my earlier suggestion (with mods):

Code: 
P2K SASI              SCSI
--------              --------------------------
 2 D0 ---------------  2 D0
 4 D1 ---------------  4 D1			
 6 D2 ---------------  6 D2
 8 D3 ---------------  8 D3
10 D4 --------------- 10 D4
12 D5 --------------- 12 D5
14 D6 --------------- 14 D6
16 D7 --------------- 16 D7
18 GND       X        18 PARITY
20 NC --------------- 20 GND
22 NC --------------- 22 GND
24 NC --------------- 24 GND
26 NC        X        26 TPWR
28 NC --------------- 28 GND
30 NC --------------- 30 GND
32 NC        X        32 ATN
34 GND -------------- 34 GND
36 /BSY ------------- 36 /BSY
38 /ACK ------------- 38 /ACK
40 /RES ------------- 40 /RST (reset)
42 /MSG ------------- 42 /MSG
44 /SEL ------------- 44 /SEL
46 /C/D ------------- 46 /C/D (control / data)
48 /REQ ------------- 48 /REQ
50 /I/O ------------- 50 /I/O (input / output)

1 - 49 (Odd) GND ---- 1-49 (Odd) GND

About the discrepancies:-

  • TPWR - If the drive has this pin powered I can't connect the line, but I think I will need active termination (hence power). According to this page, which has the instructions for the drive I bought, the drive itself can provide active termination (jumper setting), in which case the TPWR line can be disconnected.
  • PARITY - If it is grounded at the host end does it mean "no parity"? It seems to be a bidirectional signal for (parity) checking the correctness of data transfers.
  • ATN - The P2000C will ignore this so it will remain unconnected.

Does this look reasonable? It is starting to look like a pin - pin correlation (with the exception of TPWR). In other words, just attach the drive with a straight through cable. :D

Thanks
JonB
 
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I'd give it a try--SASI is close enough to SCSI that you won't get any magic smoke. Whether or not they're sufficiently compatible on the command level, is a roll of the dice. The ATN signal in particular may be a stumbling block. SCSI added the bus protocol after SASI.
 
Last edited:
Well, I paid for the drive, so nothing ventured, nothing gained. The drive should be here in a couple of days and all I will need is a 50 way IDC Edge connector to try it out (on order). I should be able to power it directly from the P2000C floppy drive supply.
 
Well, whaddya know? I'm just looking at the edge connector and I discover that, contrary to the schematic, there are no connections for pins 20 - 32. There is a gap on the edge connector where the connections would be. The wiring chart above is updated. So I need only worry about the parity line, which is grounded. No blown diodes for me!

Edit: This line appears to be a way to signal a parity error:

SCSI Hardware Bus Specification
25 signal lines; 25 ground lines
Signals may assume true or false values. Signal assertion (true)is 0 v.
Signal negation (false) is 2.5 to 5.25 V dc
( Bus driver chips typically invert the signal so bus voltage
levels are typically the reverse of normal logic levels input to the
on chip or input to the bus drivers where
+5 V is true.)
SCSI BUS SIGNALS
Busy indicates the bus is in use
Select used by the host to select the drive
Control/Data This signal is driven by the target (like a disc drive
and indicates whetehr CONTROL or DATA information is
on the DATA bus. A true value indicates a control
signal is on the bus.
Input/Output
This signal is driven by the target (disc drive) and
controls the direction of data movement on the DATA
bus with respect to the host.
A true value indicates input to the host.
Message This signal is driven by the target (disc drive)
during the message phase.
Request This signal is driven by the target (disc drive) and
indictes a request for a data transfer handsake acknowlegement.
Acknowledge This signal is driven by the host and acknowleges a
a data transfer handshake.
Attention This signal is driven by the host to indicate a
condition needing attention.
Reset This forces a reset of devices on the bus.
Data lines (8)
Parity line (1) This line may be used by the devices to signal a
parity error in the received data or command.
Click to expand...

So I may be OK...
 
Last edited:
I have received the drive and connected it. The computer is reporting "Hard Disk not ready" each time I try to access it. Hmmm. A couple of things spring to mind:

  • Termination - The drive is jumpered for active termination (not via TERMPWR, it is internal to the drive). Removing this jumper has no effect. The computer's service manual says passive termination is needed on ACK, SEL and RES in particular, and gives the resistor values of 220 Ohm to +5v and 220Ohm to 0v. I'm not certain the drive's inbuilt active termination matches.
  • Sector size - The drive has 512 bytes per sector, but the computer expects 256 bytes. There is no apparent way to change this on the drive. Would this mean the computer discards half of each sector?

Still, no magic smoke, so that's good!
 
Don't you mean 220/330 ohm termination? I doubt that termination is your problem, however.

The big difference between SCSI and SASI is the selection process. Otherwise, most details are very close or the same. The selection issue was introduced so enable more than one host/initiator on the bus. You have have a drive that's tolerant of SASI selection protocol. If not, you'll have to do some hardware hacking to introduce the SCSI selection protocol.

Here's a good writeup on the differences
 
I have decided to return the drive as I suspect the Apple firmware is doing something non standard. Plus, it is copyright 1993 which is probably too recent for the old P2000C SASI interface (which isn't fully implemented according to the service manual:

The SASI, as implemented on the P2000C Mainboard, does not use the full SASI specification. It is similar to the XEBEC Controller on the Hard Disk.
- Transfers can only be initiated from the Mainboard
- Only one target can be active at any time
- The Arbitration and Reselection phases are not implemented
Click to expand...

Perhaps this means I am looking for a drive with XEBEC controller..
 
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