SOME NOTES ABOUT THE STACY/MEGA STE INTERNAL SCSI

SOME NOTES ABOUT THE STACY/MEGA
STE INTERNAL SCSI HOST ADAPTER
joska@online.no 2014.
The Stacy and Mega STE has internal ACSI ports and optionally an ACSI->SCSI host adapter. This host adapter
can also be used with other ST computers. I have successfully used this with both the ordinary ST DMA/ACSI
port and the Mega ST internal ACSI port.
LIMITATIONS OF THE SCSI ADAPTER
This SCSI adapter is not the most advanced – it has several restrictions compared to more recent and advanced
adapters like the ICD Link 2.
1. Does not work with drives that requires parity.
2. Only supports one single drive.
3. The attached drive must not be terminated. Be very careful to remove any termination resistor packs,
and do not enable termination power.
4. Use a short SCSI cable, not more than around 20cm.
5. The attached drive must have SCSI ID 0 (zero).
6. Only supports up to 1Gb. You can use bigger drives, but only the first Gb can be accessed.
7. Does not support “newer” SCSI features like SCSI arbitration and initiator ID identification. I think – I
know very little about SCSI.
1 and 7 are the biggest problems when using modern SCSI drives. You can use a (relatively) modern SCSI-3 drive
with a cheap 50-pin adapter from eBay, but chances are that it won’t work because it requires parity.
Problem 1 can be solved by adding a parity generator to the host adapter.
Problem 7 – I really don’t know exactly what’s going on. The drive will be recognised and you can use it (atleast
with HD-Driver) without any problems except that you can’t boot from it.
Problem 2 can be worked around, with a little modification the host adapter can access two SCSI drives with
standard hd drivers. I have not tested this.
Problem 6 can also be worked around with a small modification but needs a special hd driver by Pera Putnik. I
have not tested this.
ACSI PINOUT
By looking up this table you can easily make cables to connect the host adapter to the internal (in case of Mega
ST) or external ACSI connector. It is a very simple pin-to-pin cable. The only exception is the power supply to
the host adapter – only the Mega STE and Stacy has power supply on the internal ACSI port. So in addition to
the ACSI cable you will have to connect the host adapter’s pins 25, 27 and 29 to the power supply ground, and
pins 26, 28 and 30 to the power supply 5V.
Table 1. ACSI pinouts.
Signal
Mega STE
ACSI
Mega ST
(30 pin internal)
(DB19 male)
(24 pin internal)
D0
2
1
2
D1
4
2
4
D2
6
3
6
D3
8
4
8
D4
10
5
10
D5
12
6
12
D6
14
7
14
D7
16
8
16
/RESET
17
12
3
ACK
18
14
11
/CS
19
9
18
A1
20
16
19
RW
21
18
22
DRQ
22
19
23
IRQ
23
10
24
GND
25
11
1
GND
25
11
3
GND
25
13
5
5V
26
GND
27
13
9
GND
27
15
13
GND
27
15
15
5V
28
GND
29
17
17
GND
29
17
20
GND
29
17
21
5V
30
EXTERNAL ACSI CONNECTOR
The external ACSI connector you find on the back of all variants of the ST and also the TT.
Figure 1. External ACSI connector.
If you take a close look at the connector you will see that the pins are actually numbered on the connector
itself.
MEGA ST INTE RNAL ACSI CONNECTOR
This is the layout of the Mega ST internal ACSI connector. It’s located on the motherboard right next to the
external ACSI connector. The location of pin 1 is labelled on the motherboard.
Figure 2. Mega ST internal ACSI connector.
STACY/MEGA STE INTERNAL ACSI CONNECTOR
This is the layout of the ACSI connector on the host adapter itself. It has two rows with 15 pins each, the pins at
the end are labelled. You can either solder the cable directly to the host adapter itself or a 15x2 1/10” pin
header.
Figure 3. Mega STE/Stacy host adapter ACSI connector.
ADDING A PARITY GENE RATOR TO THE HOST ADAPTER
This is a text I found on usenet 20 years ago or so. I have applied this modification to two Stacy host adapters
with success.
-------------------------8<----------------------SCSI Parity Generator
This is the Parity generation logic that solves the problem for those Atari users
that use SCSI adaptors without parity. As you can see it consists of one single
74HC280 chip. The easiest place to get to the SCSI signals is the harddisk itself.
You can use the four pin connector's red pin for +5V and the black for GROUND.
This parity generator has been tested with the following adaptors:
-
ICD Link 1 (the first type)
-
Megafile 44 SCSI adaptor
-
Mega STE SCSI adaptor
-
Protar SCSI adaptor
Pinout of the 74HC280
74HC280
+---\_/---+
D6 |1
14| +5V
D7 |2
13| D5
NC |3
12| D4
INPUT |4
11| D3
EVEN |5
10| D2
ODD |6
9| D1
GND |7
8| D0
+---------+
The logical schematic for added parity generation
74HC280
SCSI-bus
+--------+
+-------+
|
P8 | D0------------D0 | P2
|
|
P9 | D1------------D1 | P4
|
|
P10 | D2------------D2 | P6
|
|
P11 | D3------------D3 | P8
|
|
P12 | D4------------D4 | P10
|
|
P13 | D5------------D5 | P12
|
|
P1 | D6------------D6 | P14
|
|
P2 | D7------------D7 | P16
|
|
P6 | ODD------ PARITY | P18
|
|
|
|
P14 | +5V
|
|
+-------+
|
P3 | NO CONNECTION
|
P5 | NO CONNECTION
|
|
|
P4 | INPUT--+
|
P7 | GND----|
+--------+
|
GND
copyright (c)1996 Martin Graiter. Non-commercial use granted to all Atarians of
the world! Questions etc to: graiter@starmail.com
------------8<-------------
I did not modify the harddisk itself as suggested by Martin Graiter. Instead I soldered the parity generator
directly to the SCSI connector on the backside of the host adapter. If you bend out the pins on the 74HC280, D0
to D5 will line up perfectly to D0 and D5 on the SCSI connector. See figure 4.
Figure 4. Backside of host adapter.
CONNECTING TWO SCSI DEVICES TO THE STACY/MSTE HOST ADAPTER