uhclem@trsvax.UUCP (03/24/89)
The following information is provided by an individual and is not nor should
be construed as being provided by Radio Shack or Tandy Corp. Radio Shack/
Tandy Corp has no obligation to support the information provided in any way.
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6000 > 4000 > 3000 Vol 1, No. 2
Hard Disk Drives and your Tandy 16/6000 Computer system - Part 1
by Frank Durda IV (c) 1989 4-Mar-89
This article may be republished in part or in its entirety pro-
vided credit is given and the material is made available without
charge. A modest media charge is acceptable IF the recipient
has the option of obtaining the material at no charge by provid-
ing his/her own media. Any other use of this material must
be approved in advance in writing.
This article discusses how the different disk controllers and interfaces used
by Radio Shack on the Model II/16/12/16B/16BHD/6000/HD systems work, what
their limits are, and how you can add more and faster drives to your system.
ST506 - A Drive That Became An Interface Standard
-------------------------------------------------
The disk drives used by the Model II/16/12/16B/16BHD/6000/6000HD are ST506-
type drives. The ST506 was a disk drive which had an interface that has been
duplicated on dozens of different drives over the years. Over time that in-
terface has taken on the name of the ST506 drive. The ST506 drive was a very
low capacity drive by todays standards but the interface had enough flexibili-
ty to allow more growth. (The ST506 interface was also called the "S" inter-
face by some manufacturers for a time but that name is not commonly used to-
day.)
The ST506 interface itself consists of two cables which carry data and control
signals to and from the drives that are attached. The "Data" cable has 20
pins that carry the actual serial data stream to and from the write/read heads
along with timing information used only in older drives. Although it can be
wired in other ways, most systems run a separate "Data" cable to each drive to
reduce the possibility of noise and line loss affecting the data stream.
The "Control" cable has 34 pins and these are used to instruct the selected
drive as to where the heads should be positioned and which surface is to be
read or written at this time. The drive also reports its status on other
lines in the "Control" cable. (Drives compatible with the SA1000 series
drives used a 50 pin "Control" cable which had the same signals as the 34 pin
cable.) A single "Control" cable is attached to all of the ST506 drives on a
single controller.
The "Control" cable is terminated in the drive furthest (electrically) from
the disk controller. Most drives use a resistor pack that can be removed to
provide this termination. In systems where a separate "Data" cable is used
for each drive, signals in this cable are also terminated in the drive. Some
drives assume separate "Data" cables and do not have a way to disable its ter-
mination. Others use a single jumper to enable or disable the "Data" cable
termination. For all Radio Shack configurations, the "Data" cable must be
terminated.
The ST506 interface makes two size limitations on the drives that are attached
to it. A maximum of eight (8) heads may be present on any drive on the ST506
bus and a maximum of four (4) drives can be attached at any one time. All the
other size limitations or restrictions that exist in a given system are due to
the type of controller, support circuitry or software that is present. (If a
drive has more than 8 heads, it is not a ST506 drive. This is discussed in
more detail later.)
6000 > 4000 > 3000 Vol 1, No. 2 Page 2
Controllers and Host Interfaces
-------------------------------
Radio Shack used four different disk controllers and two host interface boards
during the production of the Model II/16/12/16B/16BHD/6000/6000HD systems.
Here is a discussion of each.
#1. The original hard disk controller was only used with the 8" 8 Megabyte
hard disk drives. It was based on the 8x300 processor. This card can
be identified by the presence of an edge connector for the host inter-
face cable. The 8X300 CPU was being used to emulate the 1000 disk
controller chip-set that Western Digital had under development at that
time. The firmware on most of these controllers only allowed 512 byte
sectors, which prevented them from being used directly with the Model
III/4 operating systems. This adapter provided connections for 34 and
50 pin "Control" cables so that SA1000-compatible drives could be
used. The 8 Meg drive is a Shugart SA1004, which requires a 4.34
Mbits/sec transfer rate. This rate is incompatible with the 5.0
Mbits/sec rate used by all ST506/ST412 drives.
The host interface adapter (the card that goes in the CPU card cage)
that was used with this controller can be identified use of power
strips, which appear as red strips of plastic-coated metal running
under the integrated circuits.
#2. A modified version of the first controller. First designed to be used
with 5" TM602 (5 Megabyte) ST506 hard disk drives on the Model III.
This board also used the 8X300 controller but had newer firmware that
would allow both 256 and 512 byte sectors. This and all subsequent
controller boards only provided a 34 pin "Control" connector and the
transfer rate was changed to be 5.0 Mbits/sec.
A 50 pin vertical header connector is used on this controller for the
host interface cable. The host interface cable pin-out was modified
so that the cable could be plugged directly into the expansion inter-
face on a Model III and the then-under-development Model V (later
renamed IV and then just 4). A small interface box was required to
attach this controller to a Model I.
When a 12 Megabyte drive (TM603) became available, a new host inter-
face board was designed for the Model II/16/12/16B systems which pro-
duced signals that looked like those produced by a Model III/4 EI.
This host interface card is incompatible with the original disk con-
troller (#1). The newer interface card can be identified by the lack
of the red power strips.
Because the interface signals look just like the output of the Model
III/4/4P/4D, add-on devices for these systems could be attached to
II/12/16/6000 systems. For example, a Network 4 board can be attached
to a Model 16 by using this interface card. The interface card can be
jumpered so its Counter-Timer-Chip (CTC) can respond at different port
ranges. The disk controller has similar jumpers so that the ports
numbers that it uses can also be changed, allowing a system to have
more than one interface card installed at a time. This capability was
never used by Radio Shack.
6000 > 4000 > 3000 Vol 1, No. 2 Page 3
#3. A new half-sized disk controller board was created when the 1010 chip
set became available. Although the 1010 controller was initially ad-
vertised as software compatible with the 8x300 emulation, new releases
of all operating systems were required to deal with the differences
between the two designs. This controller used the same host interface
board as controller #2.
#4. A special disk controller board was developed for the 16B HD and was
used later in the 6000 HD. This controller fit into a slot in the
16B/6000 and contained the host interface and disk controller on a
single board. This board also used the 1010 controller. Due to the
limited space on the board, it was decided that only two drives would
be allowed to be attached to this controller. The extra "Data" con-
nectors and line drivers were omitted from the design. Apart from the
number of drives, this controller was designed to be programmed the
same as controller #3. Slight timing differences and hardware race-
conditions resulted in another round of operating system updates to
support this controller.
This table summarizes the above controllers and what they allow:
Name Cntrl Max Host Interface Radio Shack Drives
type Drives type allowed
#1 WD1000 8x300 CPU emulation 4 Incompatible 8 Meg
#2 8X300 8x300 CPU emulation 4 Model III/4 5, 12, 15, 35, 70 Meg *
#3 WD1000-TB1 1010 state-machine 4 Model III/4 5, 12, 15, 35, 70 Meg *
#4 8898798 1010 state-machine 2 Combined in HDC 5, 12, 15, 35, 70 Meg *
* Note that on controllers #2, #3 and #4, marketing reasons caused subsets of
these drives to be supported by certain controllers although all were
hardware-capable of being used. For example, Radio Shack only offered a
TM503 (15 Meg) drive as the internal drive on the 16B HD/6000 HD (which
used the internal controller #4), although larger drives could certainly
have been used and probably would have reduced power requirements, noise
levels and seek times, resulting in more reliable and faster systems.
6000 > 4000 > 3000 Vol 1, No. 2 Page 4
Hard Disk Drives
----------------
The following tables contain information that will be useful in comparing per-
formance and power requirements of the various drives. Fields that are blank
were not published by the manufacturer.
Model SA1004 TM602 TM503 TM603 Q540 1325
Manufacturer Shugart Tandon Tandon Tandon Quantum Micropolis
Sold by R/S Yes Yes Yes Yes Yes Yes
Formatted Size 8 Meg 5 Meg 15 Meg 12 Meg 35 Meg 70 Meg
Media Size 8" 5.25" 5.25" 5.25" 5.25" 5.25"
Cylinders 256 153 306 230 512 1024
Heads 4 4 6 6 8 8
Seek TK to TK 19msec 18msec 18msec 18msec 10msec 6msec
Seek Full Strk* 150msec (336msec) (170msec) (450msec) 80msec 62msec
Seek Average 70msec 168msec 85msec 225msec 45msec 28msec
Latency Avg. 9.6msec 8.34msec 8.33msec 8.34msec 8.5msec 8.33msec
Interface SA1000 ST506 ST506 ST506 ST412 ST412
RWC from host 128 128 No 128 No No
Write Precomp 128 128 128 256 No
Rotational Spd 3125 3600 3600 3600 3600 3600
MTBF (in hrs) 8,000 11,000 10,000 20,000
Power Requirements in Amperes
Model SA1004 TM602 TM503 TM603 Q540 1325
Manufacturer Shugart Tandon Tandon Tandon Quantum Micropolis
5VDC Start 1.2 0.9
5VDC Run 2.0 0.8 0.8 0.8 0.7 0.9
5VDC Max 3.6 1.2 1.0 0.9
12VDC Start --- 5.0 5.0 5.0 4.5 3.9
12VDC Run --- 1.5 1.5 1.5 2.0 2.1
12VDC Max --- 2.0 2.4 3.3
The SA1000 drives did not use 12VDC, but did require these additional voltages:
24VDC Run 0.2 --- --- --- --- ---
24VDC Step 2.8 --- --- --- --- ---
-5VDC Run 0.2 --- --- --- --- ---
115VAC 60Hz .75 --- --- --- --- ---
(spindle motor)
Typical Watts** (102) (22) (22) (22) 23 10
6000 > 4000 > 3000 Vol 1, No. 2 Page 5
Model ST225 ST251 ST251-1 ST4096 94205-51 ST151
Manufacturer Seagate Seagate Seagate Seagate CDC Seagate
Sold by R/S No No No No Yes No
Formatted Size 20 Meg 40 Meg 40 Meg 80 Meg 40 Meg 42.5 Meg
Media Size 8" 5.25" 5.25" 5.25" 5.25" 3.5"
Cylinders 615 820 820 1024 989 977
Heads 4 6 6 9 5 5
Seek TK to TK 20msec 8msec 8msec 6msec 5msec 8msec
Seek Full Strk* 190msec 95msec 70msec 55msec 65msec 44msec
Seek Average 85msec 40msec 28msec 28.0msec 28msec 24msec
Latency Avg. 8.33msec 8.33msec 8.33msec 8.33msec 8.33msec 8.33msec
Interface ST412 ST412 ST412 ST412 ST412 ST412
RWC from host No No No No No No
Write Precomp 300 No No No 128 No
Rotational Spd 3600 3600 3600 3600 3600 3600
MTBF (in hrs) 50,000+ 50,000+ 50,000+ 30,000+ 30,000 45,000+
Power Requirements in Amperes
Model ST225 ST251 ST251-1 ST4096 94305-51 ST151
Manufacturer Seagate Seagate Seagate Seagate CDC Seagate
5VDC Start 1.2 1.5 1.0
5VDC Run 0.8 1.0 1.2 1.5 0.4 0.5
5VDC Max 1.5 0.6
12VDC Start 2.6 4.0 4.5
12VDC Run 0.9 0.5 0.5 1.5 1.5 0.45
12VDC Max 2.4 2.0 2.5 2.5 2.0 2.0
Typical Watts** 14.8 11 12 23 (20) 8
* Maximum seek rates in parentheses were not published and have been computed
as twice the average access. Note that some manufacturers compute the
average access time as the time it takes to seek 1/3rd the distance across
the drive, which means the actual maximum seek time could be higher.
** Typical watts values in parentheses were not published and have been com-
puted with the formula W = V x I summed for each voltage used by the drive.
+ MTBF values for these drives have increased since they originally became
available. Values shown are as of Seagate Pub 1000-002, March 1989.
6000 > 4000 > 3000 Vol 1, No. 2 Page 6
Incompatibilities And Other Strangeness
---------------------------------------
Starting with controller #2, Radio Shack changed the "Cable Open" signal (line
7) in the "Data" cable into a signal to activate the power supply in the
secondary drive cabinets. When +12 VDC is present on this line, the secondary
power supplies are activated. The reason this was implemented was that the
old 8" system required the user to turn a keyswitch on all the drives (up to
four) to start the system and this was considered a nuisance. This local
variation of the ST506 standard can be found in all subsequent Radio Shack
disk controllers, even on the latest ones built for the IBM AT-compatible
line. Not all disk drives handle this deviation from the ST506 standard; some
have line 7 grounded, which can cause a resistor on the disk controller to
burn out. Some use line 7 for factory diagnostic control or some other test
signal. Many start-up failures are caused by connecting one or more of the
data cables upside down, which will short +12 to ground via that resistor. On
the IBM AT-style controllers that Tandy sells, this resistor is usually
strapped across the solder side of the board. The "Data" cable for the pri-
mary drive (drive 0) does not have +12 on line 7 since that unit has the
keyswitch or is mounted inside the computer.
So if you buy a drive that was not sold by Radio Shack and you use it
as a primary, it will probably work fine. If a second drive of the same type
is bought and attached as a secondary, then the smoke may start pouring out.
To avoid this possibility when adding an additional drive, peel line 7 out of
the "Data" cable and run that wire over to the control relay in each secondary
cabinet. Then the drive will not have to contend with this non-compliance
with the ST506 standard but the remote power-up feature will work.
Radio Shack also implemented some other features that are really nice but are
literally tacked onto the drive with bits of wire. In all but the internal
drives on the 16B HD and 6000 HD, it is possible to write protect an individu-
al drive and get a visual indication of which drive is currently selected.
The connections are as follows:
+----------------------+
| | -|- = No Connection
Drive Active Lamp | -+- = Connection
| |
+----------------------|------------ 5 Red +5 (to controller
| +------------ 4 Purple in primary drive
Write Protect Lamp +--------- 3 Black Gnd or to lamp driver
| | +------ 2 Yellow board in second-
+----------------------+ | | +--- 1 White ary drives)
| | | | |
Write Protect Switch | | | |
| | | | | These lines were attached to test
| | | | | points or components on the drive:
| | | | |
+----------------------|--+ | +--- A Not Seek Complete
| +------ B Not Active
+------------ C Write Protect "Data" Line 5
6000 > 4000 > 3000 Vol 1, No. 2 Page 7
Signals "A", "B" and "C" are attached to points on the disk drive electronics
and these points change from drive to drive. Signal "C", or Write Protect, is
attached to line 5 which on most drives is listed as "Reserved", although some
Tandon drives listed it as "Test/PK". The ST506/ST412 drives themselves use
the Write-Fault signal (Control cable, line 12) to alert the controller to
problems when attempting to write to media. The write protect signal is held
high by using the lamp as a pull-up resistor, but so little current is drawn,
the lamp never illuminates. When the write protect switch is closed, the
write protect lamp is lit and the signal line is pulled low. All four disk
controller boards monitor line 5 in the "Data" cable. The state of this sig-
nal can be can normally be read in port 0xc0. This port is not a part of the
1010/2010 controller chip and is implemented with external components. All
four drives' write-protect status is in the upper bits. Bit 7 is for Drive
Select 1, Bit 6 is DS2, Bit 5 is DS3 and Bit 4 is DS4. As suggested for line
7, you may want to peel line 5 out of the cable in case the drive you have has
it tied to ground.
Signals "A" and "B" are combined and when the drive is Active and Seek Com-
plete is true, the Active light is lit. The light goes out briefly during
seeks (Seek Complete false) and would stay off when another drive was select-
ed. Many drives provide a similar output capable of driving a LED, but a
marketing decision selected the incandescent lamps and a lamp driver became
necessary. The lamp driver consists of a 75453 or a 75452 with an additional
gate from a 7414. Here is that circuit:
Circuit from controllers #1 and #2 Circuit from controller #3
+------+ +------+
from 5 ---- +5 -------|8 7 | from 5 ---- +5 -------|8 7 |
drive/ 4 ---------------|3 S5 | drive/ 4 ---------------|3 S5 |
lamp 3 ---- Gnd ------|4 N4 | or lamp 3 ---- Gnd ------|4 N4 |
inter- 1 ---------------|1 5 | inter- 1 ---------------|1 5 |
connect 2 ---------------|2 3 | connect 2 ---+ +\ +---|2 2 |
+------+ | | \ | +------+
+-| >o-+
| / SN7414
+/
Some users who have installed their own drives have removed the incandescent
lamp and installed the LED from the drive in the ACTIVE socket. Then the
light defuser is removed and the old wiring is replaced with a direct connec-
tion to the lamp contacts on the drive. Some of these arrangements only indi-
cate that the drive is selected (won't blink while seeking), but they are sim-
ple to install.
6000 > 4000 > 3000 Vol 1, No. 2 Page 8
The Other Limits
----------------
As mentioned earlier, the ST506 interface limits the number of drives and
heads. The 8X300 and 1010 controllers used by Radio Shack limit the number of
cylinders to 1024 that can be accessed on an attached drive.
However, the owner of controller #3 or #4 can use drives with up to 2048
cylinders by replacing the 1010 controller with a 2010 controller. On some of
these boards, the 1010 is even socketed, making the upgrade even easier. The
difference between the 1010 and the 2010 that makes this possible is an addi-
tional bit in the Cylinder Number High register:
7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+----+---+---+
| x | x | x | x | x | x |(9)|(8)| | x | x | x | x | x |(10)|(9)|(8)|
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+----+---+---+
WD1010-05 WD2010-05
XENIX 3.2 supports the 2010 controller. (If you are still running TRSDOS-II,
don't bother getting a 2010, just keep using the Dumont.) The following patch
must be made to the diskutil utility so it will allow formatting of drives
larger than 1024 cylinders. DO NOT apply this patch if the 2010 controller is
not installed. Entering a cylinder count greater than 1024 if a 1010 controll-
er is installed will result in unusable media. (Reformatting will recover if
you attempt to do this anyway.)
This patch is only valid for the version of diskutil that came with
XENIX 3.2.0. The version message in diskutil must say: 3(42) 3-Mar-87
# patch /diskutil
byte offset (<ENTER> to exit) ? 16be [ENTER]
16be: 04 |.| > 08 [ENTER]
16bf: b7 |.| > q [ENTER]
byte offset (<ENTER> to exit) ? [ENTER]
#
The WD2010-05 controller is available in single quantities from Hallmark in
Dallas Tx. The phone number is 214-553-4300. It is not cheap; it runs about
$85 (and Hallmark has a $100 minimum order requirement), although the ability
to increase total disk space may make it worth it. Hopefully there is a
cheaper source out there. It may be cheaper to get a used WA2 AT disk con-
troller and take the 2010 off of it. Many of these use the 2010. Then put
the 1010 on the WA2 and sell it to someone who doesn't need drives that big.
Another factor in favor of buying a 2010: the 1010 and 8X300 controllers limit
the top seek speeds to a rate lower than the speeds high performance drives
available today can provide. In other words, a 19 msec drive will probably
not actually run that fast if driven by a 1010 or 8X300 controller. Even
drives as slow as 28 msec can be slightly inhibited by the speed of these con-
trollers. The 2010 allows the higher seek rates to be achieved if software
changes are also made. These changes will be discussed in detail in a future
article.
6000 > 4000 > 3000 Vol 1, No. 2 Page 9
ST412 And The Death of Reduced Writing Current
----------------------------------------------
The big difference between the ST506 and the ST412 is that the ST412 drive
does not have to be told by the disk controller when it should use a lower
writing current. Reduced writing current is used on the inner tracks of
drives because the bits are spaced closer together and if the writing current
is too high, the resulting magnetic field from the write head may alter bits
that were just written in addition to the bit that is supposed to be written
at a given instant. The host indicates when to use reduced writing current
(RWC) by setting line 2 on the "Control" cable to true (LOW).
All four controllers above use the "Write Precompensation Register" to control
both precompensation and reduced-write current. The assumption these con-
trollers made was that if the drive required precompensation, the write
current should be reduced at the same point. On most drives, this was usually
not the case.
A few ST506 and all ST412 drives use their on-board electronics to monitor the
head position and reduce the write current at the point that is correct for
that drive. Line 2 on the "Control" cable is ignored. In newer drives, the
controller need only get the write precompensation starting point right, and
in some of the latest drives, write precompensation is not needed either.
(See ST251, ST251-1 and ST4096 on page 5.)
Lose Write Current, Gain A Head Select
--------------------------------------
When the hard disk drives started handling the reduced write current manage-
ment, line 2 in the "Control" cable could be used for something else. On the
larger drives line 2 has been designated as the fourth head select (2^3),
which will allow up to 16 heads to be present in the drive. An example of
this usage is the Seagate ST4096 which has 1024 cylinders and 9 heads. This
drive provides a formatted disk capacity of 80 Megabytes.
If you attempted to use a drive like the ST4096 on your existing system or
after you upgrade to the 2010, you are still limited to eight heads. But be-
cause this drive uses line 2 of the "Control" cable, you must disconnect (or
tape over) that pin. Otherwise when you reach cylinder 512 and the reduced
write current signal is asserted, the drive will attempt to access heads 9
through 15, which will fail, resulting in 7 out of every 8 tracks being
flawed-out. Covering the pin with a small piece of electrician's tape will
work fine unless you unplug and reconnect the cables frequently.
Since we have reached a semi-logical stopping point, this discussion will be
continued in a future article. That article will discuss getting that extra
boost of speed from the 2010 if you own fast drives and ways to go beyond 8
heads.
-fdiv-
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If you have questions about this article, please send email to:
Frank Durda IV @ <trsvax!uhclem>
...decvax!microsoft!trsvax!uhclem
...hal6000!trsvax!uhclem