PDP-11 Controller needed to use a couple of RX33 drive
Jerome H. Fine
jhfinedp3k at compsys.to
Tue Jun 30 18:34:46 CDT 2009
Sorry for the delay in answering this topic. I finally found the
altered 10-pin cable that was constructed about 20 years ago
(taken out of service about 15 years ago when a BA123 was
substituted for the BA23) to support both a Fixed Disk 0 and
a Fixed Disk 1 with a single BA23 box.
This post discusses the changes to the 10-pin cable which is
plugged into the J4 slot in the BA23 Signal Distribution Panel
which is located just behind the bulkhead at the rear of the two
drive compartment at the front of a BA23 box. These changes
support the use of a (second) Fixed Disk 1.
CRITICAL INFORMATION: The Fixed Disk 0 MUST be
selected as DS3 and the Fixed Disk 1 MUST be selected as DS4.
Failure to do so will almost certainly result in a loss of FORMAT
for both of the Fixed Disk drives. (Been There - Done That!)
I realize that this response is very long and provides a lot of detail.
However, it seems better to provide too much than too little.
I am holding a 4-button front panel for the BA23 in my hands:
REV. A2 ECO
MFG. CAN 43/85
After removing the 4 bolts on the back, the PCB board:
BA23 FRONT PANEL
KA 54-15610-00 C1/88
Left-Hand Number which is vertical is 5015609
Right-Hand Number which is vertical is 5415610
The labels for this panel are:
Fixed Disk 0
Write Protect Halt
Disk @ @ o
1 O Ready Restart
2 O @ @ o
where: @ is a push-button switch, O is a red LED and o is a green LED
The above front panel (without any alterations in the wiring) supports
ONLY a single Fixed Disk 0 and two floppy disks. However, by altering
the 10-pin cable which is connected to the J4 slot in the BA23 Signal
Distribution Panel, TWO Fixed Disks can be supported PROVIDED
(a) Both Fixed Disks are either READY / NOT READY at the same time
(b) Both Fixed Disks are either WRITE PROTECT / NOT WRITE
PROTECT at the same time
Note that the 10-pin cable which plugs into the J4 slot in the BA23
Signal Distribution Panel is one-half of the other end of the 20-pin
cable which plugs into the Front Panel at the front of the BA23 box.
The two alterations to the 10-pin cable are (lines are numbered from
line 1 to line 10 with the red cable being line 1):
(a) Cut the 5th line just before it enters the J4 slot and splice the
free end to the 1st line (without cutting the 1st line)
(b) Cut the 6th line just before it enters the J4 slot and splice the
free end to the 2nd line (without cutting the 2nd line)
In order to be able to easily remove the above alteration, a short
6" length of 10-pin cable was first attached to a male and female
pair of headers. The female end of the substitute cable was inserted
into the J4 slot and the female end of the standard 10-pin cable
which normally plugs into the J4 slot is plugged in the the male
header on the substitute cable. After normal operation has been
verified, the above two hardware changes are made. A second
Fixed Disk 1 is then cabled to the specified connectors on the
BA23 Signal Distribution Panel AND with the drive select jumper
set as DS4!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
In order to make sure that a little can go wrong as possible, it
is suggested that the XXDP format program be run using ONLY
Fixed Disk 0 set for DS3 when the second disk drive is being
formatted (as well as removing the patched cable). Preliminary
testing to show that this drive is working is also best carried out
in this configuration.
It would be appreciated for feedback to be posted by anyone
who attempts to implement the above changes to the 10-pin
Also, if Tony Duell can dig up the schematics for the BA23
Signal Distribution Panel to confirm the the reason why the
above changes actually work, that would be very helpful.
A second post describing the 6-button front panel for the
BA23 which correctly supports both a Fixed Disk 0 and
a Fixed Disk 1 along with 2 floppy disk drives will be posted
separately immediately after this post. This post describing
the 6-button BA23 front panel should put to rest the question
of whether or not a BA23 can support 2 fixed disk drives.
>Pete Turnbull wrote:
> On 19/06/2009 17:38, Johnny Billquist wrote:
>> This was definitely in a 4-butten front panel BA23. However, I'm not
>> sure the front panel is the only place to blame.
>>> *Maybe* the fact that the Ready/Write Protect lines
>>> for the 2nd drive are left floating, is somehow confusing
>>> the RQDXn controller, but that seems doubtful.
>> The RQDX wasn't confused. It was the drives... When the RQDX started
>> writing to #0, the write circuitry of #1 also went active. Or if it
>> was writing to #1 which also activated the circuitry of #0.
>>> Those lines merely report status of the 2nd drive.
>>> The Drive Select and Head Select lines don't go
>>> anywhere near the front panel, and there are
>>> separate signal lines for drives 0, 1, 2, and 3.
>> Exactly. Which is why I wonder if the front panel really is to blame
>> for the problem.
> It's not to blame at all. It has nothing to do with the drive selects.
> It might stop a drive working reliably, but it definitely cannot cause
> two to be selected at the same time. All it can do is tell the RQDX not
> to use the drive.
>> Hmm. As far as I can remember, there are no DS jumpers on the drives.
>> If you have an RQDXE, you have a bunch of jumpers on that in order to
>> set up which drive is which, and all that.
> Not entirely. It's intended to provide sufficient connectors and to
> route the some DS lines (etc) to the external unit. That doesn't
> remove the need to set the DS jumpers on the drive(s) correctly.
>> But the backplane of the BA23 don't have any jumpers, as far as I can
> That's right, the backplane doesn't.
>> And in this case, both drives were RD53s. Properly formatted, DEC
>> branded and everything.
> Well, those drives definitely have Drive Select jumpers. Look at the
> legend on the PCB by contact numbers 32 to 26 on the 34-way connector.
> They're the drive select lines, and you'll see they're labelled, and
> they go to a set of jumpers right beside the connector position, next to
> the terminator resistor pack. If they were "as supplied by DEC" and the
> person who installed them assumed the arrangement for a BA23 should be
> the same as for a BA123 and didn't change one, they would both be set to
> DS3, and so of course they both were selected at the same time. User
> error, pure and simple.
> The microPDP-11 Maintenance manual says where they are, and moreover
> it says:
> Always place the first fixed-disk drive in port 0 (left mass
> storage slot of the enclosure containing the RQDX controller.
> Set the device select to DS3 on any fixed disk installed in port
> 0 of the enclosure or expansion unit.
> Set the device select to DS4 on any fixed disk installed in port
> 1 (right mass storage slot) of the enclosure or expansion unit.
> This rule also applies to any subsystem installed in a BA23
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