M9301 Repair

The M9301 is a ROM/Terminator board for Unibus PDP11's.  In the PDP11/04 the PROMs on this board provide the boot loader and programmers console code.

The PROMs are mapped into two address blocks starting at 773000 and 765000 respectively.

Using the front panel I was able to read bytes from the 773xxx address block but not 765xxx which caused a "Bus Error".  

Initial testing with a logic probe showed that the Pin 8 of E17 (7420) was not going low when addresses in the 765xxx block were requested.

E17 was replaced and the board retested.

After this the logic probe showed activity on Pin 8 of E17 (7420) but reads to the 765xxx addresses still caused a Bus Error.

The following capture shows the the traces from E17 (7420).  It doesn't show the signal labels but starting from the top they are pins 8,9 (A12),13 (A11), 12 (A10) and 10 (A9).

Notice the "glitch" on pin 8 which is why the logic probe showed activity suggesting that the decoding was correct.  

Pin 9 (A12) is the wrong state for the address 765000.  It should be High (5v) rather than Low (0v).

Tracing this back through the M9301 ended up at E58 (74175 flip flop) on the M7859 controller board for the programmers panel.

The following trace shows the input and output of the A12 signals on E58 (74175) at the point when it is clocked.

You would expect that the Pin 2 matches Pin 0 after the clock and that it would be Low (0v) but instead High (5v) is going to the M7859 Unibus driver input.  

This gets inverted by the M7859 Unibus driver and then inverted twice on the M7859 so Low (0v) ends up at M9301 E17 (74175) Pin 9 when it should be High (5v).

Requests to 773000 work fine because A12 is '1' in that address.  Same for the DL-11W serial boards that live in the IO space around 7775xx.

So there you go... can't trust anything with this machine.  This is the second 74175 I have replaced on the M7859.  

Replacing the 74175 solved the problem and requests to the 765xxx address block now work.

The other unexpected behavior while testing was the high byte was always reading with all bits set.  

Removing the PROMs from their sockets and inserting them again fixed this problem so it was caused by poor socket/PROM contact.  A lot better outcome than the PROM being faulty.  I will replace the 4 sockets to prevent future problems.


KY-11LB SN#0416541 Repair

The KY-11LB Programmers Panel had been working well and being critical to testing the various boards in the PDP-11/04 it was disappointing to power it up and find the 7 segment displays not updating correctly.

The left most (most significant) digit was very bright and all the other digits very dim.  The digits did show the correct values entered on the keypad.

The display is multiplexed using control signals from the M7859 control board.  The issue with the left digit suggested that the drive control line was permanently "on" rather than cycling.

This was confirmed with a logic probe that showed no activity on E7 (7417) Pins 12 and 13.  E7 is on the front panel board and driven from E72 (7417) on the M7859.  Pins 8 and 9 on E72 also showed no activity.


Tracking back through the schematic showed E72 being driven by pin 6 of E57 (74175 Flip Flop).   Activity on pin 5 (D) of E57 was reflected on Pin 7 (Q) but not Pin 6 (/Q).

Replacing E57 (74175) fixed the problem and the display is back to normal.  

All that remains to complete the KY-11LB is replacing one of the HP-5082-7730 7 segment displays which has a faulty segment.


Kaypro / FreHD Adaptor - Part 3

A friend from out of town was visiting so he was able to being his Kaypro II with him to we could try the Kaypro '83 FreHD SHIM adaptor.

The physical arrangement of the Kaypro II PCB (81-110-n) is very similar to the 4/83 (81-240-n) so the SHIM does physically fit.

Interestingly this Kaypro II is fully socketed so I needed to add a spacer to ensure the shim would fully clear the other chips.  I used the same machine SIL strips that I use as sockets for the ROM and Z80.  Inserting these between the SHIM and the PCB sockets worked. The higher socketed chips were well cleared and the case would still close!

Kaypro FreHD SHIM installed in a Kaypro II (81-110n)

SHIM with FreHD attached

CP/M running from the FreHD

I discovered two changes required to the SHIM to work with the Kaypro II.

The Kaypro II has a 2716 EPROM (v 2732 on the 4/83).
Pin 21 on the 2716 is VPP but on the 2732 it is A11.  The Kaypro II PCB pulls pin 21 to VCC.  Not good for ensuring the ROM is addressed properly because the SHIM expects pin 21 to be A11.
This was easy enough to handle.  I just removed the pin on the SHIM and routed A11 directly from the Z80.

With that sorted the stock ROM would boot but Kayplus reported "ROM Error".

I had seen this in the ROM when I disassembled it during the SHIM development.  The ROM does a checksum so "ROM Error" was because the full 8K was not being addressed.

This is because U60 (74LS138) on the Kaypro II has A11, A12 and A13 as inputs.
On the 4/83 only A12 and A13 are used with the unused input tied to VSS.
Also different is the sequencing of the address lines to the U60 inputs.
On the Kaypro II it is A11 (A), A12 (B) and A13 (C).  On the 4/83 it is A12 (A), A13 (B), VSS (C).

To access the full 8K I needed to connect U60 pin 13 (in addition to Pins 14 and 15) to the GAL and make a small change to the equations to generate the /OE signal for the larger ROM.

Good thing the SHIM uses a 22V10 GAL and had a couple of spare pins.

Kaypro II (81-110-n) ROM Address Decoding

Kaypro 4/83 (81-240-n) ROM Address Decoding



DL11-W SN#1014374 Repair

This board was accessible from the front panel when installed in the PDP11/04 but the values read from the receive and tramsit data registers were not as I expected.

On initial power up the receive data register would report 377 octal and the status register 200 octal.

Receiving a character from the attached terminal would change the data register.

Sending a * from the terminal should load 052 octal in the data register.  This board would display 352.

The board was also inconsistent with some characters not being recognized.

Unlike DL11-W SN#1727869 the UART on this board is a genuine M5303 and is socketed.  There is no service tag on the board so I don't know if this came from the factory socketed or has been replaced in service.

Investigating the board with a logic probe shows that the values displayed on the front panel did match the data out pins on the M5303 UART suggesting a fault with the UART.

The M5303 UART is still available from some eBay vendors (2014) but the prices are quite high.  

An alternative part is the General Instruments AY5-1013A.  There were several vendors for this part (2014) with a wide range of prices (USD3 to USD50).

I ordered two from Bulgaria for USD3.  They both worked well when installed.

The board is now operational and will send and receive characters.


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Contact Andrew Quinn

jaquinn@ihug.co.nz http://twitter.com/jaquinn