TRS80 Model 4P FreHD Installation

Following on from the post about the Model 4 FreHD installation, here are a few pictures showing the installation of a FreHD Hard Disk Emulator in my TRS-80 Model 4P.

The mounting chassis is the frame from a dead Miniscribe 8425 hard drive.  The FreHD is mounted on a piece of plastic attached in the drive frame.

The FreHD is connected to the Model 4P expansion connector with a long 50 way ribbon cable.  Installation required a lot more disassembly of the machine than for the Model 4. The ribbon cable routes from the expansion connector across the 4P motherboard and comes out at the front of the chassis (but inside the plastic case) before it can get into the drive bay.

An SD Card extender (available on eBay) was used to make the SD Card accessible outside the Model 4P case. In this installation the SD Card socket is accessible in the modem port bay at the back of the 4P.

 

 

FreHD in recycled Miniscribe Drive Chassis

4P front view replacing a 5.25" Floppy

4P rear view showing SD Card slot and expansion cable

 

 

TRS80 Model 4 Internal FreHD Install

A few pictures showing the installation of a FreHD Hard Disk Emulator in one of my TRS-80 Model 4's.

The mounting chassis is the frame from a dead Seagate ST4766N hard drive.  I picked up a box of these in the early 2000's and that sat in the corner of the workshop waiting for a project just like this.  The FreHD is mounted on a piece of plastic attached in the drive frame.

The FreHD is connected to the Model 4 expansion connector with a long 50 way ribbon cable.  Installation required removal of the Model 4 PCB so the ribbon cable could be routed between the PCB and the metal chassis frame.  The cable comes out at the top of the Model 4 chassis and then attaches to the connector on the FreHD.

An SD Card extender (available on eBay) was used to make the SD Card accessible outside the Model 4 case for the few times I want to change the installed software.  I don't change the software on the cards often so having the SD card accessible from behind the machine rather than from the front isn't really a problem.  The SD Card extenders work well but the FreHD doesn't recognize that the card has been removed and inserted because it relies on a physical switch on the SD Card socket.  Best practice is to power down the machine before changing the SD Card if you use an extender.

FreHD installed in Model 4 Drive Bay. Expansion cable visible.

Alternate view showing SD Card extender cable

Not as shipped from the factory but a tidy installation

 

 

Computer Specialists Ltd - CS-16/64 Single Board Computer

A while back I was given a Z80 single board computer by Keith (ZL1BQE) when he found out that I really liked the Z80 microprocessor.

The board was made in New Zealand by Computer Specialists Ltd and sold as a "controller".  Apparantly it was also sold as a basis for a complete Z80 machine with a disk controller and could run CP/M.

I don't know the history of this particular board.  There are hand written comments on the documentation about "Pye TV's" and "Sheraton Rotorua" and I had an email discussion with Mark Eaton at Compuspec (the manufacturer still exists) and he said some of these boards were used to drive information screens in hotels.

I only have the main PCB (not the video or disk controller described in the documentation) which had been burgled for a few parts but thanks to the documentation it was actually pretty easy to get going.

It needed a replacement 4.9152 mhz xtal for the STC (AM9513 System Timing Controller).  This is used for the real time clock but more importantly the timing clock for the SIO/0 (Z80 SIO/0 serial IO controller).  Also a clean up around the reset circuit.  Looks like a reset switch or something was removed so the board by someone with a plumbers soldering iron so it wasn't in great shape there.  

The serial level converters needed to be replaced (MC1488 and MC1489) although they may never have been fitted because the serial port CTS pullup resistors were not fitted to the board (more on that below).

It took some digging into the ROM to work out why the CPU seemed to run and the STC was configured to generate the correct SIO clock frequency but nothing appeared in the terminal.

Two things.... the console is actually on port B of the SIO (not directly stated in the documentation although implied by the pinout on the serial header).

Also because the CTS pullup resistors were not fitted.  These pull the CTS lines at the serial connector to +12V which is inverted by the MC1489 to be "active low" for the SIO /CTSA and /CTSB signals. Without the resistors these signals are +5V and the SIO won't transmit anything.

So it works. Monitor ROM and Tiny Basic both run and work as documented.

 

Documentation for the CS-16/64

Technical Manual Schematic for CPU Board Schematic for Video Board Layout for CPU Board Schematic for Memory Board

 

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.

 


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Contact Andrew Quinn

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