TRS-80 (IV) help?

From: Neil Breeden II <>
Date: Mon Oct 11 23:50:49 2004


  You are correct that BREAK at power up will bypass the attempt to boot
from floppy, the Cass? / Memory? Prompts here are normal.

  You are correct that without the floppy controller it will boot directly
to BASIC in ROM. Again Cass? And Memory are normal. Cass? is asking for the
cassette baud rate (L=500Baud or H=1500Baud). Memory? Reserves high memory
for use by drivers etc.

  When the system at startup says 'Diskette?' it thinks it's sensing a disk
in the drive, for me this typically happens when the drive door is open with
a disk in the drive.

  The best web site for info, docs and software is,
you should find the service and disk owners manuals for download. Ira can
send you bootable floppies for the system; he's great to work with and knows
a lot about these old systems.

  When I power up my Model III I hear the heads step back to track 0 thus
you are correct that this should happen.

  I'm more familiar with the MODEL III however I believe the following
applies to the MODEL 4 as well:

  When you removed the floppy drive for cleaning did you reconnect it to the
same position on the floppy cable? (Cable position defines the drive
number!) Assuming this is a standard RS floppy controller and cable then the
following is true: Radio Shack removed pins from the floppy drive connector
on the cable to control selecting the drive (typically either DS0/DS2 or
DS1/DS3 will be missing on the floppy controller side of the cable), they
jumper DS0-DS3 as all active on the disk drives (if you reset this then turn
them all back on). The internal floppy connector brings only DS0 and DS1
out. The external connector brings DS2 out as DS0 and DS3 out as DS1. Again
this is ONLY 100% true for RS controllers, some 3rd party controllers used
normal pinouts on the connectors so your mileage may vary.

  The LAST drive on the floppy cable MUST be terminated (150 ohm typically)
otherwise the drives won't work.

  Finally - if the drive is ALWAYS selected with the power on them you have
one of the cables on upside down, most likely the end to the controller PCB
as it's typically not keyed.

Hope something here helps. :)


Message: 24
Date: Mon, 11 Oct 2004 21:45:57 -0400
From: Dave Dunfield <>
Subject: TRS-80 (IV) help?
Content-Type: text/plain; charset="us-ascii"

Hi Guys,

Picked up another big load this weekend, and am just starting to go
through it - Item#1 is a TRS-80 Model IV.

When I first powered it up, it sucked on the Empty bottom drive for
a couple of seconds, then prompted with "Cass?" and entered ROM basic.
Tried a Model IV boot disk and go the same result - so I removed the
drive and cleaned it (machine has been sitting a long time).

Now, it powers up, selects the drive and "hangs" - does not move on
to the "Cass?" prompt - I've checked all connections and socketed
chips, and everything is clean and looks OK. Have reseated everthing
several times.

A few of observations:

- If I hold RESET while I power up the machine, the drive motors still
  come on, but I do not get a select - the select appears to be happening
  under software control.

- If I disconnect the disk controller board ribbon cable, the machine
  powers up at the "Cass?" prompt - in this case, I believe the ROM
  startup sees that there is no disk system and enters ROM basic,
  thinking that it is a diskless unit.

- If I power-up/reset while holding BREAK, it also gives the "Cass?"
  prompt - after briefly selecting the drive - looks like BREAK very
  soon after power-up can interrupt the boot process, and ROM BASIC
  comes up normally.

- I never see the drive seek - it selects, but does not seek.
  If I manually move the drive head out during power-off, it DOES NOT
  seek back to track-0 after selecting the drive when I power it on.

- Twice so far in a couple of hours of working on it, I have seen the
  prompt "Diskette?" (no cursor) after powering on the machine (with
  disk controller connected and not holding BREAK). I cannot make this
  prompt come up reliably.

It looks to me as if something in it's interaction with the disk controller
is hanging the machine.

Anyone here knowlegable in TRS-80's?

Can anyone tell me what the Model IV is *SUPPOSED* to do on power-up, both
with and without a boot disk in the drive (I have no documentation at all).

Any pointers to service documentation? ROM listings? Other information on
servicing a model IV - any idea how I can further diagnose this problem?

Any help at all would be greatly appreciated!

dave04a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools:
com             Vintage computing equipment collector.
Received on Mon Oct 11 2004 - 23:50:49 BST

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