TRS-80 Model 4 cable

HxC Floppy emulator support for all others computers...
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Posts: 1
Joined: Fri Dec 06, 2019 6:32 pm

TRS-80 Model 4 cable

Post by spt87a »

Hi all,

Proud new owner of an HxC 2001 Rev C. I'm using it with a TRS-80 Model 4 connected to the external drive connector.

So far I've had a strange cabling problem. I do have it working using an old PC floppy 34 wire cable that has 2 IDC connectors and onc PCB connector. The drive select twist is between the two IDC connectors- between the PCB connector and first IDC connector there is no twist. I removed the second IDC connector (which was after the twist) to use elsewhere (I'll get back to that). Currently the HxC has DS0 jumpered as A, DS1 as B and is configured for Generic Shugart with 2 drive support disabled.

With the PC cable plugged into the Model 4 so that the cable exists toward the front - the way the cable is wired pin 1 is on the wrong side if it exits toward the back of the mod 4 - the HxC works like a champ. With it plugged in this way I roll the cable back under itself to come out the rear of the mod 4. This cable is pretty short so the HxC has to sit right at the back of the machine.

I have a couple 2 drive external cables with PCB connectors - no pins removed - that are probably 3 to 4 feet long or so total. I've attempted to add the IDC connector removed from the cable above to these longer cables but the HxC just won't work. Depending on what I've tried, I get either an "A" on the HxC display all the time and the display stays "awake" but it doesn't otherwise do anything or I get no response from the HxC at all. I've made sure pin 1 is lined up with the red line 1 wire. I've tried reversing the IDC connector so that pin 1 on the IDC is lined up with wire 34. Used a multimeter to check that all pins are connected and getting signal from the PCB connector which attaches to the TRS-80, checked between the pins for shorts - all looks good. I used a regular external drive on the cable without the HxC attached and it works fine.

Could there be a cable length issue? I've tried connecting the external floppy with the HxC also connected but the HxC doesn't work and the external drive doesn't work either (even though it does without the HxC attached). The external drive light stays on and the motor spins. Tried the external drive both with/without the terminating resistor installed. Tried removing the DS1 jumper on the HxC as well with the external floppy attached.

I supposed there could be a problem with that IDC connector moved from the other cable - but like I said with a continuity test it all seems connected and no shorts. I've ordered fresh ribbon cable, IDC connectors and PCB Edge connectors so soon I can try to build a cable new.

Any thoughts? Anyone have pictures of a TRS-80 drive cable modified to add an IDC connector I could use as a reference?

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Joined: Fri Jan 10, 2020 10:06 pm

Re: TRS-80 Model 4 cable

Post by joeyd »

Drive Ready signal grounded?

Posts: 47
Joined: Thu Feb 28, 2013 3:47 am

Re: TRS-80 Model 4 cable

Post by ldkraemer »

If you dig out the Schematic for the TRS-80 Model 4 you will readily see that the FDC has PULLUP Resistors
installed for the Internal Floppy Drives. But, the External Floppy Drives DO NOT have PULLUP
Resistors Installed. This Means that you need to connect the LAST floppy Drive on the End Connenctor
with a Terminator Resistor (PULLUP Resistor). This will correctly terminate the Signals. Once you have
the Floppy Drive working on the End connector, then you can add the HxC on the Connector between
the Floppy and the TRS-80 Model 4. The cable should be a straight through cable and the Drive
Selects will need to be set as DS0 and DS1. Note that the Floppy Drives inside the Computer have Signals
marked as "DIR*-E & DIR*-I" to denote Internal & External. It shouldn't take long to get the HxC working
correctly. (If the HxC has the Terminator (PULLUP Resistor(s)) installed on the Signals going to the FDC
(INDEX*, TRK00*, WPT*, and RDATA*), then it should work out of the box. I don't readily have that
information handy for the HxC.

Note: Pin 34 READY Signal Not Used on the Model 4.


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