Still Hoping to Exhibit at VCF West

[high...@lightspeed.net]

Hi everyone,

I am still hopeful to get my Heathkit H89A running in time for VCF West on August 1st. I am working on a project that uses my H89A as a terminal. I recently removed the motherboard to change the DIP settings for the baud rate on the terminal logic board, something I have safely done many times before. However, after reassembling the computer and powering it on, I heard only one beep. I took it back apart to inspect it and believe I may have accidentally misaligned the pins on a daughter board during reassembly, causing a short.

 My computer was almost a factory virgin when I picked it up. The power supply has been upgraded and the original hard sector controller card was replaced with a soft sector controller card, but I have never removed any of the custom chips. Since the incident, I have replaced what I consider the motherboard's boundary defenses. This includes the Z80A CPU, the UART, U509 74LS241N, U510 74LS373N, U511 74LS373N, U512 SN74LS74AN, U513 74LS241N, and U515 74LS02N. The machine still gives just one beep. The hardware gates are cleared, the system logic is verified clear, and the data bus is floating correctly, but the CPU remains blind. All voltages have been checked and are correct.

 Because of this, I highly suspect one of my custom PROMs has failed. My motherboard is revision 85-2549-1. Looking at my board, the populated custom logic chips are 444-83 at U516, 444-66 at U517, 444-84 at U518, and 444-61 at U550. The H89A was running perfectly prior to the short. I do not currently own a chip programmer, so I am hoping someone in the group might be able to help me verify which ROM failed or help me source replacements. Since U516 is a 74S188 bipolar ROM requiring a specialized fusible-link programmer, I would especially appreciate any assistance from members who have the gear to burn those or who might have working spares available to buy.

My goal for the show is to have this H89A functioning as a terminal paired with my EPA Micro 68b. I have already done most of the communication work between the two machines, but this motherboard issue has completely stalled progress. Please let me know if you have any insights or if you can help me burn a set of replacement chips to get this machine back on its feet.

 Thank you,

 Ken

[norberto.collado koyado.com]

To use the H19A as a terminal, you just needed to change the monitor to the MMS monitor which provides support for this configuration.

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From: se...@googlegroups.com <se...@googlegroups.com> on behalf of high...@lightspeed.net <high...@lightspeed.net>
Sent: Monday, 29 June 2026 13:31:46
To: SEBHC <se...@googlegroups.com>
Subject: [sebhc] Still Hoping to Exhibit at VCF West

 

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[Joseph Travis]

Easter egging / guessing is a poor troubleshooting technique.  There is good troubleshooting info found in the H89 assembly and operation manual.  A logical approach is the best method.

It would be helpful if you could provide good, clear photos (front and back) of the CPU board.  You mentioned that you hear one beep.  Do you have a cursor displayed?  Can you press the OFF LINE key and see characters typed on the keyboard being echoed to the screen?

Joe 

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[high...@lightspeed.net]

I think I checked everything that is mentioned in the Service manual before I started swapping in new chips. I got to the point where I was checking activity on the CPU and found none. I do not have a chip tester (yet) so I decided to change out the most likely boundary chips. The computer displays a cursor, but no H prompt. The keyboard types and displays text in the off-line mode only. The terminal logic board is fully functional. I have not removed the shield/heat sink from the back of the CPU board. I see no damaged traces or visible damage to any components. 

[high...@lightspeed.net]

I don't yet have a ROM burner. I would like to buy a MMS EPROM.

[Glenn Roberts]

Since you only plan to use the machine as a terminal, and you’ve already ascertained that the TLB works it seems to me you’re done. Connect a DB-25 cable to the TLB connector and feed it out the back? Save the CPU board repair for a rainy day…

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On Jun 29, 2026, at 6:48 PM, high...@lightspeed.net wrote:

I don't yet have a ROM burner. I would like to buy a MMS EPROM.

To view this discussion visit https://groups.google.com/d/msgid/sebhc/b0ea1d97-d9ac-49f3-873a-392194d77abbn%40googlegroups.com.

[high...@lightspeed.net]

I was told that will need to use the H89A CPU board to:

1. Program the Handshake Register

To pull both DTR (Bit 0) and RTS (Bit 1) high—along with enabling the UART's internal interrupt routing gate (OUT2, Bit 3)—you need to write the octal value 013 (which is 0B in hexadecimal or 00001011 in binary) directly to that register.

From the H89A H: monitor prompt, type the output command for your corresponding port:

     If using port 330, type: O 334 013 and press Return.

    If using port 340, type: O 344 013 and press Return.

 2. Switch to Terminal Mode

     Immediately after entering the port command, press the OFF LINE key to drop the H89A back into pure terminal mode.

    The 8250 UART latch will remain programmed with those lines held high until the machine is power cycled.

    Turn on the Micro 68b, tap the Spacebar on the H89A, and the MIKBUG * prompt at E000 will now successfully map through your Micro 68 Schematic paths to the terminal screen.

I this is incorrect please let me know. Has anyone here used an H89A in off-line mode as a terminal with a 6800 machine?

[Glenn Roberts]

The h19 portion of the machine seems to be working. Remove the cable connecting the terminal logic board to the cpu and plug in an appropriate db25 cable - pretty sure you can just use one of the ones from your serial hookup. For your application all you need is the h19 portion of the machine.

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[Joseph Travis]

If all you really want to do is use the H89 as a terminal, then you can do as Glenn suggested.  You don't need to over think the handshake lines.  You can "cancel" the handshake lines by jumpering pins 4-5 and 6-8-20 on each end with pins 2,3,7 going end to end.  I use this setup with several computers including my ET-3400 w/ ETA-3400 and Altair 680, all of which use 6800 CPUs.

Joe

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[Mark Garlanger]

Ken,

  Those instructions you have, will not work. Updating either of the 330 or 340 serial ports, won't have any effect after you press the off-line key. Both of those ports are on the add-on serial port board, which are not accessible in the off-line mode. Sounds like the best option is to do what Glenn and Joe is suggesting and just use the TLB board, connecting out the back. In order to use ports 330 or 340, you will need to boot either HDOS or CP/M and use a modem program to use those ports.

Mark

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[high...@lightspeed.net]

I have the same jumpering and pins as you suggest. I do have a MAX232 in the middle. I am now seeing activity on the Micro 68 from the H89A, but nothing from the Micro 68 to the Heathkit. I did use the serial cable as Glenn suggested. I have limited information on my Micro 68 computer. I have the paper user manual, schematic of the main board and some original catalogs, price sheets and a brochure. My computer has the TTC3 serial interface, 8K RAM, PROM expansion board and original wood case with backplane and power supply. There is some really good information at https://k7mem.com/VC_Overview.html with multiple pages. I don't really need to repair the H89A CPU board right now, but I do want to get it back working again. I'm pretty sure I have all the dips on the terminal logic board set correctly. Because I am seeing one-way communication, I am thinking that the cable wiring could be incorrect on the Micro 68 36-pin cinch connector. After following Glenn's advice, this is the first time I have seen any communication. I am early in the learning curve. Thanks to everyone who is helping. Ken