Friday, November 17, 2017

Built boot loader for HPdrive disks, installed it; starting RTE is much easier now


Creation of 12992H Boot Loader ROM for my HP-IB emulator

I brought the ROM binary image and a new PROM chip over to have it burned. When I install it in my machine, I can begin booting with the push of a few buttons instead of toggling in 64 words with an average of 9 button presses per word - that is roughly 585 actions I can avoid, both from the data entry and the related keypresses. Makes bootup quite a bit less painful. 

We completed the burning of the image on a MB7114H 1K PROM chip I bought on ebay, verified the content and packed it way to install in my machine when I get home. Eureka. We took the opportunity to burn another copy for Marc and then to download and burn other versions

Creation of character set PROMS for HP 264x terminals

The HP Display Enhancement Board allows for the addition of several additional character sets to a basic terminal. Line drawing is one, where you can create any drawing using characters that are composed of lines and arcs. Math symbols is another. 

The complication is that HP has two types of PROMs - alphanumeric and microvector - where the microvector ones can form contiguous ranges of pixels (e.g. lines abutting) while alphanumeric have blank pixels on the left and right for spacing. 

The microvector character sets require a 9 bit ROM - since each screen character position is a 9 by 15 matrix of pixels. HP reads out 16 9 bit elements while it is painting the character, ignoring the last of the 16 elements since it only needs 15 rows. As some of you may be saying to yourself, "I never heard of a 9 bit ROM device". 

We took one example of a microvector ROM and dumped it. The difference between the 8 bit and 9 bit ROM chips that HP used was only expressed in one pin. On the normal (8 bit) devices, there are three enable pins. The 9 bit chip repurposed one of the enable pins to deliver the 9th bit.

Marc did some pin bending and jumpering to take a pass through the ROM chip, substituting the 9th bit (enable) to the 8th bit socket with the real 8th bit pin bent up. Combining that with the dump of the chip taking its first 8 bits with no weird pin bends/jumpers, we recreated the contents.

We can't find any part numbers for 9 bit ROMs. Without a part to look for, we certainly won't find one. However, we have some ideas for workarounds. A complex solution involves a PC board in the shape of a DIP chip, with a small surface mount larger capacity chip, say 16 bit out, to deliver just the 9 bits. The simple solution is a stack of two ROM chips, with the second chip delivering just one bit to the socket position for the repurposed enable bit, but both following the same address and enables. 

Next week I will bring my Display Enhancement board that contains all the character set PROMS, allowing us to download them and burn new copies. 

Attempted alignment of repaired Diablo disk drive

Today I set everything up to do an alignment of the heads on the Diablo drive, after the heads had been carefully cleaned and replaced. This involved a setscrew to move the head position, cabling changes and scope probes on test points inside the drive. 

With that ready, it was time to spin up the alignment cartridge, let the drive warm up and stabilize, then move out to Cylinder 105 and adjust the heads for the right pattern. This is done twice, once for each of the heads. 

First, I let the heads land on a known good cartridge just to verify that we had no problems that would cause another crash. That went fine so I spun down so we could switch to the official alignment cartridge that has prerecorded patterns used to precisely locate the heads. 

However, as I tried to spin it up, the alignment cartridge platter was wobbling up and down between the heads, with a bit of a screeching sound, so I never let the heads load. I tried that previously read and known good cartridge, which had worked before, but it exhibited lesser but still noticeable wobble. Something happened to the drive and I don't feel safe loading heads until we understand it.


HPdrive emulation facility

I installed the boot loader PROM we created at Marc's today - 12922H - for booting the HP-IB integrated controller disc drives hooked via HP-IB cable to the 12821A controller card. I pushed just a few buttons and immediately booted up into RTE IV B. 

RTE environment

I am now able to run the RTE IV B environment, taking advantage of the firmware assists for this that are mounted on my Firmware Enhancement Module (FEM). With the increased memory I installed, I could also try to run the newer RTE 6/VM software. 

However, I know there are firmware modules for RTE 6 that are mutually exclusive with the RTE IV B chips, since they both use the same microcode address ranges. If I can locate, download and burn these RTE 6 assists, I can install them in spare sockets on the FEM and then use the enable/disable slide switches to alternate which assist chips are active on the board, based on the OS I wish to run. 

This is a project for later. Right now I will dig into RTE IV a bit and learn how to work with it better. When I have it somewhat mastered, I can look at RTE 6 and attempt to learn that as well. 

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