SYNERTEK MBC020 Video Output and More RAM

After figuring out the location of the video output on the MBC020's P4 connector (pin 20), I noticed that the video and ground signals are also connected to some large through-hole pads nearby. These may be solder points for a video coax cable, or some sort of connector.

I hoped to avoid soldering directly onto the board, so I first tried some "9mm Through-Hole Loop Test Points". These fit nicely in the holes in the board, but I struggled to get them connected to a coax video cable. Withe the added weight of a cable, they pulled out too easily and were generally awkward to solder and secure.

The obvious choice for this was a PCB mount RCA/phono jack for the composite video. Unfortunately, there was just no clearance on the front of the board since an IC and the board ejector interferred with the placement. Only after my failure with the test points did I realize that, after bending up one of the ground legs, there was plenty of space on the back of the board:

With the RCA/phono jack inserted, I booted it up. The video displays "SE" until the serial connection is made, and then is shows the exact same information as the terminal. Unfortunately, some testing uncovered that the top 1/4-1/2 of the screen has some apparent tearing and sync issues which improve progressively down the screen. The bottom 1/2 of the screen is very clear 80 column output. I am not sure why this is happening, but I would assume it is a damaged component on the board. Moving around the video jack to get a better connection did not improve the video quality. Since this monitor has proven to be compatible with many vintage video devices, if there is a compatibility problem I think it must be the board that is way out of spec.

So, video is almost working.

Next, I tried adding the unmodified Mikul 6218 board with extra RAM installed in place of the usual ROMs. Although I did not test the I/O, the RAM addition worked great! Using SERVOMON commands, I was able to edit memory areas that were previously unassigned. This confirms my belief that the Mikul 6218 is a really nice and reasonably priced memory board for a variety of EXORbus systems, especially because the memory sockets are very configurable for a variety of RAM and ROM chips.

Next time I will document the memory map further and check out the keyboard connection to try to make it a complete terminal!

MIKUL 1MiB Mods (final)

It did not take long for me to realize that the virtual address jumper board for my MIKUL 6218 was not a great long-term soloution. It was ugly and was not very physically secure with jumper wires running everywhere. Instead, I decided to remove the jumper board and add a 2*5 pin header to the top of the memory board, where it belonged. As a result, I would need to program a GAL to go back in place of the jumper board, handling the conversion from virtual to physical addresses. Since I now have a working GAL programming pipeline, this was no longer a serious impediment.

Header

Adding the pin header was a simple matter of drilling holes in the board in a .10" grid, supergluing a 2*5 male header (with latch) to the board, adding some copper tape connected to ground, and soldering on jumper wires connecting to the A16-A20 address lines. Although I would have preferred to use some mounting screws on the header, both of the mounting holes ended up right on top of VCC traces.

GAL Program

With the board soldered up, I had some initial success using it with my CMS 9639 and Microware OS9 Level 2. However, I soon noticed that there was a block of memory that was not being identified at $C000-$DFFF every 64K. This is an odd range of addresses to have a problem with, since it can't be attributed to bad connections on an address line or two. I quickly identified that the GAL in U10 will disable the RAM and enable the I/O in that range, regardless of the state of the high virtual address lines. Although this is fine for the MIKUL 6809-5 board it was designed for, it is not consistent with the CMS9639's expectation that I/O is only enabled in the $00FF60-$00FF9F address range. So, I had to reprogram the U10 GAL and make some adjustments to the program in the U13 GAL to get everything working properly.

Conclusion

Here is the final product, a relatively clean looking 1 MiB EXORBus RAM and 3xVIA I/O card working great with the CMS 9619 and OS9 Level 2 on the CMS 9639:

MIKUL 1MiB Mods (continued)

With the virtual address jumper for the MIKUL 6218 sorted, I still needed to simplify the address decoding and memory chip selects so that 1 of the 4 SRAM sockets will be selected depending on the state of our virtual A19 and A20 addresses. Since the three I/O ICs (6522 VIAs) share the data bus with the memory, I also need to disable the memory when the I/O is active, and signal the GAL in U10 to select the appropriate 6522 VIA.

Fortunately, the CMS 9619 and 9639 use several signals (VMA, VUA, or UTIL_DECODE) to indicate when the processor board is addressing external memory or I/O. This greatly simplifies the decoding from the original MIKUL arrangment which had the GAL fully decoding all of the address lines from A6 through A15. But, since the MIKUL 6218 board does not use those CMS/EXORbus decode signals, I had to cut a few of the unneeded low address lines to the GAL and replace them. With this simplified arrangement, I had hoped to use a few standard 74LS logic ICs to select the RAM and I/O. But, after a few attempts reduce the number of logic chips I needed, I decided to bite the bullet and just get a GAL programmer.

Here is my new setup which seems to work great with macOS:

• XGecu TL866II plus USB programmer - programs the GAL
• minipro - reads and writes JDEC files to the TL866
• GALasm - turns logic equations into JEDEC file

The biggest obstacle was that the first programmer I ordered (an older model TL866A) did not work due to a faulty pin 10. Unfortunately, this has been my luck with new tech items from China, including the cheap logic analyzer I bought a while back. These kinds of issues take A LOT of time to figure out.

Once I received a functioning programmer, I managed to get everything working well with my CMS 9619. So, now I can switch in 16 blocks of 32K RAM (512K) into the lower half of the CMS9619 address map, using the low nibble of its PIA output (at $FFC4) as a register to drive the virtual address lines. The CMS 9619 does not output a signal on its PIA for A20, so I can only use 2 of the SRAMs. Also, since the RAM addresses overlap the CPU addresses, half of each 512K SRAM chip can't be accessed (when A15 is high). This could be easily fixed by modifiying the U13 jumper board that I made previously, or even the CMS 9619 address decoder. Honestly, the original configuration was probably better for the CMS 9619 because it did not switch out the lower 8K of RAM that an OS would use. However, since my end goal is to use this with the CMS 9639 CPU and its integrated MMU, I will keep it as-is. I haven't fully tested the I/O and the VIAs yet, but I will get to those soon enough.

512K RAM on a MC6809!
Note the rainbow virtual address jumper and the
additional jumpers to route A19 and A20

More of the techincal details below, after the break...

MIKUL 1MiB Mods

A18 Addition

I really want to use my two 512KiB SRAM chips on this board to maximize the usable RAM. Fortunately, the MIKUL 6218 has 32 pin DIP sockets, so my SRAM physically fits. However, pin 1 (A18) of the socket is connected to VCC, limiting each socket to 256KiB (2^18) of memory. This is strange because four 256KiB (2MBit) SRAM chips would be perfect for this board (and my needs) but are quite unusual and are actually more expensive than four 512KiB (4MBit) SRAM chips.

To increase the socket capacity to 512KiB, the trace connecting A18 to VCC for each memory chip must be cut. Unfortunately, this is a wide power trace that is hidden under the end of the sockets. A few seconds with an 1/8" drill at a 45° angle cut through the A18 pull-up trace without too much collateral damage. Although I slightly cut into the socket, I barely avoided cutting into the next thin address line down.  

A18 to VCC severed.

Then, I added a few wire jumpers to connect pin 1 of each socket to each other to give a common A18 line. This address line, along with A16 and A17, need to be connected to a new virtual address header. A19 and A20 will be connected from this header to a decoder to create the memory chip select signals.

Bank Address Eliminator

With the physical connections made, it's time to simplify the MIKUL 6218's memory bank select system.

This little jumper board just connects each real address to its respective memory address line (A11-A15), overriding the bank switching latch and logic at U13. The virtual addresses (A16-A20) connect to the 10 pin header which will connect to the main CMS 9639 processor board. The 3 pin header on the right will connect A19 and A20 to a decoder to select the correct memory chip.

With the RAM and this board in place and the virtual address lines pulled high, the board works exactly the same way as it did before and has the same memory map. However, the bank register functionality (which I could not test anyway) has been eliminated. But, without the virtual addresses, I can only access some of the RAM.

Next time ... the RAM chip select Decoder

MIKUL 6218 Memory and VIA EXORbus board

I haven't spent much time working on my CMS 9639 SBC because, unlike the CMS 9619, it does not have any usable on-board RAM and does not include a monitor/debugger in ROM. Instead, it is designed to use an external memory board (which I don't have) on a back plane (which I now have) and boot OS-9 from a disk drive (which I don't have). What is nice about the device is the built-in memory manager which uses up to 1MiB of RAM. Unfortunately, EXORbus RAM cards are still prohibitively expensive on eBay and only have 16K to 64K of RAM capacity. Since the components would be far less than the price of those boards, I took a stab at designing a 1MiB SRAM board with a bonus additional I/O expansion.

I had just started to get components inserted into an EXORbus prototype board when I noticed the MIKUL 6218 Memory and VIA boards appearing on eBay at reasonable prices (<$45 shipped). Let's take a look at the board:

Click below for more details...