TeleVideo Systems Part IV - TS-801

TeleVideo TS-801 "System I"

The TeleVideo TS-801 was TeleVideo's first entry into the single-user CP/M computer market. It was also usable as a client system off of one of their distributed systems, which were centered around one of the TS-806 or TS-816 "Service Processor" systems.

The system is not expandable but arrives as a fairly well-configured system.  It was released in 1981 and listed for $3295 with a 4MHz Z-80, 64KB of ram, two floppy drives, a couple of serial ports, and a Centronics-compatible parallel printer interface.

Outside appearances

The TS-801 has a pair of double-sided/double-density Tandon TM100-2A floppy drives with up to 360KB of storage each, plus a red reset button on the front.  On the back, there's a DB-25 terminal port for the console terminal, a DB-25 serial printer port, and a 36-pin Centronics parallel port, along with a DA-15 RS-422 port to connect to a Service Processor for use in a distributed system.

Due to how the machine is set up, you can either use the RS-422 port or the serial printer port.  To switch between the two, there are a set of jumpers on the motherboard that need to be cut and re-soldered differently.  This is probably the worst way I could think of implementing this.  However, I'm sure that it was expected that you would only change this at most once when the system was initially installed.

Disassembly

The inside of the TS-801 is fairly simple and designed to be serviceable. The major chips are socketed, and the floppy drives are mounted to a platform that hinges out of the way for access to the system board.  The power supply is a fairly simple open-frame unit, which provides +5, +12, and -12V to the system board and floppy drives.

I have noticed that there are some differences between the two TS-801 systems that I have. The second system has several chips soldered into the "spare" locations on the board, and a bunch of fly-wire connecting them into various parts of the board.

Also, there are the set of jumpers on the motherboard which need to be adjusted to switch between using the 2nd (printer) serial port, or the RS-422 port.

System Design

The system overall is fairly similar to the TS-802 that came out a few months later.  There seem to be two main differences: The system requires an external terminal, versus the integrated terminal in the TS-802.  The system also has one less SIO chip, which means that there are fewer serial ports available.

The main chips in the system are:

• 4MHz Zilog Z80A CPU
• Z8410 DMA
• Z8442 SIO/2 - RS-232 terminal and either RS-232 printer or RS-422 link
• Z8420 PIO - Centronics serial interface
• Z8430 CTC - Time of day interrupt, baud rate generation
• WD1793 - FM/MFM floppy drive controller
• 16 x 4164 64k x 1 150ns DRAM 
• 2732 - 250ns, 4K x 8 EPROM

The same code will (mostly) run on either system, because they both use the same controllers, at the same IO port addresses.  The TS-802 moved the floppy (and an optional hard drive controller for the TS-802H) off to a daughter card, which made things a bit more modular.

Trying to get the system working, I ran into several issues.  I'm still not convinced anyone had these systems actually working before I got them - the hardware seems flaky, but the hardware mods to the second system seem to make them slightly more stable.  It would randomly reset occasionally while running CP/M programs.

Knowing they were similar to the TS-802, I grabbed the 802 service manual off of Bitsavers and started comparing it to the system board.  Most of the schematic matched up, but some parts around the memory latch -- the circuit that selects between having the boot EPROM or DRAM selected in memory -- was different.  

I hooked up a logic analyzer, and sure enough, the latch seemed to randomly be flipping between EPROM and DRAM.  Sitting at the CP/M command prompt, this wasn't really noticeable, because it only affects the bottom 16K or so of RAM, and CP/M and the SSP command interpreter lives up in the top 16K of RAM.

Comparing the wiring to the schematic, It appeared that the system controlled the latch based off decoding the address bus, and the buffered /IORQ line. On the 802, the system uses a signal it calls /IOWR, generated from the buffered /IORQ and /WR lines.

At first glance, just using /IORQ seems like it should work, but will set the latch anytime you do an IN/OUT instruction to address 04 (really, 4 or 5, because the bottom address line isn't decoded), and re-set the latch to enable the EPROM at address 6.

However, in processing an interrupt on the CPU, /IORQ and /M1 are used to signal an interrupt acknowledge and get the interrupt vector from the interrupting device. If this occurs when the processor is executing code with the bottom 8 bits of the address in the range 04-07, it'll trip the latch to potentially change state.  The system has a periodic timer generating interrupts to track the time of day, which will occasionally cause this to happen.  If that happens when you're executing code in the bottom 16K of RAM, it will instead execute code from the PROM - either crashing the system or (more commonly) resetting it.

This problem manifested itself as randomly rebooting the system when I tried to execute CP/M programs.  If the latch was set to use the EPROM, when CP/M tried to jump to the program at address 100h, it would instead jump to the EPROM at address 100h.

I was able to fix this by severing a trace for /IORQ, and running a jumper wire to connect /IOWR as a control input to the latch instead.

After this change, I was able to get this second system to mostly work, using the RS-422 port to boot off of a service processor.

Thoughts

These systems were pretty rough, though they meant to be cheap business systems.  They came as un-expandable systems but were fairly maxed-out for a single-user system, with a full 64KB of RAM, and two floppy drives.  TeleVideo quickly released the TS-802 for about $200 more, which included an integrated terminal, and made the TS-801 obsolete.  The TS-801 seems fairly obscure, and I doubt many people bought them with the lower priced TS-802 (and later, TS-803 and TPC-I systems) coming out over the next couple of years.

Today, they're sort-of nice because they're easy to connect to a service processor and serial terminal server to development work on without needing physical access to the system.  The biggest problem is that they still seem to have design issues which make the big red reset button on the front is important to have access to.  The Tandon floppy drives seem to be fairly unreliable; I'm not sure if that's due to age, or just their generally low quality.  The Teac FD55 drives in later machines are less expensive, smaller, and in my experience are way more reliable.