TeleVideo Systems Part III - TS-1605 PC/XT Clone

TeleVideo TS-1605 PC/XT Clone

The TeleVideo TS-1605 is one of their first PC clone systems, and unlike the earlier TS-1603, it includes expansion capabilities using an ISA card slot, and optional ISA expansion chassis.

I ended up picking up one of eBay with a few known faults, hoping that I could fix them. The two errors were obvious in the eBay listing, a 201 and 601 error. Based on IBM's standard numbering this meant there probably was a memory fault and a floppy disk fault of some sort.

TeleVideo TS-1605 computer

Outside appearances

This system, based on the 970 terminal and TS-803 before it, has a nice 14" green phosphor tilting screen, and a base that swivels nicely if it's placed on a hard surface.

The front has a modular 6-pin coiled cable coming out of it to plug a keyboard into. This appears to be a mostly IBM PC/XT compatible keyboard but powered using 12V instead of 5V. I haven't quite figured out if it can be made to be compatible with a PC/XT keyboard, and my initial attempts didn't work out with supply 12V power the keyboard and connecting it to an IBM 5160 PC/XT.

The front also shows to vertically stacked 360KB (DSDD, 48tpi) TEAC FD-55B floppy drives, which are the same as TeleVideo used in the TS-803 CP/M system.

Back of TS-1605

The back of the system shows some integrated peripherals:

• RCA jack and DE-9F RGBI output, compatible with an IBM CGA monitor. 
• DB-25M RS-232 serial port from an 8250 UART 
• DB-25F IBM-compatible Centronics interface parallel printer port 
• DIP switches for setting, easily accessible from the back of the case 
• A built-in ISA expansion slot, for connecting one card, or an expansion chassis, which I pulled through the expansion slot hole to make it easier to connect cards. 

Disassembling the system

System with side cover removed

The system is fairly easy to get the cover off with only 3 screws at the bottom of the right side holding it on. I figured that before I would power the system up, I should pull it apart, and test out the power supply outputs, and check that everything physically looked OK. 

RAM card close-up

Inside there was a 256K ram expansion card installed into the card slot. The slot itself is mounted on a cable that plugs into a 64 pin header on the board, making it easy to remove to connect something external to.

System board close-up with 8088 CPU / 8087 socket and connectors for floppy drive, hard drive controller, and 64-pin ISA slot header.

Removing that shows off some features on the system board:

• Intel 8088 CPU 
• Slot for 8087 math coprocessor 
• 4 memory banks and an option MUX chip to allow upgrading the stock 128K of ram to 256K (4 banks of 4164) or 512K (2 banks of 41256) -- the system only allows 4 banks of 256K RAMs 
• 32K of static ram in 51400 chips for video memory 
• TeleVideo gate array and ROM chips for the CGA-compatible video card and system BIOS 
• An NEC 765 and FDC9229B floppy controller and data separator 
• 34-pin IBM-compatible floppy drive connector 
• An onboard 6-pin modular jack for the keyboard cable 
• 40-pin WD1000 compatible hard drive interface, which is the same as used on various Z80 systems, including the TS-802H, 803H, 806, and 816. 

Debugging memory

System board RAM, 4 banks of 8 chips, and BIOS EPROM

First, I tried to find and eliminate the memory fault. I started by pulling out the memory expansion card, which didn't help at all. All of the onboard memory was socketed, so I went through and tried to find which chips were bad. The system still tried to boot with a reduced amount of memory and the "201" memory error, so I hoped to boot up DOS and used DEBUG to find the bad memory.

This didn't go quite as well as I planned. The floppy drives didn't work either, so I was trying to boot off an XT-IDE card. However, as soon as it displayed the boot menu, the system would freeze, and not display any input. I first figured this was due to the memory issue, but it turns out that it wasn't. Fortunately, I had another 8-bit IDE card which I was able to use, and boot off an SD card in an IDE to SD adapter. It's a "Centos, Inc. CI-1020", as far as I can tell not related to RedHat.

Once I got booting sorted out, I was able to boot up MS-DOS 5.0, run DEBUG, and poke around memory. I discovered that a section of RAM had one bit that was stuck on. I didn't have schematics, so I went through and replaced memory chips one bank at a time until the error went away. Once I did that, I put back memory chips one at a time until the error re-appeared. That helped isolate the one bad 4164 chip. Fortunately, I have a stack of spare 4164 from Quadboard and AST Six Pack expansion boards.

Debugging floppy drives

Floppy controller and video memory/chips

The next problem I went to diagnose was the "601" disk error. Booting off of a floppy disk didn't work, and the drive didn't respond from within DOS either. However, the drive light and motor did turn on, so at least some parts were working.

At this point, I had found a schematic in the technical manual for the machine on eBay. So, I could see that the drive select/motor lines were driven by a latch chip on the motherboard. The rest of the drive controller was integrated, and the way that it was all integrated, it would have been a major effort to replace the onboard controller with an ISA card. So, I pulled out my oscilloscope and started looking at things.

First, I looked at the data line from the drives to the controller. With a disk in the drive, and the system trying to use the disk (I had to do this from within DOS since the BIOS didn't even try to boot off of the failed controller) I saw pulses that looked like flux transitions on the disk. So far so good - the drives were probably OK.

The system uses an NEC D765 floppy controller, and an FDC9229B data separator, to convert the data stream into clock and data pulses for the controller. So, one of these two chips were likely the problem. Hooking the scope up to pins on the NEC chip, it looks like it was receiving its chip select line, clock, read/write, etc, ok. However, I noticed that the chip wasn't responding to commands, and the data line from the FDC9229B was stuck low.

I wasn't sure at this point if it was the D765 or FDC9229B. However, both were available on eBay for reasonable prices, so I got both. A few days later, I had received both of them and continued with debugging. Replacing the D765 didn't change anything until I had also replaced the FDC9229B. With it working correctly, I put back the original D765, and that worked as well. So I had narrowed it down to the FDC9229B and pitched the old one.

It was fortunate that both chips were socketed, so it was easy to swap them out. Otherwise, the repair process would have taken much longer (I would have had to replace both of them, as it's difficult to desolder chips from boards without damaging one of them). And in the end, they would have ended up with sockets anyways.

Expanding I/O

ISA expansion

The system came with only one ISA slot but was also offered with an expansion chassis for additional card slots. I thought I'd try an easy solution to get more slots. On eBay, I was able to find an ISA riser card with multiple slots, which had a normal ISA card-edge connector on it. The one I found was called a "Generic CTK-303" like these on eBay.

I was able to pull the ISA slot cable out through the hole at the back of the case and plug it in. Note that it's a 16-bit riser card, where the computer has the original IBM 5150 PC style 8-bit slots. This doesn't really matter, as long as I don't try to use any cards where the 16-bit "extension" slot gets in the way. Few cards had that problem, such as the IBM original CGA card. However, with the built-in CGA compatible video, I wasn't going to be using a CGA card in this either.

I found a collection of cards to plug in, including a 3com 3c503 Ethernet card, 8-bit IDE card, IBM 3270 emulator card, RAM expansion, and serial port card, and booted the system back up. Everything seemed functional (I didn't test every card, but I didn't see any signs that I had overloaded the bus).

The Future!

Going forward, there's a few things I'd like to work on. I'd like to use the system with the IBM 3270 card and IBM's PCOMM as a 3270 terminal, mostly because it looks cool. I'd also like to stick the ISA riser card into an external chassis with a separate power supply, and cable back to the host. Since the connection is just a standard 64-pin header connector on the system, this should be relatively easy. It'd also help getting the memory up to 640K or so. I have purchased the memory MUX chip and have a stack of 41256's, which would let me at least increase the memory on the system board to 512K.

TeleVideo Systems Part II - Keyboards

TeleVideo 965 Keyboard Emulation

The Terminal

TeleVideo 965 terminal with a PS/2 keyboard

I purchased a set of TeleVideo 965 terminals for a relatively good deal on eBay, hoping that I could make them work with existing TeleVideo keyboards I have.  I had some reason to suspect this - older terminals both used a 12V power supply to the keyboard and a TTL-level RS-232 connection between the keyboard and terminal logic.

Unfortunately, they weren't quite compatible, as I noticed the 965 has a 4-pin (handset-style) modular jack, and the slightly earlier 970 terminal I have has a 6-pin (RJ14 type) modular cable.

Older Terminal keyboards

TeleVideo 970 keyboard, missing F1 and CHAR INSERT

TeleVideo's original detached-keyboard terminals, the 925 and 950, both use a 4-pin modular jack. Going through the 925 schematics available on Bitsavers, it's apparent what the pinout is, and the supply voltage. The maintenance manual also lists the baud rate of 1200 baud for this keyboard.

The later model 970 keyboard added pins for a shield (likely to appease the FCC) and -RESET line to the terminal (the same keyboard was used on the TS-803 computer, and possibly others). Looking through the 970 technical manual on Bitsavers, you can see that the supply is 12V, like the earlier keyboard.  However, the baud rate is increased to 9600 baud, as I was able to verify with an oscilloscope.

Keyboard protocol analysis

I figured the best place to start was to decode what data the 970 keyboard was sending to the terminal.

TeleVideo 970 keyboard data capture on a 'scope

Hooking an oscilloscope to the keyboard pins, using a couple of spare modular jacks to connect pins through, I was able to verify the voltage of the connection, and the data coming out of the keyboard was 2 bytes of TTL level serial, at 9600 baud, 8/N/1.  I then connected the output of the keyboard up to a Linux PC using a generic TTL serial adapter.  From there, I was able to write a small program to log key presses and let me input what I was pressing, and dump the output into a CSV file for filtering through.

Keyboard decoding hardware setup

I discovered that the basic protocol is a byte to indicate the status of shift/modifier keys, such as shift, caps lock ("alpha lock"), control, and function, along with a second byte to indicate the key pressed, which was mostly the ASCII version of the key pressed.  This made the keyboard fairly simple to understand.  All I had to do is see how the modifier keys modified this second byte, and what all of the special function keys on the keyboard did.

At this point, I hoped that the terminal didn't have to send any data to the keyboard to initialize it.

Keyboard interface analysis

I assumed that the 965 keyboard interface would be similar to the older 970 interface, and used a multimeter to test the pins coming out of the terminal.  I quickly discovered that the terminal ran off of 5V instead of 12V, and was hoping that the baud rate was still 9600 baud.   

Keyboard simulator using "echo"

My first test was to use my Linux box to simulate a keyboard and send two-byte sequences to the terminal.  This worked perfectly, and I was able to get things I typed to show up on the terminal screen after looping back the primary serial port Tx/Rx pins.

Testing with an adapter

Keyboard with modular jack adapter

I grabbed a couple of surplus modular jacks I had laying around, connected wires between them, and attempted to run this, with just by-passing the 7805 voltage regulator on the keyboard.  This didn't quite work, as the voltage regular drew too much power, so I instead found a spare 12V DC wall-wart power supply to connect to my modular jack setup to power the keyboard.

To my surprise, this worked pretty much perfectly! So my earlier effort reverse-engineering the 970 keyboard protocol gave me exactly what I needed to replace the 965 keyboards.  I used the TTL serial adapter to record the commands that the terminal sent to the keyboard, and slowly figured out what some of them were -- reset, set/disable key repeat, etc.

Arduino PS2Keyboard library

My first thought at an adapter was to finally one of the Arduino boards that I had picked up at a Pi-day sale at Sparkfun.  With minimal searching, I found the PS2Keyboard library that was available for Arduino and worked on getting it to work with a PS/2 keyboard (I was using an IBM Model M since I had several of them laying around). After some work, I was able to get the example code to work.

My initial tests didn't work out, because I had messed up the pin-out on the PS/2 jack, and swapped +5V and the clock signal around, which gave strange results from the keyboard and attached oscilloscope confirmed there was a problem.  Fortunately, PS/2 interfaces are pretty hardy, and I didn't manage to break things.  If I had reversed data pins and the 12V power line to the TeleVideo keyboard, I'm sure it wouldn't have worked out well.

Re-writing PS2Keyboard

After digging into what the library would do by itself, I quickly discovered what its limitations were.  There were a few things that I wanted to do with my adapter, such as support the non-alphanumeric keys on the keyboard and modifier keys, which were not really supported with the Arduino library.  So, I decided to scrap most of what the library included, and just focus on reusing the library parts that directly interacted with receiving data from the keyboard and turning the bit stream into PS2 scan codes.  From there, I could use some translation tables to convert that data into the TeleVideo keyboard protocol modifier bytes.

In researching this, I spent lots of time reading every reference I could find on the PS/2 keyboard protocol. The MCS electronics app note on interfacing to an AT keyboard was useful, along with this article on the PS/2 keyboard protocol, along with another article from the same author. I used a few different references on keyboard scan codes including this osdev wiki page and this other list. And a few more general references, including the IBM PS/2 Hardware Interface Technical Reference manual.

I decided on some translations to do for keys that differed between the two.  PS/2 keyboards don't have a "SET-UP" or "SEND" key, but both of those are important keys to support on the TeleVideo terminal. I was able to get this mostly working, which was fairly easy to debug by moving the RS-232 output on the Arduino between my Linux box and a terminal.

After getting this all mostly working, I was able to add in support to send commands back to the keyboard, mostly so that I could control the LED keyboard lights and make the interface a bit more user-friendly.

Schematics and Code

The connections to the Arduino were fairly simple, just wiring up to RS-232 from the terminal, and connecting power, ground, clock and data to the PS/2 connector.

Schematic of keyboard converter 

I have uploaded the code to Github here.

The future!

I have re-created the device using an Arduino Pro Mini, so that it can fit into a small enclosure and be a small adapter. I'd like to extend it to support other keyboard interfaces which are RS-232 compatible, like the earlier TeleVideo terminals, and DEC LK-201/401 keyboards.

TeleVideo Systems Part I

TeleVideo Systems, Inc.

Who was TeleVideo?

Televideo Systems, Inc, was a bay area company that started in the 1970s by producing CRTs and monitors.  Using this experience, they then produced dumb terminals, including terminals emulating the popular Lear Siegler line of ADM terminals, Digital VT-52 and VT-100, and many others.  They then produced single-user and then multi-user Z-80 and CP/M based systems in the early 1980s.  Once the IBM PC became popular, they produced 8088-based PC clone systems, including systems that interoperated with their Z-80 multi-user systems, and then multi-user based 80x86 PC-compatibles.

TeleVideo terminals

TeleVideo's first terminal was the 912, which looked much like the Lear Seigler ADM-3A which it emulated.

TeleVideo 925 terminal

TeleVideo later produced the popular 925 and 950 terminals, which were similar looking with different features.  They used a 6502 CPU to control the CPU, with a pile of random logic (and later, gate chips) to create the display image from the screen buffer.  On earlier terminals, this meant that changes in display attributes (blink, underline, reverse video, half intensity, etc) took up a display position.  Doing this helped to reduce the memory required for the display buffer, and reduce the cost of the terminals.

TeleVideo 970, rebadged by BT

The 970 had a sleek look with an adjustable tilt 14-inch green phosphor CRT, and ANSI, VT-100, and VT-52 emulation modes.  The ANSI mode included a form-based mode which worked much like the IBM 3270-style block mode.  The system sent a form down, laying out fields on the display, and what type of data could be entered in the fields, which the terminal would verify.  When the user was done entering data, they'd hit the SEND key on the keyboard, which would transmit some or all of the screen back to the host.  This helped to minimize the interaction with the host system (which with a single Z-80 serving multiple users wasn't particularly fast) and made the system more responsive and user-friendly.

TeleVideo 965 Terminal

Later terminals such as the 965 would emulate dozens of other terminals.  TeleVideo moved away from making straight clones (like the 912 that looked like the ADM 3A that it emulated) towards making their own unique, nicely styled, sleek products.

The 965 is a fairly versatile terminal, which used a 65816 CPU (the 16-bit version of the 6502, just like what the Apple IIgs used) to emulate many different terminals.

TeleVideo pt100 Personal Terminal

One of the more unique terminals that I have found is the TeleVideo pt100 "Personal Terminal", which was a small easily portable all-in-one terminal, with a small amber CRT, mini keyboard, optional modem, and emulated a VT-52 and VT-100.

One of the features that I've noticed while digging into TeleVideo systems is that TeleVideo liked to standardize on parts. Large chunks of TeleVideo terminals and computers used the same or fairly similar keyboards, for example.  Since the keyboards are hard to find, that helped me make use of the systems and terminals that I received without a keyboard.  I was able to use that info to build a keyboard converter for the 965 terminals that I didn't have a compatible keyboard for based on information gathered from keyboards for the earlier 970 and 925 terminals that I do have.

Z80 systems

Televideo TS-801 computer

TeleVideo made several single-user systems, some of which could be used with a central host to make a multi-user system. They communicated over an 800K-baud RS-422 serial link, which used the SDLC protocol to serialize packets of data to communicate.

The TS-801, picture above, had two 5.25" floppy drives.  All of their systems came with 64K (sometimes 128K) of ram standard, as at that point it was clear that maxing out the memory was a good idea.  It has a Z80A running at 4MHz for a central processor, included the Z80 DMA chip, a Z80 SIO/2 to run the serial ports and an optional RS-422 port, and a WD 179x / NEC D765 floppy controller.  The TS-801 was a "cheap" model, which lacked an integral terminal that their other early models had (like the TS-800 / TS-802).

Televideo TS-803 computer

The systems were based around Digital Research's CP/M.  Since they were fairly late in the CP/M timeframe, they used a fairly standard and easy to inter-operate with 5.25" double-density, double-sided, soft-sectored floppy format.  The disks are 18 sectors, at 256 bytes per sector, 40 tracks per side (48tpi), using MFM encoding, and can be easily read and written on a PC floppy controller.

The TS-803 above added an integrated "terminal", which supported character mode, using the logic array chip from a TeleVideo 925 terminal, and a graphics mode, which used the Synertek 6545 CRT controller and 32K of dedicated video RAM.  It allowed 64K or 128K of bank-switched RAM. on the main CPU, and had a slot for an add-on card with a Z80-SIO/2 and line drivers for a host RS-422 interface.

Some of the systems included a 5-40MB hard drive, using a controller based on the Western Digital WD1000 controller.

The single user systems would have zero - two floppies and up to two hard drives.  Systems without any drives, such as the TS-800A/TS-800R are designed to only be used as terminals or user stations on a central host that would provide storage.

Televideo TS-816 computer

Two host systems were available.  The TS-806 was a smaller host system, which provided ports for 6 terminals, a Centronics parallel port for a printer, a couple of RS-232 ports for a printer or modem and a console terminal, along with a single 5.25" floppy drive and a 10 / 10MB 5.25" full height hard drive.  It could be expanded with another hard drive of the same size, and an optional QIC tape drive to do back-ups to.  It used a 4MHz Z80A CPU, 64KB of RAM, Z80A-SIO/2 chips for the user station links, a Z80A DMA interface chip to manage data transfer to/from devices and user stations, and Western Digital floppy and hard drive controllers.

Back of TeleVideo TS-816, showing user ports

The TS-816 was an expanded version of the TS-806. It allowed up to 16 user stations over RS-422 links, an optional Centronics parallel printer port, three RS-232 connections for printers/etc and a maintenance terminal, and a QIC tape drive and 8" hard drive that stored 25 or 40MB. It used the same Z80A CPU, but had 128K of bank-switched RAM, plus the same Z80A-SIO/2 chips for user station links, DMA and WD hard drive controller.

Both host systems had fairly ugly beige and black cases that were designed before TeleVideo switched to more modern looking cases (apparently after they hired someone to do product design for them).

TeleVideo TPC-I missing the keyboard

One of the more interesting systems was the "TPC-I" TeleVideo Portable Computer. There was a TPC-II version which was introduced a little later as a PC compatible computer.  The TPC-I had a Z80, 64K of ram upgradable to 128K, the same graphics mode as on the TS-803 and included the same mouse port.  The TPC-II is notable as it was the first computer I had growing up (but sadly was thrown out when my parents moved out of their house).

The TPC-I I acquired had two 5.25" double-sided, double-density (TEAC FD55-B) floppy drives. There was an available option of an RS-422 port (using the same adapter as the TS-803), but that was not installed in the system I received.  It was also, unfortunately, missing the keyboard.  However, it is compatible with the PC keyboard from the TS-1605 and TPC-II models, so I was able to use the system with that keyboard.  The keyboard and lack of a hard drive interface seem to be the two main (software-affecting) differences between it and the TS-803 -- the TS-803 boot image loads on the TPC-I, although the keyboard doesn't work.

PC compatibles

TeleVideo TS-1605 IBM PC/XT compatible

TeleVideo produced a few 8088-based PC compatibles. After the first model (TS-1603), they started producing systems with an ISA card slot, and optional expansion chassis, which allowed system expansion using the now standardized IBM PC expansion bus.

From the first systems, they moved into 286 and 386-based systems, before mostly giving up the clone market, which had been greatly diluted with competitors by the time, and focused back on terminals.  They produced a few Windows Terminal Server compatible terminals, before eventually selling off parts of the company, filing for bankruptcy, and ending terminal sales completely in 2011.