HP 27201A Speech Output Module, first speech synthesizer designed by a major computer manufacturer!

In this video, I reverse engineer the protocol used by the HP 27201A.

The “major computer manufacturer” claim comes from the HP museum page for the SOM. There certainly were other speech synthesizers that were contemporary with this one, or perhaps even predated it. For example, the TI99/4A speech module. As such, I think it may come down to your definition of “major”.


I bought this thing off ebay, not really knowing that much about it. The documentation on the web is really sparse, there’s basically on page and a half article that gives a few details.

HP Journal Article on the HP 27201A

The article was enough information to give me a couple of details:

  • The module is designed to sit inline, between an HP 1000 or HP 3000 series minicomputer and a peripheral device, such as a terminal or line printer.
  • The interface is RS-232 serial.
  • The communication protocol is ASCII, and has 8 commands.

Unfortunately, the details about the protocol were pretty vague. For example, although I knew there was a DOWnload command, I didn’t know the arguments or syntax.


Below are some pictures of the speech synthesizer:

HP 27201A top view
HP27201A bottom view
HP 27201A front view
HP 27201A back view

It’s nice that the dip switch settings were documented right on the case, that would have made life a lot more difficult had they not been.

HP 27201A Inside View

The internal view we can see a lot of things:

  • A TMS5220 speech processor IC. Same chip as used by the popular TI99/4A speech synthesizer peripheral.
  • Five 2K static RAM ICs. This is where the speech data is stored. There are jumpers next to each IC that allow an EPROM to be placed instead. Using an EPROM would allow you to use pre-stored speech data rather than having to download it on startup.
  • A Z8 Microprocessor with a piggyback socket for a ROM. This is where the software for the speech module resides.
  • Two 8-pin RS232 line drivers, U11 and U13, UA9636ACP
  • A quad RS232 receiver, AM26LS32ACN

From the RS232 line drivers, it’s easy to learn which pin son the 15-pin connector are +12V and -12V. It’s also evident that four pins are RS232 output signals. From the AM26LS32, one can tell that there are four RS232 inputs.


By tracing the pins, I managed to figure out the following:

3Not Connected
5SerIn 1B – Z8-P33, handshake line?
6SerIn 3B – Likely serial RX from inline dev, passthrough to Pin15
7SerIn 2B – Z8-P30, Serial RX (accepts escape sequences this pin)
8SerIn 4B
12Serout U11-2
13Serout U11-1
14Serout U13-2 – serial TX (probably to inline dev)
15Serout U13-1 – serial TX (probably to host)

The key bit of information is that Pin7 is the serial input from the host and pin 15 is the serial output to the host. The other pins are either the slave device, which we don’t really care about, or handshake signals such as RTS/CTS, or power pins.

Reverse Engineering the Protocol

I pulled the ROM and downloaded it using my USB EPROM programmer and minipro software. Then using unidasm, a universal disassembler from the mame project, I disassembled the Z8 ROM.

Unfortunately, trying to understand an architecture your unfamiliar with, without any assembly comments or anything else to go on, can be a long process, and I stayed up two long weekend nights figuring out what arguments each of the commands took. This is what I came up with:

clear<ESC>&ySCLE <bank>;<ESC>&yU
download <ESC>&ySDOW <bank>.<phrase> <length> [F|V] <hex digits>;<ESC>&yU
pitch <ESC>&ySPIT [H|M|L];<ESC>&yU
reset <ESC>&yRES;<ESC>&yU
speak <ESC>&ySSPE <bank>.<phrase>;<ESC>&yU
status <ESC>&ySSTA [S|A];<ESC>&yU
transparent <ESC>&ySTRA;<ESC>&yU
upload <ESC>&ySUPL <bank>;<ESC>&yU

The download and speak commands are the most interesting. The first argument to each are two decimal numbers separated by a dot, which identify a bank and phrase. For example, 3.27 would be RAM chip #3, phrase #27. The bank numbers start at 1.

When downloading, you then specify the length in bytes. Then you put the letter F or V (no, I don’t yet know the difference) and finally you send a bunch of hex bytes, two characters per byte. Separators between the hex bytes are not necessary.

Every command (except identify, which isn’t really a command) is terminated with a semicolon.

The Status, Upload, and Identify commands for me all did not produce output unless I followed their escape sequence with an <XON> character.

Obtaining speech data

Bitsavers has a collection of HP 1000 software, and the Voice Exerciser (VX) program that would have originally been used with the speech module is present there in the type-4 and type-5 archives. Specifically, this list of files. The 27203 files are the ones you’re looking for:

27203-16001  2320  01/01  A02797   00101   00005  %VX                               VOICE EXERCISER
27203-16001  2330  01/01  A02798   00101   00005  %VX                               VOICE EXERCISER
27203-16002  2320  01/01  A02799   00101   00005  %VMNGR                            VOICE MANAGER
27203-16002  2330  01/01  A02800   00101   00005  %VMNGR                            VOICE MANAGER
27203-16003  2320  01/01  A02801   00101   00004  VSCHMA                            IMAGE SCHEMA FILE
27203-16003  2330  01/01  A02802   00101   00004  VSCHMA                            IMAGE SCHEMA FILE
27203-16006  2320  01/01  A02803   00101   00004  VDBLD                             VMNGR COMMAND FILE
27203-16006  2330  01/01  A02804   00101   00004  VDBLD                             VMNGR COMMAND FILE
27203-16007  2320  01/01  A02805   00101   00004  SOMWRD                            DICTIONARY WORDS
27203-16007  2330  01/01  A02806   00101   00004  SOMWRD                            DICTIONARY WORDS
27203-16008  2320  01/01  A02807   00101   00004  SMWRD1                            WORDS FOR FLOPPIES
27203-16009  2320  01/01  A02808   00101   00004  SMWRD2                            WORDS FOR FLOPPIES (
27203-16010  2320  01/01  A02809   00101   00004  VXVERF                            VX COMMAND FILE
27203-16010  2330  01/01  A02810   00101   00004  VXVERF                            VX COMMAND FILE
27203-16011  2320  01/01  A02811   00101   00004  *VINST                            INSTALLATION FILE
27203-16011  2330  01/01  A02812   00101   00004  *VINST                            INSTALLATION FILE
27203-16012  2320  01/01  A02813   00101   00004  VDBLF                             VMNGR COMMANDS - FLO
27203-16013  2320  01/01  A02814   00101   00004  *VINS1                            INSTALLATION - FLOPP
27203-16014  2320  01/01  A02815   00101   00004  *VINS2                            INSTALLATION - FLOP
27203-17001  2320  01/01  A02816   00101   00004  "VXHLP                            VX HELP FILE
27203-17001  2330  01/01  A02817   00101   00004  "VXHLP                            VX HELP FILE
27203-17002  2320  01/01  A02818   00101   00004  A27203                            SP LIB SOFT NUM CAT
27203-17002  2330  01/01  A02819   00101   00004  A27203                            SP LIB SOFT NUM CAT
27203-17003  2320  01/01  A02820   00101   00004  "VMHLP                            VMNGR HELP FILE
27203-17003  2330  01/01  A02821   00101   00004  "VMHLP                            VMNGR HELP FILE

The two files with the speech data are 27203-16007_Rev-2330.src and 27203-16009_Rev-2320.src. These two files together contain approximately 1700 English words and their associated speech data.

Note that the VX software could be run on an actual or simulated HP 1000 series computer and that would have yielded the protocol by simple snooping of the serial interface. That was my backup plan had I been unable to disassemble and understand the Z8 ROM. I did have a lengthy chat with Dave Bryan who developed the Simh HP2100 simulator, and learned that loading complex software that requires a database, such as the voice exerciser, may be a daunting project. It’s still my hope, with some help from Dave, that I might be able to get this running on the simulator and try it out in the environment it was intended to work in.

My python program

I wrote a python program to communicate with the speech synthesizer and it’s present in my github repository linked below. There are two programs:

  • build_vocab.py. Builds a vocabulary file to download to the module, acceptings words from stdin and writing the vocabulary to stdout.
  • hpcli.py. A command-line tool to interact with the speech module, with commands for initializing, speaking, etc.

The video at the top of this blog post shows me running the python programs to interact with the SOM


Comments (2)

  1. Thank you so much for this nice resurrection of ancient digital voices.
    I am collecting everything about speech synth and of course about the TMS5220. Here is my collection (I just added the hp27201a files you dug) of LPC files
    I use a slightly different format to stay compatible with the one used in Arduino and https://github.com/ptwz/python_wizard
    Keep going!

  2. Henk Gooijen says:

    Cool find on eBay!
    Whether this unit was the first from a major computer manufacturer is (as always) a “dangerous” assumption. For example, D|I|G|I|T|A|L (DEC) had the DECtalk 1. It used a TI DSP processor for the sound generation and had a 68000 as interface. You send text serial to the unit, and speech comes out. We al *know* the DECtalk 1, because this unit generated the voice for Stephen Hawking (RIP).

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