Running a Multiuser PDP-8 Emulation as a Service

Created: 10th Sep 2026

Introduction

This article describes how to setup a PDP-8 emulation on a standalone Linux machine, running TSS/8 the muliuser operating system using real serial devices.

The Linux machine used in this example small footprint Lenova device running Fedora Linux 43 and open-simh v4.0-0. The machine has a single hardware serial port fitted.

In this example the console is set up for use with the hardware serial terminal and up to four users can connect using a Brainboxes four port USB to Serial device. These ports are connected to the SIMH emulated PDP-8 TTIX device.

Configuration

The pdp8.ini file shown below will produce the machine configuration specified below. Note that the machine is setup for running TSS/8 the muliuser operating system.

Actually connected and doing something:

CPU: 32K words of core (needs the KM8E memory-management hardware to bank-switch beyond the base 4K), plus a KE8-E Extended Arithmetic Element — hardware multiply/divide, a genuine performance option most installations paid extra for.
KW8-I real-time clock crystal controlled clock (CLK, 60Hz).
Console teletype (TTI/TTO) — the primary operator console, KSR (keyboard send-receive), uppercase-only, mark parity — this is connected to the machines hardware serial port.
A terminal multiplexer (TTIX/TTOX) — this lets multiple users share the machine simultaneously. This is attached to four USB serial ports.
RF08 fixed-head disk (RF, 262KW) — this is the TSS/8 system disk — everything else is present but empty.

Present (the controller/interface exists) but nothing loaded — the equivalent of an empty drive bay:

PTR/PTP — a high-speed paper tape reader and punch, sitting idle (no tape in the reader, a 'backup.ptp' tape in the writer).
LPT — a line printer, no paper/output file attached.
RK8E controller with 4× RK05 disk drives (RK0–RK3) — a whole second disk subsystem fitted, but no cartridges mounted in any of the four drives.
RX8E controller with 2× RX01 8" floppy drives (RX0/RX1) — floppy disk capability, empty.
TC08 DECtape controller with 8 transports (DT0–DT7) — a large DECtape installation, all empty reels.
TM8E magtape controller with 8 drives (MT0–MT7) — reel-to-reel magnetic tape, all empty.

Not fitted at all (hardware options this particular machine doesn't have):

TSC8-75 — no ETOS-style timeshare clock, not used in TSS/8.
FPP8A — no floating-point coprocessor, most languages handle floating point well without it.
RL8A and DF32 — two other disk subsystem types, neither present.
TD8E — an alternative DECtape controller variant, not fitted the TC08 is filled instead.
CT — no cassette storage.
DPY — no graphics display.

pdp8.ini

; ============================================================
; pdp8.ini — TSS/8 timesharing configuration
; ============================================================

; --- CPU ---
set cpu 32k		; 32KW of core memory (max addressable with MC8/KM8E extension)
set cpu eae		; enable Extended Arithmetic Element (KE8E) - hardware MUL/DIV
set cpu noidl		; disable host-CPU idle detection; TSS/8 is multi-user/interactive
; and its idle loop pattern isn't one SIMH auto-detects well

; TSC (TSC8-75) provides timeshare control for DEC's ETOS operating
; system - unrelated to TSS/8, which does its own multiplexing via
; the KM8E memory-extension hardware. Leave disabled for TSS/8.
set TSC disabled

; The FPP8A was a real add-on hardware coprocessor for floating-point math — it existed
; on some physical ; PDP-8/E systems, but it's a pure performance accessory. TSS/8 itself
; has no dependency on it; the OS boots and runs its monitor entirely without it. Whether
; you'd want it depends only on what user programs you plan to run under TSS/8:
;
;  BASIC-8, FOCAL    — do their own software floating point, don't call the FPP hardware at all.
;  FORTRAN D / ALGOL — these are the languages that could, in principle, use FPP-accelerated
;                      math on real hardware.
set FPP disabled

; --- Console teletype (TTI/TTO) ---
; KSR, uppercase-only, mark parity is the simulator default for TTI/TTO;
; set tti 7b            ; would allow lowercase/full 7-bit ASCII through

; --- Extra terminals (TTIX/TTOX) — the timesharing lines ---
; 4 lines, attached as a Telnet listener on port 4000 so additional users
; can log in as separate TSS/8 jobs
;attach ttix 4000

; 4 real RS232 terminals via USB-serial adapter, 1200 baud 7E1
; lines 0 and 1 have modem protocols enabled, lines 3 and 4 don't 
set ttix lines=4
attach -V ttix Line=0,modem,connect=/dev/pdp8-term1;1200-7e1
attach -V ttix Line=1,modem,connect=/dev/pdp8-term2;1200-7e1
attach -V ttix Line=2,nomodem,connect=/dev/pdp8-term3;1200-7e1
attach -V ttix Line=3,nomodem,connect=/dev/pdp8-term4;1200-7e1


; --- RF08 fixed-head disk — TSS/8's system/swap disk ---
set rf enabled
attach rf tss8_rf.dsk

; --- add a tape for use as a backup
attach ptp backup.pt

; --- add DEC tapes
attach dt0 tape0.tap
set dt0 writeenabled

attach dt1 tape1.tap
set dt1 writeenabled

attach dt2 tape2.tap
set dt2 writeenabled

attach dt3 tape3.tap
set dt3 writeenabled

; console detached from tty - admin-only telnet, separate from user lines
;set console telnet=6000
;set console telnet=buffered
; set console speed 1200

; serial console
set console serial=/dev/ttyS0;1200-7e1

; The set remote command opens a second listener purely for feeding SCP
; commands (like exit) to the simulator over the network, independent of
; the emulated machine's console. This is used by the Systemd service
; to cleanly shut down everything.
set remote telnet=6001

; --- Devices present but unused for this TSS/8 session ---
set df disabled        ; DF32 disk - not used by this TSS/8 image
set rl disabled        ; RL8A - not fitted in this configuration
; PTR/PTP/LPT/RK/RX/DT/MT/CT/DPY are left at their defaults (enabled,
; not attached) - TSS/8 doesn't need them for this boot, but they're
; available if you later want to attach paper tape, an RK05 pack, etc.

; automate the unattended boot sequence
expect "ETC?" send "START\r"; continue
expect "MONTH-DAY-YEAR" send "01-01-85\r"; continue
expect "HR:MIN" send "00:00\r"; continue

; --- Load the bootstrap and start ---
load tss8_init.bin
run 24200

Configuring Serial Ports

In the example described here, four serial ports have been added to the server by a four port serial port device made by Brainboses. And can be seen in the output of the lsusb command.

# lsusb

Bus 001 Device 004: ID 05d1:7031 Brainboxes, Ltd USB2Serial 4xRS232
Bus 001 Device 005: ID 05d1:7032 Brainboxes, Ltd USB2Serial 4xRS232
Bus 001 Device 006: ID 05d1:7033 Brainboxes, Ltd USB2Serial 4xRS232
Bus 001 Device 007: ID 05d1:7034 Brainboxes, Ltd USB2Serial 4xRS232

To ensure they all bound correctly to the appropriate driver (ftdi_sio in this case), and came up in a defined order, a udev rule was created.

This was placed un a file called /etc/udev/rules.d/99-brainboxes.rules.

# Brainboxes USB2Serial 4xRS232 — bind ftdi_sio and create stable symlinks
ACTION=="add", SUBSYSTEM=="usb", ATTRS{idVendor}=="05d1", ATTRS{idProduct}=="7031", RUN+="/sbin/modprobe ftdi_sio" RUN+="/bin/sh -c 'echo 05d1 7031 > /sys/bus/usb-serial/drivers/ftdi_sio/new_id'"
ACTION=="add", SUBSYSTEM=="usb", ATTRS{idVendor}=="05d1", ATTRS{idProduct}=="7032", RUN+="/sbin/modprobe ftdi_sio" RUN+="/bin/sh -c 'echo 05d1 7032 > /sys/bus/usb-serial/drivers/ftdi_sio/new_id'"
ACTION=="add", SUBSYSTEM=="usb", ATTRS{idVendor}=="05d1", ATTRS{idProduct}=="7033", RUN+="/sbin/modprobe ftdi_sio" RUN+="/bin/sh -c 'echo 05d1 7033 > /sys/bus/usb-serial/drivers/ftdi_sio/new_id'"
ACTION=="add", SUBSYSTEM=="usb", ATTRS{idVendor}=="05d1", ATTRS{idProduct}=="7034", RUN+="/sbin/modprobe ftdi_sio" RUN+="/bin/sh -c 'echo 05d1 7034 > /sys/bus/usb-serial/drivers/ftdi_sio/new_id'"

# Stable symlinks keyed to each port's unique serial number
SUBSYSTEM=="tty", ATTRS{idVendor}=="05d1", ATTRS{serial}=="R5CL0R2E", SYMLINK+="pdp8-term1", TAG+="systemd", ENV{SYSTEMD_ALIAS}="/dev/pdp8-term1"
SUBSYSTEM=="tty", ATTRS{idVendor}=="05d1", ATTRS{serial}=="R5UCARK7", SYMLINK+="pdp8-term2", TAG+="systemd", ENV{SYSTEMD_ALIAS}="/dev/pdp8-term2"
SUBSYSTEM=="tty", ATTRS{idVendor}=="05d1", ATTRS{serial}=="R5UCARK8", SYMLINK+="pdp8-term3", TAG+="systemd", ENV{SYSTEMD_ALIAS}="/dev/pdp8-term3"
SUBSYSTEM=="tty", ATTRS{idVendor}=="05d1", ATTRS{serial}=="R5CL0R2G", SYMLINK+="pdp8-term4", TAG+="systemd", ENV{SYSTEMD_ALIAS}="/dev/pdp8-term4"

This can be applied with the following.

sudo udevadm control --reload-rules
sudo udevadm trigger

The serial ports now turned up as

root@s1:/etc/udev/rules.d# ls -la /dev/pdp8-term*

lrwxrwxrwx 1 root root 7 Sep  9 19:26 help -> ttyUSB0
lrwxrwxrwx 1 root root 7 Sep  9 19:26 /dev/pdp8-term2 -> ttyUSB1
lrwxrwxrwx 1 root root 7 Sep  9 19:26 /dev/pdp8-term3 -> ttyUSB2
lrwxrwxrwx 1 root root 7 Sep  9 19:26 /dev/pdp8-term4 -> ttyUSB3

As a sanity check reboot the machine and then do a check e.g. Not that in my case the ttyUSB names have been assigned in a different order but SIMH TTIX has attached its connections to the correct ports.

root@s1:/home/john# ls -la /dev/pdp8-term*

lrwxrwxrwx 1 root root 7 Sep 10 13:43 /dev/pdp8-term1 -> ttyUSB0
lrwxrwxrwx 1 root root 7 Sep 10 13:43 /dev/pdp8-term2 -> ttyUSB3
lrwxrwxrwx 1 root root 7 Sep 10 13:43 /dev/pdp8-term3 -> ttyUSB1
lrwxrwxrwx 1 root root 7 Sep 10 13:43 /dev/pdp8-term4 -> ttyUSB2

journalctl -u pdp8.service --since "13:43" | grep -i ttix

Sep 10 13:44:02 s1.glasstty.local pdp8[977]: ./tss8.ini-38> attach ttix Line=0,/dev/pdp8-term1;1200-7e1
Sep 10 13:44:02 s1.glasstty.local pdp8[977]: ./tss8.ini-39> attach ttix Line=1,/dev/pdp8-term2;1200-7e1
Sep 10 13:44:02 s1.glasstty.local pdp8[977]: ./tss8.ini-40> attach ttix Line=2,/dev/pdp8-term3;1200-7e1
Sep 10 13:44:02 s1.glasstty.local pdp8[977]: ./tss8.ini-41> attach ttix Line=3,/dev/pdp8-term4;1200-7e1

Running as a service

Create a pdp8 user and add to the dialout group.

# useradd --system --no-create-home --shell /sbin/nologin --comment "PDP-8 TSS/8 simulator" pdp8
# usermod -aG dialout pdp8

Create a unit file for the service as /etc/systemd/system/pdp8.service.

[Unit]
Description=PDP-8 TSS/8 timesharing simulator
After=network.target
After=dev-pdp8\x2dterm1.device dev-pdp8\x2dterm2.device dev-pdp8\x2dterm3.device dev-pdp8\x2dterm4.device
Wants=dev-pdp8\x2dterm1.device dev-pdp8\x2dterm2.device dev-pdp8\x2dterm3.device dev-pdp8\x2dterm4.device

[Service]
Type=simple
User=pdp8
Group=pdp8
WorkingDirectory=/opt/pdp8
ExecStart=/opt/pdp8/bin/pdp8 tss8.ini
ExecStop=/bin/sh -c 'printf "exit\r\n" | timeout 5 /usr/bin/nc localhost 6001'
Restart=on-failure
RestartSec=5
TimeoutStopSec=10
SupplementaryGroups=dialout

[Install]
WantedBy=multi-user.target

Start the service.

# systemctl start pdp8.service

Check its status

# systemctl status pdp8.service

If all looks OK and you can connect to the remte console (see below), then enable it so that it starts automatically on a system boot.

# systemctl enable pdp8.service

Connect to the Remote Console

From a local machine connect using TELNET e.g

$ telnet <server name/ip addreaa> 6001

You should see the following response.

Connected to the PDP-8 simulator REM-CON device

PDP-8 Remote Console
Enter single commands or to enter multiple command mode enter the ^E character
Simulator Running...

Entering the SHOW SERIAL command should return the following. Note that the SIMH> prompt only appears when the command is typed in. DO NOT use Ctrl-E to get to the prompt.

sim> show serial
Serial devices:
 ser0	/dev/pdp8-term1 (1200-7e1)
 ser1	/dev/pdp8-term2 (1200-7e1)
 ser2	/dev/pdp8-term3 (1200-7e1)
 ser3	/dev/pdp8-term4 (1200-7e1)
 ser4	/dev/ttyS0     
 ser5	/dev/ttyS4     
 ser6	/dev/ttyUSB0   
 ser7	/dev/ttyUSB1   
 ser8	/dev/ttyUSB2   
 ser9	/dev/ttyUSB3   
Open Serial Devices:
 TTIX	Ln00 /dev/pdp8-term1;1200-7e1 (1200-7e1)	Config: 1200-7e1
 TTIX	Ln-1 /dev/pdp8-term2;1200-7e1 (1200-7e1)	Config: 1200-7e1
 TTIX	Ln-2 /dev/pdp8-term3;1200-7e1 (1200-7e1)	Config: 1200-7e1
 TTIX	Ln-3 /dev/pdp8-term4;1200-7e1 (1200-7e1)	Config: 1200-7e1
 CON-TELNET	Ln00 /dev/ttyS0;1200-7e1	Config: 1200-7e1

Transferring Text Files to the Machine

Create a Paper Tape File

A paper tape file can be created using the txt2ptp utility e.g.

txt2ptp <lander.bas > lander.ptp

Attack the new file to the high speed reader e.g. (from SIMH> prompt or pdp8.ini file)

ATTACH lander.ptp PTR

Cut and Paste

Use character delay an line delay of around 30ms and 200ms respectively when sending text files from a terminal emulation program.

Transferring Directly

Alternatively text can be be send directctly using tio e.g.

cat /home/john/Downloads/LunarLander.txt | tio /dev/ttyS0 --baudrate 1200 --databits 7 --parity even --output-delay 30 --output-line-delay 200

The tio package can also be used as a serial terminal e.g.

tio /dev/ttyS0 --baudrate 1200 --databits 7 --parity even --output-delay 30 --output-line-delay 200

`Telstar Status`

Line 1: (01756 664433)
Status: OK

Line 2: (01756 664434)
Status: OK

TCP 1: (glasstty.com:6502)
Status: OK

TCP 2: (glasstty.com:6503)
Status: OK

`What's in a Name`

Telstar is named after a BBS with the same name that operated during the 1980s. That early BBS was created the author and a friend called Terry Slater. Telstar is an anagram of T Slater.

Prestel Servers were typically named after authors that originated from the area the server was located. The Telstar server names, Currer, Ellis and Acton are of course the pseudonyms used by the Bronte sisters, who like the author of Telstar, originated from Yorkshire. These have been selected as the names of the three Telstar servers.

`GlassTTY`

In the 1970s, character mode terminals e.g., ADM3A, VT100 etc. were considered preferable to the large cumbersome teleprinters. These new all electronic devices with their glass screens were often referred to as "Glass TTYs".