Port switches, buffers and converters

You have one computer and a few printers. Or one printer and a few computers. A very frequent problem, especially in 1990s offices without flexible computer networking (or "network" based on floppy disks). What to do? Use printer switch! A small box switching one port between few sockets. With such boxes many computers can be connected to one printer or many printers to one printer port. First, they were purely mechanical, but later became electronic and sometimes had additional buffers or converters.

Mechanical switches  
Manufactured by: Various

Type: Port switches
Sockets: DB25 parallel port
These switches were among the cheapest ones, being the most simple parallel port switches. It consists of casing, connectors, rotary switch and lots of wiring inside. Nothing more. This simplicity gave them flexibility, so it was possible to use it not only for printers, only proper cables were needed. Even 5-position switches have been made. Unfortunately, being mechanical constructions with moving parts, they failed quickly.
If your mechanical switch fails, use ohmmeter on "buzzer" function to locate failing pin in connectors. Then open the case and check its wires with ohmmeter. It may have broken off as all connections between sockets and switch are soldered. If it is only solder problem, it's easy to fix. The problem is if you trace to the missing contact in switch.
Then you have to use a special spray for cleaning contacts, it's usually isopropyl alcohol. Just spray it in contact's PCB, make sure it goes inside switch. Turn it back and forth. This should clean contacts. If after a few repetitions there is still no contact, the last resort is to re-solder wires and use another unused pin, e.g. from some redundant ground pin. After all modifications and IPA cleaning check other pins with ohmmeter as dirt removed by a flush may block another contact!

Switches in my main computer. The left one controls parallel port between printer, old computer and Willem EPROM programmer (the free one is used for bsd programmer to AVR chips). The right one controls serial port between IC tester, 8-bit CP/M machine and 2 free connections. The second picture shows how DB9-to-DB25 serial converters are used in old parallel port switch to make it switch serial port. You can use commercial-grade changers or just copper rods of specific thickness - remember to secure them properly as they may like to fall inside holes.

Lindy digital switch  
Manufactured by: Lindy?

Type: Parallel port switch
Sockets: DB25 paralle port (Centronics)
Information: A more advanced parallel port switch, based on digital circuits. It has a simple monitoring circuit which constantly scans all ports and, if data is coming on one, stops on it. This circuit can be deactivated and switching is then done manually.
Inside it has circuit based on TTL 74-series chips. Although 3 chips have their markings scratched off, one is definitely 74123, while other two look like similar 74s, not microcontrollers or memory. Contrary to their mechanical equivalents, these switches rarely failed. Powered with 9V DC.

Serial-Parallel converter  
Manufactured by: ??

Type: Port converter
Sockets: DB25 (serial port), Centronics (parallel)
This small adapter, powered by 9V DC, allows to connect serial printer to parallel port or vice versa. Its buffer allows to cope with bandwidth problems for a moment. I don't have more information about its operation. Inside, it's based on Intel 8031 microcontroller with external EPROM and 64kB of RAM for buffering incoming data. These devices were rarely used and mostly in industrial systems, where it was needed to replace old serial printer with parallel one.

ALMA Printer buffer  
Manufactured by: ALMA Gdansk

Type: Printer buffer/switch
Sockets: DB25 (Computer), 2x Centronics (Printer)
This is something unusual. A printer buffer is a tool to temporarily keep printed content for a time the other computer is printing. This one was buiilt ca. 1993 by Polish company ALMA from Gdansk. Today there is no sign of this company and many small companies are located in building (address in string in ROM).
The device allows to switch between two printers, keep documents for later printing and print contents of buffer in specific number of copies, maximum 15. To print copies, just click "SELECT", tle COPY LED will blink slowly. Then press PLUS to make it blink fast once. Add amount of copies by pressing PLUS button, it will blink fast 2, 3 or more times with pauses. Confirm by pressing SELECT and now you can print to buffer and START to print.
The case is very well made. Markings are also professional. Unfortunately when you peek inside it becomes a less professional.
Inside, there is a complete Z80 system with 16kB of ROM (dump). Like a Spectrum. Memory is aligned in 64K banks. More, although the quality of PCB is quite normal as for Polish technology, the mounting technique is below all standards. Lots of alloy, rosin left around pins and in one case broken pin soldered "until it sticks" reveals that it was mounted totally by hand. Supplying power, give it a good 200mA 9V supply.

Manufactured by: Contec

Type: Data logger
Sockets: DB9 Serial port in/out,
This is for general modem connectivity, not for printers but it fell to the category as it's a buffer. Although this device can buffer serial port's data, it's primary role is to perform logging of critical data from networks and keep it secured in battery-backed (or later Flash) memory for diagnostic and backup purposes. Typical usage: Imagine that telephone patch's computer fails and information about connections are not written to it. This buffer gives additional chance of recovery. Data is written in character or binary strings, stored in FIFO way. This device is still manufactured (only with LAN interface and larger memory) and used in wide range of applications.
My unit was used in industrial system working as an equivalent of airplane's "black box" - recording reasons why the machine stopped.
Inside, it's driven by Intel microcontroller + Lattice PLD. Serial port lines states can be seen in LEDs.
Description in Polish
http://contec.com.pl/download/bu-mikro-i-mini-instrukcja-wydanie-1/ - Manual