RS232 and RS422

The confusing world of RS422 and RS232

How do Lumberjack and Takelog talk to my recording machine(s)?

Most computers are fitted with one 9-pin RS232 serial port. However, the standard Sony P2 machine control format used by Lumberjack and Takelog requires an RS422 port. The names may sound similar, but they are different! The main difference is that RS232 is an unbalanced system, with two wires for the data (one to send, and one to return), whereas RS422 is balanced, with four wires (a 'positive' and 'negative' send, and a 'positive' and 'negative' return).

Lumberjack has the added benefit/problem of being able independently to control up to 4 machines - which requires 4 serial ports.

There are two main ways of dealing with these problems:

Fit one or more RS422 ports to your system

This is the best approach. RS422 ports are available in numerous different guises:

1-port to 4-port PCI cards for desktop computers

1-port to 2-port PCMCIA cards for laptops

1-port to 4-port USB boxes for any computers

If your computer has the capability, I would recommend a USB solution. USB boxes can be "hot-plugged", are easy to install, and very convenient.

Please note, for all these solutions, that the computer industry standard wiring convention for 9-pin RS422 ports is not the same as the broadcast industry standard. Some manufacturers (unfortunately only a very few) provide RS422 cards/boxes with the broadcast standard wiring. See below for the broadcast pinout. If you use a non broadcast industry standard device, you will need a custom adaptor cable.

The cheapest PCI solution I have found so far is from Data Solutions, who can supply the excellent DC-0032 4-port RS422 PCI card for £91.

The cheapest USB solution I have found so far is from Inside Out Networks. Their reseller (B&B Electronics) can supply the EDGEPORT/422 4-port RS422 USB module for €299.

Both of these products are strongly recommended. Custom cable wiring for each of these solutions is shown here.

Use an external RS232 to RS422 converter

This will let you use the existing RS232 port on your computer. This is only really suitable for Takelog users who won't be upgrading to Lumberjack, and only need to control one machine. Very few computers have more than 2 serial ports, and so Lumberjack users almost certainly need to fit extra ports - and if you're fitting extra ports, why fit RS232 and convert them to RS422 when you could fit RS422 in the first place?

There are a frightening number of options when choosing such converters - the world of RS232 is a nightmare - but the following pointers may help.

It is only necessary to send the data lines only - no handshaking lines are needed.

The RS232 side of the converter will normally be configured as a DCE (Data Communications Equipment) rather than a DTE (Data Terminal Equipment).

The maximum required speed is 38.4kbps.

Asynchronous, four wire, full duplex, data only operation is required.

An alternative is to build a converter yourself. Click here to see the circuit diagram of a simple converter that has been used successfully at Abbey Road Studios for many years. Note that the RS232 handshake lines are looped straight back (DTR to DSR, and RTS to CTS).

Important Note about Line Termination

Some recording machines, such as the Sony 7030 range of DAT machines, have a 100R termination resistor fitted in their 9-pin input (i.e. where the data comes in from the computer). This is a fairly low value, which results in a reasonably high current flowing through the line.

The net result of all this is that the power supply of the RS422 driver needs to be capable of supplying this current. Many of the commercially available RS232 to RS422 converters are powered from the RS232 port on the computer, which generally do not have the guts to cope with powering a converter and a high current line.

In my experience, if you are driving a machine fitted with a 100R termination resistor using an RS232 to RS422 converter, then the converter will always need an external power supply, rather than being powered from the computer's RS232 port.

Most of the Sony video machines do not have terminating resistors, and these can be driven using "port-powered" RS232 to RS422 converters. The Tascam DA-88 tape machine also does not have terminating resistor. Fostex DAT machines have a switchable terminating resistor - I suggest leaving it OFF.

Power supply considerations for the RS422 driver circuitry are especially important if you opt to drive more than one machine from a single port (see below).

Connection Details

The following tables shows the pin connections for Sony P2 compatible tape machines, and standard IBM-PC 9-pin RS232 serial ports:

Sony P2 RS-422 Pin Connections

DB9 Female Socket

2 - Tape Machine Transmit +ve

3 - Tape Machine Receive -ve

4 - Transmit and Receive Common

6 - Transmit and Receive Common

7 - Tape Machine Transmit -ve

8 - Tape Machine Receive +ve

IBM-PC RS-232 Pin Connections

DB9 Male Plug

2 - Data Out to computer

3 - Data In from computer

4 - Data Terminal Ready DTR (from computer)

5 - Signal Ground

6 - Data Set Ready DSR (to computer)

7 - Request To Send RTS (from computer)

8 - Clear To Send CTS (to computer)

Running more than one machine from a single RS422 port

It is possible to control more than one machine from a single port. The general principle is to send commands from the computer to multiple machines, but only let the computer receive return data from one of the machines (the "master"). Thus the displays in Takelog and Lumberjack (e.g. machine position and transport status) will only show information for the "master" machine. But if the software sends a command (e.g. "Play"), all listening machines will respond.

You will need to make up a custom cable, where pins 2 and 7 of the RS422 line (the recording machine's Transmit lines) are only connected for the designated "Master" machine.

If the machines used do not have terminating resistors on their incoming lines (pins 3 and 8 - the Receive lines) there is no practical limit on how many machines can be controlled. However, if the machines do have 100R terminating resistors, then please be very careful. Connecting 4 machines together like this will result in the computer's RS422 driver trying to drive a 25R load - almost a short circuit. With some cards/computers/converters, this can destroy the RS422 driver circuitry. You have been warned! The do-it-yourself circuit shown here has been used to drive 4 terminated devices in parallel for many years without failure.

Companies manufacturing and selling RS232 and RS422 products

Company

Notes

Data Solutions

Very cheap and very good ISA and PCI RS422 cards

The DC-0032 4-port RS422 PCI card (£91) is highly recommended (non broadcast standard pinout)

Blastronix

ISA and PCI 1- to 4-port RS422 Cards (including broadcast standard pinout models)

B&B Electronics Ltd

USB and PCI cards

The EDGEPORT/422 4-port RS422 USB module is €399

Quatech

USB, PCI and PCMCIA cards

	DSU-200/300 2-port RS422 USB module
	QSU-200/300 4-port RS422 USB module
	QSP-200/300 4-port RS422 PCMCIA card

Silicon Valley Bus Co.

Multiport USB 4-port RS422 module

Digi International

AccelePort USB 4 EIA-422 4-port RS422 module

KK Systems

Broadcast standard Sony P2 DB9(RS232) to DB9 (RS422) converters for just £39. These will almost certainly need an external power supply to drive a 110R terminated machine.

Antona Corporation

Broadcast standard Sony P2 DB9(RS232) to DB9 (RS422) converters for just $49. These will almost certainly need an external power supply to drive a 110R terminated machine.

Imagine Products Inc.

Broadcast standard Sony P2 DB9(RS232) to DB9 (RS422) converters for just $20. These will almost certainly need an external power supply to drive a 110R terminated machine.