By JPK Systems Limited

E-mail: info@jpk.co.nz

1. Introduction

The DMX transceiver by JPK Systems Limited provides a simple and effective means of implementing DMX512 based lighting control systems, be it laser display beam table control or just plain old theatrical lighting. Through the use of an optically isolated RS-232 serial port it simplifies connection to a host computer and no longer ties you into using an IBM PC or compatible. Other DMX512 transceivers need a spare ISA slot or a parallel printer port, on a notebook computer there simply are no slots and people keep telling horror stories about how the printer port on an expensive laptop was destroyed by accident.
 
 

Through the use of multiple serial ports several DMX transceivers may be connected simultaneously to the same host computer without having to become concerned with timing at all. Each transceiver has three microcontrollers, one for DMX transmit, one for DMX receive, and one for host communication, all three being linked together completely asynchronously by dual port memories. Only 256 of the possible 512 channels have been implemented, which is sufficient in most applications, and the transmitter may be switched to 128 channel output if faster refresh rates are required.
 
 

Receive only and transmit only versions are also available, although all versions have bi-directional communication with the host computer. Figure 1 shows the DMX transceiver viewed from the front.



Figure 1. The DMX Transceiver

2. Connection to host

The DMX transceiver is wired as DCE (Data Communications Equipment) and hence is connected to the host computer RS232 port (DTE, Data Terminal Equipment) with a 1:1 cable, i.e. no cross-over. A total of 7 wires are necessary for successful operation, they are detailed in table 1. Note that there is no problem using a cable that has all 9 connections present. Gender of plug for DMX transceiver is male, host computer is female.
 
 

Host Computer RS232 Port (DTE)		DMX Transceiver RS232 Port (DCE)
RS232 Name	Pin number (25 way)	Pin number (DMX box)	RS232 Name
RD	2 (3)	2	TD
TD	3 (2)	3	RD
RTS	7 (4)	7	CTS
CTS	8 (5)	8	RTS
DSR	6 (6)	6	DTR
DTR	4 (20)	4	DSR
GND	5 (7)	5	GND

Table 1. RS232 connection to DMX transceiver





Settings for the host computer RS232 port are in table 2, note that although RTS/CTS and DTR/DSR handshaking appear to be implemented they are merely linked in the DMX transceiver. This is to ensure that RTS and DTR are high so that power can be derived from them for the isolated RS232 interface built into the DMX transceiver.
 
 

Host computer RS232 port settings
Baud rate	115200
Data bits	8
Parity	None
Stop bits	1
RTS/CTS handshake	ON
DTR/DSR handshake	ON
DCD	OFF
XON/XOFF	OFF

Table 2. RS232 port settings on host computer

Two basic protocols have been implemented in the DMX transceiver, one of them is ASCII character based and the other is a block transfer. The ASCII protocol allows operation from a terminal package, no need to have special software completed, and allows useful results to be obtained straight out of the box. When using a terminal package to communicate with the DMX transceiver make sure the COM port settings are correct and the ASCII setup is for local echo and append line ends to line feeds (i.e. sends CR+LF when ENTER is pressed).
 
 

All ASCII commands start with the character 'C', followed by a three ASCII numbers that describe the channel number of interest unless a special function command is to be accessed. The DMX512 specification describes channels numbers as 1 to 512, the DMX transceiver accepts channel numbers from 000 to 255 which correspond to channels 1 to 256. If an 'L' is send after the channel number then the three ASCII numbers that follow describe the level to set the channel, range is 000 to 255. If a question mark '?' is sent after the channel number then the DMX transceiver will return the value of that channel as 'Lxxx' where xxx is the level (000 to 255).
 
 

ASCII Command Examples:

• Send C000L255, the DMX transceiver will set channel 1 to level 255 and also send the ASCII character 'G' back to the host to indicate that the data was received and acted upon. Note that it is not necessary to press ENTER, this may be confusing when operating from a terminal but will save two extra characters being sent from an application program.
• Send C109L019, this will set channel 110 to level 19 with the 'G' character also being returned.
• Send C000?, the DMX transceiver will return Lxxx, where xxx is the last valid level of channel 1 received by the DMX receiver section. If that level was 128 then the command would return L128. Note that there is no CR+LF sent after the level number, this may prove confusing when operating from a terminal but once again saves 2 characters (saves time).

ASCII Special Commands:

• Sending C? will return 'G' if the DMX transceiver is operating normally, and can serve as a test to check if it is connected and powered.
• Sending CS will return 'G' and set the DMX transmitter to 128 channel output.
• Sending CB will return 'G' and set the DMX transmitter to the default 256 channel output.

If your application requires updating of several consecutive channels then the block transfer protocol should be used as it is far quicker. Instead of sending each channel and level in ASCII characters the protocol reduces this to one byte per channel plus a start header that describes which channel to start from and how many channels will follow. A checksum is used to provide a simple means of detecting transmission errors.
 
 

Block transfer to the DMX transmitter:

Send the following to the DMX transceiver.

0xFF (Block transfer header)

0xFF (Block transfer header)

0xC0 (BOF)

start channel low byte (0x00 to 0xFF corresponds to DMX channels 1 to 256)

start channel high byte (0x00 for this DMX transceiver as only 256 channels are implemented)

number of channels - 1 (0x00 to 0xFF, start channel + (number of channels - 1) <= 0xFF)

first channel data byte (0x00 to 0xFF corresponds to level 0 to 255)

..

..

last channel data byte

checksum byte (add all channel data bytes, take low byte of answer)

0xC1 (EOF)

Response from the DMX transceiver is 'G' if the block was received without error.
 
 

Block transfer request from the DMX receiver:

Send the following to the DMX transceiver.

0xFF

0x00

0xC0 (BOF)

start channel low byte (0x00 to 0xFF corresponds to DMX channels 1 to 256)

start channel high byte (0x00 for this DMX tx/rx, 256 channels implemented)

number of channels - 1

0xC1 (EOF)
 
 

Response from the DMX transceiver will be:

0xFF

0xFF

0xC0 (BOF)

first channel data byte (start channel requested)

..

..

last channel data byte (start channel + number of channels requested)

checksum byte

0xC1 (EOF)
 
 

Note that the start channel + number of channels can never exceed 0xFF as this is outside the range of possible channels. If it is exceeded the DMX transceiver will ignore the command. Likewise any ASCII commands that specify channel numbers or addresses outside the range 000 to 255 will be ignored. Standard C notation for hex numbers has been used, 0xFF means FF Hex which is 255 decimal.
 
 

Examples of block transfers:

• To set DMX transmit channels 10,11,12,13,14,15 to levels 100,120,140,150,255,10 respectively the following would be sent to the DMX transceiver. Note the channel numbers all have 1 subtracted from them to make the first channel equal to 0 instead of 1, hence channel 10 the start channel becomes channel 9 when executing the block transfer.

FF FF C0 09 00 05 64 78 8C 96 FF 0A 07 C1 (Command sent)
 
 

Response to this command will be an ASCII 'G' to signify that it was received okay and the channels are now set to the stated levels. Note that the channel levels have been converted into Hex numbers.
 
 

• To read what the latest received levels for DMX channels 129,130,131,132 are the following command is sent to the transceiver and a response it returned. In this example assume that the levels for these channels are 200,200,0,255 respectively.

FF 00 C0 80 00 03 C1 (Command sent to DMX transceiver)
 
 

FF FF C0 C8 C8 00 FF 8F C1 (Response from DMX transceiver)
 
 

It is not possible to keep up with the 250000bps data rate of DMX512 transmissions using an interface that operates at only 115200bps. Because it is unlikely all channels will need to be altered at the same time this slower RS232 interface can be used without any adverse effects, although some care must be taken when writing software to ensure the desired update rate is achieved. Due to the completely asynchronous nature of the DMX transceiver a new channel level may not actually occur until up to one complete DMX512 transmission cycle, which is 5.78ms for 128 channel output and 11.41ms for 256 channels. These update rates are sufficient for all DMX applications including moving lights and laser display beam control systems.
 
 

DMX512 output continues regardless of what the host computer interface is doing, there are no breaks in transmission between channels. If the unit becomes unplugged from the host at some stage the last channel levels will continue to be output by the independent DMX512 transmitter as it stores the last channel levels internally. Likewise DMX512 reception will continue regardless of the host computer interface, all channel levels are stored and are updated only upon reception of a new DMX512 data stream. Unplugging the DMX receive cable will result in no further reception but the last good data will still be held internally and returned upon request from the host computer.
 
 

Care must be taken when writing any software for the DMX transceiver, keep in mind that there are many different fault conditions that may exist such as RS232 cables being unplugged during data transmission or reception. It is recommended that regular checks be made to ensure the unit is still present and operating, sending the 'C?' command should result in a 'G' being returned. If it isn't then clear all serial port buffers and send 264 NULL (0x00) characters to the serial port, this will terminate any command that may have been corrupted due to a cable being momentarily unplugged or some other fault. Then try the 'C?' command again, if there is no response then the unit is not present or not powered. Be sure to check the 'G' characters being returned after some commands, this indicates a good data transfer.
 
 

Commands send to the DMX transceiver have no special timing requirements, characters may be sent with no delay between them. Once a DMX transmit command has been sent the next one may be sent immediately following the previous one, although it is recommended that the 'G' character be received to ensure the previous command was received okay. This will introduce a 1 character delay as it will be sent immediately upon the DMX transceiver receiving the last character in the command from the host computer. After sending a DMX receive request the channel level(s) will be returned, during this period no further commands will be accepted until all bytes have been sent to the host computer.

4. Specifications

Table 3 below contains the specifications for the DMX transceiver.

Table 3. Specifications for DMX transceiver