Tony Withers: Lab: Data Acquisition

RS232/RS485 wiring

Introduction

The purpose of this page is to store some information about how the computers in the lab interface with experimental hardware for control and data acquisition. It also contains links to some useful information about how serial communications work, which will aid in setting up future configurations and understanding the existing set-up.

The 'front end' that we use use for data aquisition in the lab is described here.

For some general information about how serial communications work, check out the Wikipedia entries for RS-232, RS-485 and RS-422 protocols.

Here's a useful table of decimal and hexadecimal codes for ASCII characters:

Dec	Hex	ASCII	Dec	Hex	ASCII	Dec	Hex	ASCII	Dec	Hex	ASCII
0	0	NUL	32	20	SP	64	40	@	96	60	`
1	1	SOH	33	21	!	65	41	A	97	61	a
2	2	STX	34	22	"	66	42	B	98	62	b
3	3	ETX	35	23	#	67	43	C	99	63	c
4	4	EOT	36	24	$	68	44	D	100	64	d
5	5	ENQ	37	25	%	69	45	E	101	65	e
6	6	ACK	38	26	&	70	46	F	102	66	f
7	7	BEL	39	27	'	71	47	G	103	67	g
8	8	BS	40	28	(	72	48	H	104	68	h
9	9	HT	41	29	)	73	49	I	105	69	i
10	A	LF	42	2A	*	74	4A	J	106	6A	j
11	B	VT	43	2B	+	75	4B	K	107	6B	k
12	C	FF	44	2C	,	76	4C	L	108	6C	l
13	D	CR	45	2D	-	77	4D	M	109	6D	m
14	E	SO	46	2E	.	78	4E	N	110	6E	n
15	F	SI	47	2F	/	79	4F	O	111	6F	o
16	10	DLE	48	30	0	80	50	P	112	70	p
17	11	DC1	49	31	1	81	51	Q	113	71	q
18	12	DC2	50	32	2	82	52	R	114	72	r
19	13	DC3	51	33	3	83	53	S	115	73	s
20	14	DC4	52	34	4	84	54	T	116	74	t
21	15	NAK	53	35	5	85	55	U	117	75	u
22	16	SYN	54	36	6	86	56	V	118	76	v
23	17	ETB	55	37	7	87	57	W	119	77	w
24	18	CAN	56	38	8	88	58	X	120	78	x
25	19	EM	57	39	9	89	59	Y	121	79	y
26	1A	SUB	58	3A	:	90	5A	Z	122	7A	z
27	1B	ESC	59	3B	;	91	5B	[	123	7B	{
28	1C	FS	60	3C	<	92	5C	\	124	7C	\|
29	1D	GS	61	3D	=	93	5D	]	125	7D	}
30	1E	RS	62	3E	>	94	5E	^	126	7E	~
31	1F	US	63	3F	?	95	5F	_	127	7F	DEL

Piston Cylinder A

THESE ARE JUST NOTES SO FAR; NEED TO FILL THIS IN WITH THE CORRECT INFORMATION

Control computer is a PC (Piston-A on the network). All comms are 4-wire RS-422 or 2 wire RS-485.

Serial Port A of the PC is connected to a duTec RS-232/485 serial converter using a null modem cable.

The RS485 network is connected to the controller, a Eurotherm 818, the process controller for the pressure control system, an Omega..., and a CR Magnetics voltage and current transducer that reads primary voltage and current. Current is stepped down via a current transformer.

PID settings: Pb = 112.0 °C; ti = 10 s; td = 0.5 s

Piston Cylinder B

Control computer is a PC (Piston-B on the network). Serial port is connected to a duTec RS232-RS485 serial converter with a null modem cable.

The RS485 network is connected to the controller (Eurotherm 818) and a CR Magnetics voltage and current transducer that reads primary voltage and current. Current is stepped down via a current transformer.

PID settings: Pb = 500.0 °C; ti = 4 s; td = 1.0 s

Future improvements: needs a pressure transducer and pressure controller.

Multi-Anvil

Computer: PC. Multi-anvil on the network.

Serial ports: 1 onboard plus 3 via RocketPort USB serial hub. Port #3 ?? of the serial hub doesn't work.

If the extra serial ports 'disappear' (check device manager) you may need to restart the computer or to disconnect and reconnect the usb cable.

Hardware:

Fluke multimeter (used to measure secondary voltage or with a bridging circuit to measure resistance for in situ indentification of electronic transitions)
Eurotherm 2404 controller, has ? COMMS card added (RS-232), settings are ?
Heise pressure transducer with built in RS-232 interface, connected to onboard serial port.
Home made RS-232 relay switching hardware as part of motor speed control circuit.
Voltage output module (DAC), measurement computing model CB-7024 connected to RS485 network.
CRmagnetics true RMS voltage/current transducer, 2 wire RS-485. measures primary current & voltage. Current is stepped down via current transducer (rating?).
Omega process indicator to measure anvil temperature via type K thermocouple.
PID settings for graphite furnace, 12mm TEL: Pb = 5000 °C; ti = 5 s; td = 1 s

How the relay controller works

The relay controller has serial controlled ?8 relays and 4 digital inputs. The inputs are used to sense the state of some of the alarm and limit switch relays. The 115V supply that powers the alarm lights is used to trip some isolated relays that switch a DC voltage (+12V from power supply in control panel) into the logic inputs of the relay controller.

Pressure control via computer

Pressure control is realised as an add on to the Igor Pro data logging procedure described here. The PT control procedure is called from the DL_UserBG function of the data logger within each acquisition cycle. A buffer of pressure recordings is maintained and used to calculate pressure ramp rate. Because resolution of the pressure transducer is limited to 1 psi, the algorithm uses linear regression to calculate ramp rate from a sufficient number of readings, according to the expected ramp rate.

Future improvements

needs an LVDT to measure displacement.

Links

Links to commercial sites do not imply endorsement of the vendors' products or services.

duTec (serial converters)

CR Magnetics (transducers and current transformers)

Eurotherm (how to connect COMMS cards to serial converters)

B & B Electronics

Comtrol (makers of the rocketport usb-serial hub)

RS-232 info

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The contents of this page have been neither reviewed nor approved by the University of Minnesota.