ModBus: Communication Control Blocks

ModBus is selected and configured via a suitable Communications Control Block (CCB). The following table summarizes the various parameters in the CCBs for ModBus. Entries are hyper-linked to detailed descriptions. If you are using a ModBus protocol you must set up a CCB with the specified parameters and then invoke it in your program with a COMSetCCB instruction.

The table contains a spare column so you can print it out and use it as a work sheet. Click on each entry to get a detailed expose.


Byte	Slave RTU	Master RTU	Notes
0	1	2
1	DevAddr	0
2	CharProfile	CharProfile
3	BaudRate	BaudRate
4	Not used - set to 255
5	Not used - set to 255
6	Not used - set to 255
7	Not used - set to 255
8	Not used - set to 255
9	MessageGap	MessageGap
10	0	QryTimeout
11	0	QryAttempts
12	Not used - set to 255
13	Not used - set to 255
14	Not used - set to 255
15	Not used - set to 255
16	Not used - set to 255
17	Not used - set to 255
18	Not used - set to 255
19	Not used - set to 255

Examples

The following table is a copy of the one above (with the master column removed) used as a worksheet for a slave.

Byte	Slave RTU	Example value
0	1	1
1	DevAddr	3
2	CharProfile	0 (8,1,N)
3	BaudRate	5 (9600)
4		Not used - set to 255
5		Not used - set to 255
6		Not used - set to 255
7		Not used - set to 255
8		Not used - set to 255
9	MessageGap	10 (x10mS = 100mS)
10	QryTimeout	0 (Not used in slave)
11	QryAttempts	0 (Not used in slave)
12		Not used - set to 255
13		Not used - set to 255
14		Not used - set to 255
15		Not used - set to 255
16		Not used - set to 255
17		Not used - set to 255
18		Not used - set to 255
19		Not used - set to 255

In your SPLat program this configuration would look like this:

                NVEM0           ;Start of NVEM data

ModBusCCB:

                NV0Byte         1       ;Protocol

                NV0Byte         3       ;Address

                NV0Byte         0       ;Character = 8,1,N

                NV0Byte         5       ;9600

                NV0Byte         255     ;Not used - set to 255

                NV0Byte         255     ;Not used - set to 255

                NV0Byte         255     ;Not used - set to 255

                NV0Byte         255     ;Not used - set to 255

                NV0Byte         255     ;Not used - set to 255

                NV0Byte         10      ;Message gap 100mS

                NV0Byte         100     ;Query timeout 1S

                NV0Byte         3       ;Query attempts

                NV0Byte         255,255,255,255,255,255,255,255 ;Not used - set to 255