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UNIX for Dummies Questions & Answers
how to read and write device registers
Post 94818 by Perderabo on Wednesday 4th of January 2006 07:58:30 AM
01-04-2006
It's bad enough when someone asks a shell scripting question but can't be bothered to mention what computer, what os, what language, etc. But this really takes the cake. 
Some systems have a separate I/O address space with special instructions to access them. Those special instructions are usually encapsulated into routines to be called by the driver. I don't remember the names but readl and writel does not seem to right a bell. However, they might be right for your os. If your computer really maps the registers into memory and you know the address, you probably can do stuff like:
char status;
char *StatusReg = (char *) 0xFF123456;
status = *StatusReg;
I thought that this technique was passé, but maybe your computer uses it. If your device uses a collection of registers and you know the base address where one of them resides, the others are usually right next to them in ascending addresses. And this tends to be true whether the registers are mapped into the same address space as memory, or have an address space of their own. But beware, some registers can be read-only while other might be write-only. If a device has both read-only and write-only registers, it is not unusual for two registers to share the same address.
Some systems have a separate I/O address space with special instructions to access them. Those special instructions are usually encapsulated into routines to be called by the driver. I don't remember the names but readl and writel does not seem to right a bell. However, they might be right for your os. If your computer really maps the registers into memory and you know the address, you probably can do stuff like:
char status;
char *StatusReg = (char *) 0xFF123456;
status = *StatusReg;
I thought that this technique was passé, but maybe your computer uses it. If your device uses a collection of registers and you know the base address where one of them resides, the others are usually right next to them in ascending addresses. And this tends to be true whether the registers are mapped into the same address space as memory, or have an address space of their own. But beware, some registers can be read-only while other might be write-only. If a device has both read-only and write-only registers, it is not unusual for two registers to share the same address.
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LEARN ABOUT DEBIAN
modbus_write_and_read_registers
MODBUS_WRITE_AND_REA(3) Libmodbus Manual MODBUS_WRITE_AND_REA(3) NAME
modbus_write_and_read_registers - write and read many registers in a single transaction SYNOPSIS
int modbus_write_and_read_registers(modbus_t *ctx, int write_addr, int write_nb, const uint16_t *src, int read_addr, int read_nb, const uint16_t *dest); DESCRIPTION
The modbus_write_and_read_registers() function shall write the content of the write_nb holding registers from the array src to the address write_addr of the remote device then shall read the content of the read_nb holding registers to the address read_addr of the remote device. The result of reading is stored in dest array as word values (16 bits). You must take care to allocate enough memory to store the results in dest (at least nb * sizeof(uint16_t)). The function uses the Modbus function code 0x17 (write/read registers). RETURN VALUE
The modbus_write_and_read_registers() function shall return the number of read registers if successful. Otherwise it shall return -1 and set errno. ERRORS
EMBMDATA Too many registers requested, Too many registers to write SEE ALSO
modbus_read_registers(3) modbus_write_register(3) modbus_write_registers(3) AUTHORS
The libmodbus documentation was written by Stephane Raimbault <stephane.raimbault@gmail.com[1]> NOTES
1. stephane.raimbault@gmail.com mailto:stephane.raimbault@gmail.com libmodbus 3.0.3 05/26/2012 MODBUS_WRITE_AND_REA(3)