wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Modbus 36-5
Request APDU
Response APDU
01 Ah Al Ch Cl 01 B D ... D
02 Ah Al Ch Cl 02 B D ... D
03 Ah Al Ch Cl 03 B Dh Dl ...
Dh
Dl
04 Ah Al Ch Cl 04 B Dh Dl ...
Dh
Dl
05 Ah Al Dh Dl 05 Ah Al Dh Dl
06 Ah Al Dh Dl 06 Ah Al Dh Dl
0F Ah Al Ch Cl B Dh Dl ... Dh Dl
0F Ah Al Ch Cl
10 Ah Al Ch Cl B Dh Dl ... Dh Dl
10 Ah Al Ch Cl
16 Ah Al M A h M A l M o h M o l
16 Ah Al
M A h M A l M o h M o l
17 A R h A R l C R h C R l A W hA W l C W h C W l D W h D W l ... D W hD W l 17 B D R h D R l ...
D R h D R l
18 Ah Al
18 Bh Bl Dh Dl ... Dh Dl
A –Starting bit/register address
C – Number (count) of referenced bit/register
Xh – High byte of 16 bit value X
Xl – Low byte of 16 bit value X
X R – Parameters for reading
X W – Parameters for writing
B – Number of remaining bytes in frame
D–Data values
M A – AND mask
M o – OR mask
FIGURE 36.4
Format of data access functions APDUs.
Note that the maximum number of addressable elements is limited by the maximum size of the
APDU. For example, for function code 0x0F, the request APDU starts off with 6 bytes containing the
function code and respective parameters (see Figure 36.4), leaving a maximum of 247 (equal to 253 − 6)
available bytes in which to send the data. The data, consisting of 1 bit elements, are packed 8 per byte,
resulting in a maximum of 1976 (247 × 8) addressable elements.
Function 0x16 (Mask Write Register) changes the value stored in a single holding register. However,
unlike the other functions, the new value is not obtained directly from the data sent in the APDU, but
rather is obtained by applying a logical operation using the current value of the holding register with
two masks, an AND mask and an OR mask, sent with the APDU.
New_value = (Current_value AND And_mask) OR (Or_mask AND (NOT And_mask))
Using this function the client can switch off certain bits and set other bits of the holding register,
while simultaneously leaving the remaining bits unchanged, in a single atomic operation.
Function 0x18 (Read FIFO Queue) allows the client to request reading a FIFO queue from the server.
The FIFO queue must be stored within the holding registers memory area in the server and consists of
a first register containing the number of elements in the queue (queue count register), followed by the
values of each element in the queue in the following registers (queue data registers). The client merely
© 2011 by Taylor and Francis Group, LLC