
DS26518 8-Port T1/E1/J1 Transceiver 
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9.9.5.3 Legacy T1 Transmit FDL 
It is recommended that the DS26518’s built-in BOC or HDLC controllers be used for most applications requiring 
access to the FDL. 
Table 9-21
 shows the registers related to control of the transmit FDL. 
Table 9-21. Registers Related to T1 Transmit FDL 
FRAMER 1 
ADDRESSES 
Transmit FDL Register (
T1TFDL
) 
162h 
REGISTER 
FUNCTION 
FDL code used to insert transmit FDL. 
Transmit Control Register 2 (
T1.TCR2
) 
182h 
Defines the source of the FDL. 
Transmit Latched Status Register 2 (
TLS2
) 
191h 
Transmit FDL empty bit. 
Transmit Interrupt Mask Register 2 (
TIM2
) 
1A1h 
Mask bit for TFDL empty. 
Note: 
The addresses shown above are for Framer
1. Addresses for Framers 2 to 8 can be calculated using the following: Framer
n = (Framer
1 
address + (n - 1) x 200hex), where n = 2 to 8 for Framers 2 to 8.
When enabled with 
T1.TCR2
.7, the transmit section will shift out into the T1 data stream, either the FDL (in the 
ESF framing mode) or the Fs bits (in the D4 framing mode) contained in the Transmit FDL Register (
T1TFDL
). 
When a new value is written to the 
T1TFDL
, it will be multiplexed serially (LSB first) into the proper position in the 
outgoing T1 data stream. After the full eight bits has been shifted out, the framer will signal the host controller that 
the buffer is empty and that more data is needed by setting the 
TLS2
.4 bit to a one. 
INTB
 will also toggle low if 
enabled via 
TIM2
.4. The user has 2ms to update the 
T1TFDL
 with a new value. If the 
T1TFDL
 is not updated, the 
old value in the 
T1TFDL
 register will be transmitted once again. Note that in this mode, no zero stuffing will be 
applied to the FDL data. It is strongly suggested that the HDLC controller be used for FDL messaging applications. 
In the D4 framing mode, the framer uses the 
T1TFDL
 register to insert the Fs framing pattern. To accomplish this 
the 
T1TFDL
 register must be programmed to 1Ch and 
T1.TCR2
.7 should be set to 0 (source Fs data from the 
T1TFDL
 register). 
The 
T1TFDL
 register contains the Facility Data Link (FDL) information that is to be inserted on a byte basis into the 
outgoing T1 data stream. The LSB is transmitted first. In D4 mode, only the lower six bits are used. 
9.9.5.4 Legacy T1 Receive FDL 
It is recommended that the DS26518’s built-in BOC or HDLC controllers be used for most applications requiring 
access to the FDL. 
Table 9-22
 shows the registers related to the receive FDL. 
Table 9-22. Registers Related to T1 Receive FDL 
FRAMER 1 
ADDRESSES 
Receive FDL Register (
T1RFDL
) 
062h 
REGISTER 
FUNCTION 
FDL code used to receive FDL. 
Receive Latched Status Register 7(
RLS7
) 
096h 
Receive FDL full bit is in this register. 
Receive Interrupt Mask Register 7(
RIM7
) 
0A6h 
Mask bit for RFDL full. 
Note: 
The addresses shown above are for Framer
1. Addresses for Framers 2 to 8 can be calculated using the following: Framer
n = (Framer
1 
address + (n - 1) x 200hex), where n = 2 to 8 for Framers 2 to 8.
In the receive section, the recovered FDL bits or Fs bits are shifted bit-by-bit into the Receive FDL Register 
(
T1RFDL
). Since the 
T1RFDL
 is 8 bits in length, it will fill up every 2ms (8 times 250
μ
s). The framer will signal an 
external controller that the buffer has filled via the 
RLS7
.2 bit. If enabled via 
RIM7
.2, the 
INTB
 pin will toggle low 
indicating that the buffer has filled and needs to be read. The user has 2ms to read this data before it is lost. Note 
that no zero destuffing is applied to the for the data provided through the 
T1RFDL
 register. The 
T1RFDL
 register  
reports the incoming Facility Data Link (FDL) or the incoming Fs bits. The LSB is received first. In D4 framing 
T1RFDL
 updates on multiframe boundaries and reports only the Fs bits.